A Comprehensive Guide: The Manufacturing Process of Industrial Valves
What is the valve manufacturing process?
Table of Contents
- What is the valve manufacturing process?
- Features of valve manufacturing
- The casting process of the valve
- Smelting of molten valve steel
- Heat treatment of valves
- Welding process of valves
- The machining process of valves
- Grinding process for valve sealing surface
- The assembly process of valves
- Testing and Inspection of Valves
- The Development Direction of the Valve Manufacturing Process
The valve manufacturing process, which is the method of manufacturing valves, generally refers to the valve parts of the blank manufacturing, machining, welding, heat treatment, and the valve assembly process.
A wide variety of valves, specifications vary greatly in size (nominal diameter as small as 1mm, as large as several meters), the types of parts materials are also more, coupled with the production conditions of the valve factory and the experience and habits of the artisans are different, so the valve parts processing process also varies widely. Even if the same valve parts, in the production conditions of the two factories in roughly the same process, often quite different, the amount of labor spent and the technical and economic results achieved are different, here there is a reasonable process of the problem.
A reasonable process can ensure product quality, improve labor productivity, and reduce product costs. The preparation of reasonable process procedures is more complex work. Because in addition to the premise of meeting the requirements of the product design drawings, determining the type of blank, processing methods, and the choice of equipment and tooling also do a variety of process options for comparison and analysis. In addition, the preparation of the process procedures is often reasonable depending on the technical level and experience of the process personnel. Therefore, it is required that personnel engaged in technical work must seriously study and learn from the valuable experience of previous generations, become familiar with the performance of the plant’s equipment, process equipment, and processing capabilities, and design the most optimal and economical process program after analysis of the product’s comprehensive performance. After analyzing the comprehensive performance of the product, they can design the most optimal and economical process plan and prepare a reasonable process document to guide the production.
Features of valve manufacturing
At first glance, the valve parts are not many, with simple structure and general precision; in the mechanical industry is a simple part, but the core sealing parts of the valves are particularly demanding; sealing fit must be zero to zero to achieve zero leakage of the gas-tight test. Therefore, its manufacturing process is complex and technically difficult, with some of the following characteristics:
- (1) In terms of manufacturing materials, due to the wide variety of valve specifications applied in various areas of the national economy, the use of its many different occasions, such as high temperature and high pressure, low temperature, deep cold, flammable and explosive, highly toxic, highly corrosive media and other working conditions, the valve material has put forward harsh requirements in addition to cast iron, carbon steel, structural alloy steel, but also a large number of CrNi stainless steel, CrMoAl nitriding steel, CrMoV heat-resistant steel, CrMnN acid-resistant steel, precipitation hardening steel, duplex stainless steel, low-temperature steel, titanium alloy, Monel alloy, Inconel alloy, Hastelloy and G0CrW carbide, etc. The casting, welding, and processing properties of these high-alloy materials could be better, which makes the manufacturing process very difficult. Coupled with the fact that most of these materials are high alloy, high strength, and high hardness precious materials, there are many difficulties in selecting, preparing, and procuring the materials. Some materials are difficult to procure and supply due to the small amount of use.
- (2) From the structure of the casting blank, most of the valve blanks used in the complex structure of thin shell castings require not only good quality appearance but also the dense internal quality and good metallographic structure, cannot have pores, shrinkage, sand trapping, cracks, and other defects. Therefore, the casting process is complex, and the heat treatment technology is difficult. In the machinery industry, the valve pressure thin shell casting blank casting difficulty is far more complex than other mechanical components of the casting, more difficult.
- (3) From the machining process, because most of the high strength, high hardness, high corrosion-resistant materials are not good cutting performance, such as high-alloy stainless steel, acid-resistant steel have toughness, high strength, poor heat dissipation, chip viscosity, and machining hardening tendency and other shortcomings, it is difficult to achieve the required dimensional accuracy and finish, to the machining of tools, processes, and equipment to bring certain difficulties. In addition, the valve sealing surface in the machining accuracy, with the angle, finish, and pairing of the requirements of the seal, is also very high, bringing great difficulty to machining.
- (4) From the valve parts of the process arrangement, the valve’s main parts are not many, the structure is relatively simple, most of the size of the machining accuracy is not high, and the external is relatively rough, which gives the impression that belongs to the simple machinery. The heart of the valve sealing parts but extremely precise; the sealing surface of the “three degrees” (flatness, finish, hardness) requirements are very high, as well as the two sealing surfaces of the sealing sub coincidence should reach zero to zero, to meet the gas-tight test of zero leakage. This rough benchmark ensures that the heart part of the precision of the zero-to-zero requirements is the valve processing the biggest process difficulties.
- (5) From the valve test and inspection, the valve is an important pressure pipeline opening and closing, regulating components, and the use of pressure piping conditions are very different, high temperature and high pressure, low temperature, deep cold, flammable and explosive, highly toxic and corrosive. But the valve manufacturing test and inspection conditions are unlikely to meet the same requirements as the working conditions; international and domestic valve test standards are close to room temperature conditions, using gas or water as the medium for the test. This is one of the most fundamental pitfalls in the normal factory test of qualified valve products; harsh conditions may arise due to material selection, casting quality and sealing damage, and other issues to meet the requirements of use, but also major quality accidents. No wonder some old valve experts have done a lifetime of work; the older, the more restrained, the drier, the more worried.
The casting process of the valve
Valve casting is an important part of the valve manufacturing process, with good casting determining a significant proportion of the success of a good valve. The following is an introduction to the casting process design and several casting process methods commonly used in the valve industry:
(A) The casting process design of the casting:
Correct and effective control of casting solidification is the first important condition for obtaining quality cast steel parts; take the correct process measures such as: pouring system, riser and cold iron, complementary process amount, etc., to form a reasonable process program. Due to their uneven wall thickness, Valve cast steel parts should take the principle of sequential cooling sequential solidification to reduce the internal stress of the casting, shrinkage, shrinkage, and other defects.
(1) Control the process measures for sequential solidification of cast steel parts:
1) Design a reasonable parting position, pouring position, and pouring system.
2) The design of the riser in the last part of the casting solidification, while playing the role of shrinkage, delays the solidification of the steel around the riser, resulting in the conditions of sequential solidification.
3) Pouring operation, when the steel rises to the riser height 1/4, change from the top of the riser pouring; its role can increase the steel pressure head and improve the riser temperature.
4) Cast steel riser size determination: its methods are the modulus method (determined by the heat capacity of the casting), volume shrinkage method (determined by the steel solidification shrinkage rate), ratio method (determined by the type of complementary shrinkage of the casting) and the thermal knot circle method (determined by the thermal knot circle of the casting). At present, to meet the convenient design requirements in the factory, the hot-knotted circle method is commonly used to determine the size of the riser.
5) The casting shrinkage rate selected: cast steel parts in the process of solidification and cooling, their volume and size will shrink to reduce the amount of shrinkage from the liquid state solidification for the solid state is generally expressed in terms of the amount of change in length – the line shrinkage rate (%).
Many factors affect the casting shrinkage rate, the casting in the casting solid shrinkage is also affected by external resistance, which will reduce its actual shrinkage, at this time known as non-free shrinkage, and the non-free shrinkage rate is always less than the free shrinkage rate. Factors affecting the casting shrinkage rate are mainly the type of metal alloy, casting structure, size length, molding materials, core tightness, etc., which also affect the casting to produce a non-free shrinkage rate.
According to the valve production practice, to facilitate mold design, shrinkage is generally selected concerning the following table:
|Steel grade||Reduced scale|
|Carbon Steel||WCB ZG25||20/1000|
|Stainless Steel||CF8 CF8M||20-25/1000|
|Chromium Molybdenum Steel||ZGCr5Mo C5||20-25/1000|
|Heat Resistant Steel||WC6 WC9||18-22/1000|
(B) Sand casting:
Sand casting is commonly used in the valve industry; the different binders can also be divided into the wet sand, dry sand, water glass sand, and furan resin self-hardening sand.
(1) Wet sand is a bentonite binder molding process; it is characterized by the following: the sand does not need to dry, does not need to go through hardening treatment, sand has a certain wet state strength, sand core, shell recession is better, easy to clean up the castings fall sand. High molding production efficiency, short production cycle, low material cost, easy to organize assembly line production. Its disadvantages are: that castings are easy to produce porosity, sand trapping, sand sticking, and other defects, and the quality of castings, especially the internal quality is not ideal.
The ratio and performance of wet sand for cast steel parts are as follows:
|Serial Number||Ratio (%)||Performance||Use|
|New sand||Used sand||Bentonite||Sodium carbonate||Dextrin||Pulp||Moisture content %||Breathability AFS||Wet compressive strength KPa|
|Particle size group||Added|
|1||10||100||9/11||0.2||0.2/0.4||3.8/4.3||100/200||56/77||Sand for small steel castings|
|2||15||50||50||3||0.4||0.6/1.2||4/4.7||100||50/75||Sand for machine modeling|
(2) Dry sand is a molding process method with clay as the binder, and slightly adding bentonite can improve its wet strength. Its characteristics are: the sand needs to be dried, has good permeability, not easy to produce defects such as sand punching, sand sticking, porosity, etc., and the intrinsic quality of the casting is better. Its disadvantage is that it needs sand drying equipment and the cycle time of production is long.
(3) Water glass sand is water glass as the binder of the modeling process method, it is characterized by: water glass encountered CO2 has the function of automatic hardening, can have a variety of advantages of air hardening method of modeling and core making, but there is poor shell collapse, casting sand cleaning difficulties and the old sand regeneration, reuse rate is low.
Water glass CO2 hardening sand ratio and performance table:
|Serial Number||Proportioning %||Performance|
|New sand||Water glass||Alkali solution 15-20%||Bentonite||Water content %||Moisture permeability||Wet compressive strength KPa||Hardening strength MPa|
|Levels of granularity||Added|
|3||21||100||4-4.5||LK-2 collapsible agent||Water 0.4-0.6||≤3.5||≥150||≥1.0|
(4) Furan resin self hardening sand molding is a casting method using furan resin as the binder. At room temperature, the sand is solidified due to the binder’s chemical reaction under the curing agent’s action. Its characteristic is that the sand mold does not need to be dried, which shortens the production cycle and saves energy. Resin molding sand is easy to compact and has good collapsibility. The molding sand of castings is easy to clean, with high dimensional accuracy and a good surface finish, which can greatly improve the quality of castings. Its disadvantages are high quality requirements for raw sand, slight irritating odor on the production site, and high cost of resin.
Furan resin self hardening sand mixture ratio and mixing process:
|Raw sand||Ratio of furan resin addition to weight of raw sand||Ratio of curing agent addition to weight of furan resin||Ratio of Silane Addition to Furan Resin Weight|
Furan resin self-hardening sand mixing process: resin self-hardening sand is best to use a continuous sand mixing machine; the original sand, resin, curing agent, etc., will be added in turn, rapid mixing, ready to mix, ready to use.
When mixing resin sand, the order of adding various raw materials is as follows:
Raw sand + curing agent (aqueous solution of p-toluenesulfonic acid) – (120-180S) – resin + silane – (60-90S) – out of the sand
(5) Typical sand casting process card:
(6) Typical sand casting production process:
Sand preparation – Moulding – Moulding – Pouring – Cooling – Sand drop – Cleaning – Inspection – Heat treatment -Inspection -Obtaining castings
Characteristics: Using sand to form a casting pattern and pouring usually refers to the sand casting process under gravity.
- Molding sand – a mixture made by mixing raw or recycled sand + binder + other additions, etc.;
- Molding – the cavity made of sand that forms the outline of the appearance of the casting is called the molding;
- Core – the solid made of core sand that forms the inner cavity of the casting (the sand used to make the core is called core sand);
- Molding – the process of making sand molds;
- Core making – the process of manufacturing sand cores.
(C) Precision casting:
In recent years, valve manufacturers have been paying more and more attention to the appearance quality and dimensional accuracy of castings. Because a good appearance is a basic requirement of the market, but also as a positioning benchmark for the first machining process.
Investment casting, also known as lost wax casting, includes pressing wax, trimming wax, forming trees, dipping, melting wax, casting metal liquid, and post-treatment. It is a casting process with very old origins, but still widely used today.
Principle of the investment casting process
The investment casting process is simply a process in which a soluble mold, also known as a molten mold, is made of a material that melts easily and is used to process metal castings.
The material used to make the molten mold can be wax or plastic. After the mold is made, it must be coated with several layers of special refractory coatings, which are dried and hardened to form a monolithic shell. For the monolithic shell, steam or hot water is used to melt off the model from it, and then the shell is placed in a sandbox and filled with dry sand around it for molding. Finally, the mold filled with dry sand is placed in a roaster and roasted at high temperatures.
The valve industry commonly uses precision casting as solution casting, which is briefly described as follows:
(1) Two process methods of solution mold casting:
- ① Using low-temperature wax-based mold material (stearic acid + paraffin), low-pressure injection wax, water glass shell, hot water dewaxing, atmospheric melting, and pouring process, mainly for general quality requirements of carbon steel and low-alloy steel castings, casting size accuracy can reach the national standard CT7-9 level.
- ② Using medium-temperature resin-based mold material, high-pressure wax injection, silica-sol mold shell, steam dewaxing, rapid atmospheric or vacuum melting and casting process, casting size accuracy up to CT4-6 precision castings.
(2) Typical process flow of solution mold casting:
The first step is to remove the wax cylinder from the insulation tank, remove the air mixed in the wax material and the generated hard wax due to long-term exposure, and install the wax cylinder on a dual-station hydraulic wax mold injection machine.
The second step is to place the mold on the workbench of the injection molding machine for positioning, and all core positions of the mold must be correct. Check whether the wax injection port of the mold is aligned with the injection molding machine’s wax injection nozzle and whether the mold’s opening and closing are smooth.
The third step is to open the mold and spray a thin layer of parting agent on the inner and outer surfaces.
The fourth step is to adjust the cycle time, injection pressure, injection temperature, holding time, and cooling time of the injection molding machine according to the technical requirements. After the cycle is completed, remove the core, open the mold, carefully remove the wax mold, and place it in a cooling water or storage tray as required.
The fifth step is to remove residual mold material from the mold. This process can only be completed with a bamboo knife, and remember not to use metal blades, as it can easily cause damage to the mold cavity and parting surface.
The final step is to close the mold and proceed with the next wax mold pressing.
(3) The characteristics of solution mold casting:
- ① Casting high dimensional accuracy, surface finish, and good appearance quality.
- ② Can cast the structure of complex shapes, difficult to achieve processing parts with other processes.
- ③ Casting materials are not limited to alloy materials such as carbon steel, stainless steel, alloy steel, aluminum, high-temperature alloys, precious metals, and other materials, especially difficult-to-use forging, welding, and cutting alloy materials.
- ④ Good production flexibility and adaptability. It can be mass production and is suitable for single-piece or small-batch production.
- ⑤ Solution casting also has limitations, such as cumbersome processes and long production cycles. Due to its limited means of casting process that can be used for casting pressure-bearing thin-shell valve castings, its pressure-bearing capacity cannot be very high.
(D) Analysis of casting defects
Any casting internal will have defects; the existence of these defects to the casting of the intrinsic quality of the great hidden danger in the production process to eliminate these defects for the welding patch will also bring a great burden to the production process. Especially for valves as thin-shell castings subjected to pressure and temperature, the denseness of their internal organization is very important. Therefore, the internal defects of the casting become a decisive factor affecting the quality of the casting.
Valve castings of internal defects are mainly porosity, slag, shrinkage, and cracking.
(1) Pores: pores generated by gas, smooth surface of the hole, produced in the casting or near the surface, the shape is more round or oblong.
The main sources of gas generated pores are:
- ① Metal in the dissolved nitrogen and hydrogen in the casting solidification process is included in the metal, forming closed round or oval walls with a metallic luster of the pores.
- ② The moisture or volatile material in the modeling material will turn into gas due to heat, forming a dark brown pore with an inner wall.
- ③ Metal in the pouring process, due to instability in the flow, the air will be involved and generate pores.
Prevention methods of porosity defects:
- ① In smelting, it should be used as little as possible or not use rusted metal raw materials, tools, and steel ladle to bake dry.
- ② Steel pouring to a high temperature out of the furnace, low temperature pouring, the steel should be properly sedated to facilitate gas floating.
- ③ The process design of the pouring mouth should increase the pressure head of the steel to avoid gas involvement and set up an artificial air path to exhaust reasonably.
- ④ The modeling material should control the water content, and gas generation, increase the air permeability, sand and sand core should be baked dry as far as possible.
(2) Shrinkage (loose): it is produced in the casting internal (especially in the hot section parts) and is a coherent or incoherent round or irregular cavity (cavity); the inner surface is rough, dark in color, metal grains coarse, more dendritic crystallization, gathered in one or more places, the hydraulic test is easy to occur leakage.
Produce shrinkage (loose) reasons: metal solidification from the liquid state for solid volume contraction, at this time, such as not enough steel supplement, is bound to produce shrinkage. The shrinkage of cast steel parts is caused by improper control of the sequential solidification process; the reason may be the incorrect setting of the riser, the steel pouring temperature being too high, metal shrinkage, etc.
Prevent shrinkage (pine) generated by the following methods:
- Scientific design of casting pouring system so that the steel achieves sequential solidification; the first part of solidification should be supplemented by steel.
- Correctly and reasonably set the riser, subsidies, and internal and external cold iron to ensure sequential solidification.
- In the steel pouring, the last top injection from the riser is conducive to ensuring the temperature of the steel and the complementary shrinkage, reducing the generation of shrinkage holes.
- In the pouring speed, low speed pouring is better than high speed pouring for sequential solidification.
- The pouring temperature should not be too high; the high temperature of the steel out of the furnace, after calming pouring, is conducive to reducing shrinkage.
(3) Sand trap (slag): sand trap (slag), commonly known as trachoma, is the appearance of incoherent circular or irregular holes in the interior of the casting; the hole is interspersed with sand or steel slag, size is not regular, gathered in one or more places, often more in the upper part of the type.
Causes of sand (slag): Slag is due to the steel in the smelting or pouring process, discrete steel slag with the steel into the casting formed. Sand is due to modeling cavity tightness is not enough; when the steel is into the cavity, the sand washed up by the steel into the castings caused by the internal. In addition, the improper operation when repairing the mold and closing the box, there are sand drop phenomenon is also the cause of sand trapping.
Prevent sand trapping (slag) generated by the following methods:
- Steel smelting to exhaust slag thoroughly, the steel out of the furnace in the steel ladle after the sedation, conducive to the steel slag float.
- Steel pouring package as far as possible without turning the package, but with a teapot package or bottom injection package to avoid the upper part of the steel slag along the steel into the casting cavity.
- In the steel pouring, take castor slag measures to minimize the steel slag with the steel into the cavity.
- To reduce the possibility of sand trapping, in the modeling to ensure the tightness of the sand, repairing the type of attention not to drop sand before closing the box to blow the cavity clean.
(4) Crack: Most of the cracks in the casting are thermal cracks, which are irregular in shape, penetrating or non-penetrating, continuous or intermittent, and the metal at the crack is dark or has surface oxidation.
The cause of cracks has two aspects: high-temperature stress and liquid film deformation.
High-temperature stress is the steel shrinkage deformation at high temperatures and the formation of stress; when the stress exceeds the strength or plastic deformation limit of the metal at that temperature will produce a crack. Liquid film deformation is a liquid film between the steel grains in the solidification and crystallization process; with the solidification and crystallization, the liquid film deformation, the deformation and deformation rate exceeds a certain limit, producing a crack. The temperature range of thermal cracking is about 1200 – 1450 ℃.
The factors influencing the generation of cracks:
- ① S, P elements in steel is a harmful factor to produce cracks; they and iron eutectics reduce the strength and plasticity of cast steel at high temperatures, resulting in cracks.
- ② Slag and segregation in steel increase the stress concentration, thus increasing the tendency of thermal cracking.
- ③ The larger the linear shrinkage coefficient of the steel grade, the greater the thermal cracking tendency.
- ④ The greater the thermal conductivity of the steel grade, the greater the surface tension, the better the high temperature mechanical properties, and the less the tendency of thermal cracking.
- ⑤ The structural design of the casting is not well crafted, such as rounded corners are too small, wall thickness disparity is too large, stress concentration is serious, and will produce cracks.
- ⑥ The tightness of the sand is too high, and the poor yielding of the core will increase the tendency of cracking by hindering the shrinkage of the casting.
- ⑦ Other such as the improper arrangement of the sprue, casting cooling speed being too fast, cutting sprue, and heat treatment caused by excessive stress will also affect the generation of cracks.
Given the above causes and factors of cracking, take corresponding measures to reduce and avoid generating cracking defects.
Comprehensive analysis of the causes of the above casting defects, finding the existing problems, and taking corresponding improvement measures, you can find a solution to the casting defects, which is conducive to improving casting quality.
Smelting of molten valve steel
The quality of molten steel in valve castings has a decisive impact on the quality of castings. Therefore, it is necessary to strictly control the steelmaking process to ensure the quality of molten steel.
The purpose and requirements of steelmaking are:
- ① Melt solid furnace charge.
- ② Make the elements in molten steel reach the specified composition.
- ③ Remove harmful elements S and P and reduce them below the limit.
- ④ Remove gases and non-metallic inclusions from the molten steel to ensure purity.
- ⑤ Overheat the molten steel to a certain temperature to ensure the pouring needs.
There are main methods for smelting, such as induction furnaces, electric arc furnaces, and vacuum refining furnaces, as well as differences between oxidation and nonoxidation smelting processes. The following focuses on introducing the basic knowledge of induction furnaces and electric arc furnace steelmaking commonly used in the valve industry.
(1) Induction furnace steelmaking: (See the figure below for induction furnace)
The principle of induction furnace melting metal is to use alternating current induction to generate eddy currents in the furnace material, and use the heat generated by the eddy currents to heat and melt the furnace material. The advantages of induction furnace steelmaking are high production efficiency and strong production mobility. The rapid induction furnace used in the valve industry in China has high power, fast temperature rise, easy temperature control, and short melting time. It only takes more than an hour to smelt a furnace of steel, resulting in high production efficiency.
The disadvantage of an induction furnace is that it can only eat refined materials during batching. As the chemical composition cannot be adjusted during induction furnace steelmaking, it can only add burnt-out elements, so its steelmaking process is just a melting process. Unlike electric arc furnace steelmaking, induction furnace steelmaking does not have an oxidation period, and its slag removal, exhaust, and C, P, and S removal functions are not strong. The quality of molten steel mainly depends on ingredient control, so the internal quality of castings is difficult to guarantee strictly.
In the past, when some factories used induction furnaces for steelmaking to improve the quality of molten steel, oxygen blowing was carried out in the early stage of molten steel melting to increase the oxidation period. However, blowing oxygen can seriously damage the furnace wall and cannot be repaired in a timely manner during the steelmaking process, so this process has yet to be widely used.
(2) Electric arc furnace steelmaking: (Electric arc furnace as shown in the figure below)
When making steel in an electric arc furnace, the heat generated by melting the furnace material and superheated molten steel is generated by the electric arc. Electric arc furnaces are divided into acidic and alkaline furnaces based on the different refractory materials of the furnace lining. An acidic furnace produces acidic slag, which cannot be desulfurized or phosphate, and requires furnace materials with low sulfur and phosphorus content. Alkaline slag produced by alkaline furnaces can remove sulfur and phosphorus, but it has low requirements for furnace materials and is widely used.
Steelmaking is a complex chemical and physical process that can be summarized into four main reactions:
(1) Oxidation reaction: The method of removing impurities during molten steel melting is to oxidize the impurities, blow oxygen or add oxidant (iron ore) to the steel, make the steel boil, and remove impurities generated by gases and oxides such as carbon, silicon, and phosphorus from the steel into the slag or furnace gas. This stage is called the oxidation period.
(2) Reduction reaction: After the oxidation process is completed, there is a large amount of residual iron oxide in the steel, which is extremely harmful to the quality of the molten steel and must be removed. The method of removal is to add a deoxidizer (reducing agent) to the molten steel, which reacts with the iron oxide in the molten steel to capture the oxygen in the iron oxide and form some new insoluble compounds that enter the slag, thereby removing the oxygen in the molten steel. This stage is called the reduction period.
(3) Phosphorus and sulfur removal reaction: Phosphorus and sulfur are harmful elements in molten steel, which will increase the tendency of steel to undergo hot and cold cracking. During the steelmaking process, attention must be paid to removing them. Phosphorus is oxidized during the oxidation boiling period and combines with calcium oxide to form stable compounds that enter the slag. Sulfur exists in molten steel as a compound with manganese, which can react with calcium oxide to form stable compounds that enter the steel slag.
(4) Slagging reaction: In popular terms, steelmaking is slag-making. Slag is a mixture of slag-making materials (fluxes) added to the furnace during the steelmaking process, as well as impurities and various oxides. The role of slag is twofold: ① protection. The slag covers the surface of the molten steel, preventing it from coming into contact with the furnace gas, reducing the intrusion of harmful gases into the molten steel, and protecting useful elements in the molten steel from oxidation and burning. ② A series of reactions carried out through slag to remove impurities from molten steel.
The typical process for smelting cast carbon steel by oxidation method:
|Period||Item||Working procedure||Key points of operation|
|Melting period||1||Power on||Power supply with maximum allowable power|
|2||Assistive melting||Pushing material for melting, blowing oxygen for melting, and adding slag ore|
|3||Sampling and slag removal||The furnace material is fully melted, fully stirred, and sampled for analysis C and P|
|Oxidation period||4||Oxygen blowing decarbonization||The temperature of molten steel reaches 1560 ℃, ferrosilicon is added, and oxygen blowing is used for decarburization|
|5||Sampling||Stop blowing oxygen, take a steel sample, and analyze C, P, Mn|
|Reduction period||6||Slag removal and deoxidation||Remove oxide slag, add manganese iron and slag material, and make thin slag|
|7||Reduction||Add reducing slag for reduction|
|8||Sampling||Fully stir the molten steel and take steel samples for analysis of C, P, Mn, S|
|9||Adjusting ingredients||Adjust the chemical composition based on the analysis results of the steel sample|
|10||Temperature measurement||Measure the temperature of molten steel and check the deoxygenation condition|
|Steel tapping||11||Tapping||The temperature of the molten steel meets the standard, aluminum is inserted, and then the steel is tapped with a large opening, and the steel slag flows together|
|12||Pouring||Calm the molten steel for 5 minutes, start pouring, and take a steel sample|
The valve body and valve stem are the main parts that require heat treatment on valves. This article focuses on the heat treatment of valve body castings and stainless steel valve stems.
Heat treatment of valve body casting blanks
(1) Heat treatment of carbon steel castings: To eliminate casting stress, refine the metallographic structure, improve mechanical properties, and improve cutting performance, carbon steel castings usually adopt an annealing or normalizing + tempering heat treatment process.
- ① Annealing: Carbon steel castings are generally made of WCB, and the heating temperature for annealing is 880-920 ℃. The thickness of the casting wall determines the insulation time and is generally 2-5 hours. After insulation, it is cooled in the furnace.
- ② Normalizing + tempering: To improve the strength of steel castings, normalizing can be used instead of annealing, which means that after annealing and insulation, cooling in the furnace will be changed to air cooling out of the furnace. However, normalizing and cooling will generate significant stress, requiring additional high-temperature tempering. The microstructure obtained by normalizing and tempering is more uniform, the grain size is finer, and the overall mechanical properties are better than those obtained by simple annealing.
(2) Heat Treatment of Austenitic Stainless Steel Castings
Austenitic stainless steel has excellent corrosion resistance, high-temperature mechanical properties, and oxidation resistance and can still maintain good low-temperature impact toughness at ultra-low temperatures.
The main defect of austenitic stainless steel is its susceptibility to intergranular corrosion. To overcome this defect, in addition to reducing the carbon content of the steel (C ≤ 0.08%) and adding stable elements (titanium, niobium) to the steel, solid solution treatment is added to improve corrosion resistance.
The heating temperature of casting solid solution treatment is an important factor affecting heat treatment quality. Although the composition of various grades of austenitic stainless steel varies, the heating temperature for solution treatment does not differ significantly, all within the temperature range of 1000-1150 ℃. The heating speed should be faster in the range of 430-820 ℃ to avoid the precipitation of the chromium carbide phase. The insulation time depends on the thickness of the casting wall and the amount of furnace charge. Generally, the insulation coefficient is 1 hour for every 25 mm wall thickness. After being discharged from the furnace, quenching should be adopted, especially when passing through the temperature range of 430-820 ℃, the cooling speed should be fast, and water cooling is generally used.
The solid solution treatment process curve of ZG1Cr18Ni9 casting is shown in the following figure.
Heat treatment of stainless steel valve stem blank
(1) Annealing treatment of Cr13 type valve rod blank: Cr13 steel belongs to martensitic stainless steel, and the commonly used valve rod material is 1Cr13 or 2rC13. The heat treatment of the 2rC13 valve rod blank is briefly introduced as the representative.
After forging the 2Cr13 valve stem blank, it is necessary to anneal it promptly, as even if air cooling is used after forging, it will harden, making mechanical processing difficult. In addition, due to factors such as forging stress, cracks can easily occur on the valve stem. Therefore, softening treatment should be carried out after forging to eliminate stress, reduce hardness, prevent cracks, improve machining performance, and make organizational preparations for subsequent quenching and tempering treatment. The softening treatment method for 2Crl3 forgings usually uses an annealing process in valve production.
The heating temperature for annealing is generally 840-860 ℃. The insulation time mainly depends on the diameter of the valve stem blank and the charging amount. When heating and air furnaces, the insulation coefficient is generally 60 minutes per 30 millimeters of diameter. After insulation, the furnace is cooled. When the furnace is cooled to 500 ℃, it can be discharged for air cooling.
(2) Cr13 valve stem blank quenching and tempering treatment: The quenching and tempering treatment combines quenching and high-temperature tempering to obtain good comprehensive mechanical properties and corrosion resistance to meet the use needs.
The most suitable temperature for quenching and heating 2Cr13 valve stem blanks is 980-1000 ℃. If the heating temperature is low.
At 950 ℃, chromium carbide (mainly Cr23C6) cannot be fully dissolved, which is not conducive to improving mechanical properties and reducing the material’s corrosion resistance. If the heating temperature exceeds 1050 ℃, the structure can easily be overheated. After quenching, the martensite structure is coarse, significantly reducing the impact toughness. At this time, even if the tempering temperature is subsequently increased, the impact toughness cannot be improved. The insulation time should be sufficient to allow the carbides to dissolve fully. The insulation time mainly depends on the effective diameter of the valve stem blank, and the furnace loading amount, and the insulation coefficient can generally be 90 minutes per 30 millimeters of diameter insulation. The critical cooling temperature of 1Cr13 is relatively low, and its hardenability is excellent, so oil cooling can be used for quenching and cooling 2Cr13 valve stems. After quenching, tempering should be carried out promptly, with a general interval of no more than 48 hours, to prevent cracking of the 2Cr13 valve stem. The tempering temperature is determined according to technical requirements. When the hardness requirement of the 2Cr13 valve stem is HB240-280, the tempering temperature is generally 600-650 ℃, and the tempering is cooled with oil.
(3) Precipitation hardening stainless steel valve stem heat treatment:
For valves used in highly corrosive media, the valve stem needs to be made of precipitation-hardened stainless steel. In addition to adding Cr and Ni, elements such as Al, Cu, Co, Ti, Mo, and Nb are added to precipitation-hardened steel.
The commonly used materials are 17-4PH (0Cr17Ni4Cu4Nb) and 17-7PH (0Cr17Ni7Al). They precipitate fine metal compounds through heat treatment for high strength and corrosion resistance. 17-4PH (0Cr17Ni4Cu4Nb) belongs to the martensite series precipitation hardening stainless steel. The time temperature structure after solution treatment is the martensite. Precipitation hardening is carried out after aging at 400-650 ℃ to precipitate copper rich precipitates to strengthen. The corrosion resistance of this steel is higher than that of 13Cr series stainless steel, and the welding process is simple and has good comprehensive mechanical properties.
17-7PH (0Cr17Ni7Al) belongs to the semi austenitic precipitation hardening stainless steel series, with a low carbon content and corrosion resistance similar to the 18-8 series stainless steel. Due to the special treatment of precipitation hardening, steel has high strength and elasticity and good cold working performance, welding performance, and high-temperature strength in the austenitic state.
The heat treatment of precipitation-hardened stainless steel is relatively complex, and a series of heat treatment specifications have been applied according to different requirements, including:
- 1) Homogenization treatment: Heat the blank to 1050-1200 ℃, keep it warm for about 2 hours, and cool it.
- 2) Solid solution treatment: Heat the blank to 1020-1060 ℃, keep it warm for about 2 hours, and then water cool it.
- 3) Austenite conditioning and martensite transformation treatment: T, R treatment, such as TH1050, RH950, CH900, etc. This series of heat treatments have different temperature control and operation specifications, so carbide in austenite is precipitated, and the metallographic structure is transformed into martensite.
- 4) Aging treatment: the aging treatment makes the martensite matrix precipitation hardening, produces a fine dispersed precipitation phase, and obtains high strength and good comprehensive mechanical properties.
- The heat treatment of precipitation-hardened stainless steel is a complex process that requires strict requirements. It must have complete equipment conditions and be carried out under the guidance and operation of professional personnel to ensure the quality of heat treatment.
Welding process of valves
Welding materials that meet the usage requirements on the substrate of valve parts as valve sealing surfaces can significantly improve the hardness, wear resistance, and scratch resistance of the valve sealing surface, thereby greatly improving the valve’s service life and saving a large number of precious metals. Therefore, valve products produced by domestic and foreign valve manufacturers mostly use surfacing sealing surfaces.
The copper sealing surface of cast iron low-pressure valves often adopts the structural form of pressure ring flanging. This section introduces several commonly used surfacing welding methods for steel valve sealing surface materials.
(1) Manual arc surfacing of Cr13 stainless steel: Manual arc surfacing of Cr13 stainless steel is the most widely used surfacing method in steel high and medium pressure valves.
The chemical composition of Cr13 stainless steel overlay welding rods is equivalent to 1Cr13 and 2Cr13, and these welding rods are commonly used at temperatures below 425 ℃; The operating pressure is 1.6-16.0Mpa; The base material is forged or cast from carbon steel or low alloy steel, which serves as the sealing surface for valves used in power plants and petrochemical industries. There should be a slight difference in the hardness of the welding material for the sealing pair of the valve body (seat) and gate (disc) in the valve, with a general difference of HRc5. The wider moving sealing surface takes a higher hardness value, while the narrower stationary sealing surface takes a lower hardness value.
The commonly used grades of Cr13 stainless steel surfacing welding rods in the valve industry include D502, D507, D512, D517, D507Mo, D577, D547, D547MO, etc. Specialized specification tables exist for their chemical composition, welding parameters, and post weld performance. Valve manufacturers can choose according to the product’s requirements and the parts’ surfacing requirements according to the welding rod’s specifications.
The basic process for overlaying the sealing surface of valves with Cr13 type welding rods is as follows:
- Before welding, the surface of the workpiece must be rough turned or sandblasted to remove oxide skin. The surface of the overlay welding must not have defects such as cracks, pores, sand holes, looseness, oil stains, rust, etc.
- Before using the welding rod, it should be dried according to the instructions for use. The welding rods should be stored in a dedicated insulation cylinder during the welding process. If the welding rod is left unused for a long time after drying, in order to avoid the formation of pores in the welding layer, the welding rod must be dried again.
- Before using D507MO and D577 welding rods for surfacing, the workpiece generally does not need to be preheated. When welding large parts with 2Cr13, preheating is required before welding.
- The welding surface of the workpiece should be kept in a horizontal position, and the entire welding process should not be interrupted. The number of welding layers is generally 2-3 to meet the requirements of welding layer height, chemical composition of the welding layer, and hardness of the sealing surface.
- In order to prevent cracks on the sealing surface of the surfacing welding, in addition to taking appropriate preheating measures before welding, it is still necessary to pay attention to post welding insulation and slow cooling. Important surfacing parts also need to undergo heat treatment.
(2) Manual surfacing of cobalt based hard alloy: The sealing surface of cobalt based hard alloy arc surfacing has better hardness, wear resistance, and corrosion resistance than the sealing surface of Cr13 stainless steel surfacing. The commonly used cobalt based hard alloy welding rods in the valve industry include D802, D812, D807, D817, etc. There are specialized specification tables for their chemical composition, welding parameters, and post weld performance. Valve manufacturers can choose according to the welding rod specifications according to the surfacing requirements of the parts.
The pre welding preparation work for the surfacing parts, such as preheating the workpiece before welding and heat treatment after welding, is shown in the table below for the temperature range:
|Base material type||Preheating temperature before surfacing ℃||Post weld heat treatment temperature ℃|
|Low alloy heat-resistant steel||300-400||680-720|
|Austenitic stainless steel||250-300||525-575|
1) Welding precautions:
- ① Before using the welding rod, it should be dried according to the instructions for use. If the welding rod is left unused for a long time after drying, the welding rod must be dried again to avoid forming pores in the welding layer.
- ② When the weldability of the base material is poor or the workpiece rigidity is large, a 1Cr18NI9 austenitic stainless steel welding rod can be used to weld the transition layer to improve the welding process performance of the base material.
- ③ Thinner electrodes and smaller currents should be used as much as possible to reduce the dilution rate.
- ④ The lateral swing amplitude of the welding rod should not be too large.
- ⑤ When the height of the deposited metal is required to be more than 3 millimeters, it is necessary to weld two or more layers.
2) Precautions for multi-layer overlay welding:
- ① It is necessary to use a grinding wheel or wire brush to polish the surface of the pre-weld metal of multiple welds smooth, flat, and free of welding slag and other dirt.
- ② Pay attention to controlling the interlayer temperature, which should be slightly higher than the predetermined preheating temperature.
(3) Tungsten electrode manual argon arc surfacing of cobalt-based hard alloys:
Cobalt-based hard alloy’s manual hydrogen arc surfacing has a shallow melting depth, less alloy burning loss, no splashing, and good surfacing formation. Due to argon gas protection, the welding layer quality is high, making it a widely used surfacing method in the valve industry. Commonly used surfacing welding wires include wire 111, 112, Co106, STL6, STL12, etc.
Precautions for manual argon arc surfacing of cobalt-based hard alloys:
- The temperature for preheating the workpiece before welding and heat treatment after welding; refer to the table above.
- Before welding, it is necessary to check the operation of the high-tech oscillator, the protective gas (argon) control circuit, the cooling water flow rate, and the welding power supply voltage for normal operation.
- The arc length should remain unchanged during the welding process.
- The swing amplitude of the tungsten electrode should not be greater than three times the diameter of the tungsten electrode.
- During multi-pass surfacing, the edges of the weld bead should be flat and not too thick to avoid poor fusion between multiple weld beads, resulting in defects such as incomplete penetration, slag inclusion, and air holes.
- When the workpiece is small, due to the surfacing workpiece’s rapid heating, the surfacing surface is oxidized, causing difficulty in the surfacing. At this point, the preheating temperature should be reduced or not preheated. Attention should be paid, especially when the base material is austenitic stainless steel.
- Generally, there are two layers of surfacing, and when the height of surfacing is required to be less than 3mm, one layer can be overlayed.
- The treatment of transition layer surfacing, multi-layer surfacing, and the beginning and end of surfacing is the same as manual arc surfacing.
(4) Alloy powder without plasma spray welding:
Alloy powder plasma arc surfacing emerged in the early 1960s and has been widely used in the valve manufacturing industry in China in recent years. It is a relatively important new surfacing process. It not only has the advantages of high welding quality and low dilution rate of tungsten argon arc surfacing but also has the characteristics of easy mechanization of the surfacing process, smooth and flat surfacing layer, accurate thickness control, and high production efficiency.
1) Working principle: Powder plasma arc surfacing is the process of making filler metal into powder and, according to the width requirements of the workpiece’s surfacing layer, feeding the welding powder into the plasma arc for deposition to form a surfacing layer. Cobalt-based hard alloy plasma arc surfacing generally adopts a transfer-type arc, which starts with a non-transfer arc. When the transfer arc stabilizes and burns off, the non-transfer arc is cut off, and only the transfer arc is left for operation. This can be achieved with an independent power source.
2) Process characteristics
- ① High temperature and high heat transfer rate are one of the main characteristics of plasma arc as a surfacing heat source, and the maximum temperature of transfer type plasma arc can reach 3000 ℃. Not only can it improve welding efficiency, but it can also weld refractory metal materials.
- ② Low dilution rate and high deposition efficiency of alloy materials are other process features of plasma arc surfacing.
- ③ The plasma arc is extremely stable and has strong directionality. Therefore, the quality of surfacing is easy to ensure.
The machining process of valves
Typical Process Specification for Valve Parts
1) The significance of typification of valve parts process regulations
The specifications and varieties of valves are diverse, with significant differences in size, and there are also many types of component materials. In addition, the production conditions of each valve factory and the experience and habits of process personnel are different, and the process of preparing valve parts could be more consistent. A reasonable process ensures product quality, improves labor productivity, and reduces production costs. Developing reasonable process procedures is a complex task, as in addition to meeting the requirements of product drawings, determining the type of blank, processing method, and selecting equipment and tooling, multiple process plans need to be compared and analyzed. In addition, the rationality of preparing process procedures often depends on the technical level and experience of the process personnel.
Due to the obvious similarity in the structure, shape, and process of valve parts, the number of valve parts is also limited. This is a favorable condition for achieving the typification of valve parts process regulations. The typification of process flow is to compile the processing process of valve parts into typical process procedures based on their similar shapes, sizes, and processes.
Typical process procedures can serve as the basis for process personnel to develop specific parts of process procedures, thereby shortening the production technology preparation work cycle and ensuring the rationality of process procedures. In addition, the typification of process procedures can also create favorable conditions for the generalization and standardization of process equipment, the organization of processing assembly lines for similar parts, and the use of specialized and efficient equipment.
Currently, most manufacturers in the valve industry in China use typical process procedures and group processing techniques for single-piece and small batch production. This method helps to simplify production organization, arrange process flow, and plays an important guiding role in ensuring the actual needs of the production site and meeting various quality certifications
2) Classification of valve parts
Preparing typical process procedures requires the classification of valve parts in advance. The shape of any mechanical part is composed of several surfaces (flat, cylindrical, conical, special-shaped, etc.). The surfaces of different shapes and parts are combined to form different shapes of various parts, and the processing methods for surfaces of the same shape are generally the same. For example, the outer cylindrical surface is usually processed by turning or grinding methods; The inner cylindrical surface is obtained through machining methods such as drilling, reaming, boring, and grinding. Therefore, there is a method of classifying parts based on the shape of their main machined surfaces, such as straight cylinder valve bodies, three-way valve bodies, frame valve covers, disc valve plates, etc. Similar parts have similar geometric shapes, often with similar process routes and similar processing processes, and can be produced using typical process procedures and group processing techniques.
Due to the wide variety and specifications of valve parts, only three major categories of parts, namely three-way valve body, frame, ball cap valve cover, and valve core closure parts, are represented below to introduce some typical processing methods.
Processing of valve body parts
The flange straight through three-way valve body is the most commonly used structural form for gate valves, globe valves, check valves, pressure reducing valves, and throttle valve bodies. This structure has many valve bodies and a wide range of applications, so the flange straight through gate valve body is a typical introduction.
1) Structure and process analysis of gate valve body: The valve body is a hollow, thin-walled shell part, mostly in the shape of a three-way pipe, used in linear pipelines. The inlet channel and outlet channel of this valve body are coaxial, so it is called a straight through type (see the figure below). The ends of the two channels are circular flanges with evenly distributed bolt holes for connection with the flanges on the pipeline. The upper end of the valve body also has a middle flange for connecting the valve cover.
The valve body comprises cylindrical surfaces, conical surfaces, spherical surfaces, and special shaped surfaces. Most external and internal surfaces do not need to be machined, so the parts are generally made of castings. The left and right cavities of the valve body are designed with high-precision cylindrical valve seat holes, and for the valve seat structure, there are also high-precision threaded holes. The valve body with a welding sealing surface has a wedge angle (mostly 5 °) and a wedge sealing area with high smoothness. To guide the gate plate to match the sealing surface of the valve body accurately, there are two symmetrical guide ribs in the body cavity. Because the valve body is a thin-walled shell under pressure, there are high requirements for the blank casting’s strength, stiffness, and internal quality.
The sealing surface of the gate valve body requires high angular accuracy and high accuracy in the distance between the centers of the two sealing surfaces (commonly known as the opening width) to ensure the normal mating position of the gate plate. There are also certain requirements for symmetry between the two sealing surfaces with symmetrical centerlines and guide ribs.
The main machined surfaces of the valve body are mostly rotating surfaces; therefore, all other surfaces are turned except for the guide rib parts that are machined on a slot or planer. Due to the high requirements for precision and smoothness of the sealing surface and the large machining allowance of the casting blank. The gate valve body can be divided into two stages: rough and fine machining. During rough machining, the three flanges are first machined, and then the sealing surface is precision turned with an angled inclined tire based on the flange. After precision turning, the sealing surface still cannot meet the drawing’s geometric accuracy and smoothness requirements and must undergo polishing and grinding. During this period, the drilling process for three flanges can be arranged.
2) Typical processes for gate valve bodies:
According to the size of the valve diameter, there are two processing methods for gate valve bodies:
The processing plant for smaller diameters: Using the three outer circulars (conical) surfaces of the valve body as the rough reference for positioning, first process the middle flange and stop, and then use the middle flange and stop positioning as the precision reference for processing the flanges, guide ribs, and sealing surfaces at both ends. The machining of three flange bolt holes is based on the end flange as its benchmark.
The processing of two sealing surfaces is usually carried out on a lathe using a rotary fixture, using the same positioning reference as the Chinese flange, so that the processing of the two sealing surfaces is completed in one installation. The fixture ensures the accuracy of the angle of the valve seat hole or sealing surface, and the symmetry of the sealing surface and the opening size is controlled by the machine tool stop in conjunction with the clamp measuring tool. This solution requires a 180 ° rotating fixture with a sloping chassis, and the valve is only suitable for smaller diameter valves with DN ≤ 100. (The rotating fixture is shown in the following figure)
|Itme||Process content||Positioning reference||Fixture features|
|1||End face, outer circle, and seam of flange in the vehicle||Outer circular (conical) surface of the three necks||Lower three V-shaped positioning seats|
|2||Flange surfaces, outer circles, and sealing surfaces at both ends of the vehicle||Middle flange end face and stop||The lower part is a flat plate rotary fixture|
|3||Planing guide bars||Middle flange end face and stop||Middle flange positioning bending tire|
|4||Sealing surface area on both sides of the vehicle||Middle flange end face and stop||The lower part is a swashplate rotary fixture|
|5||Drill the middle flange hole||Middle flange end face and stop||Drilling template|
|6||Drill flange holes at both ends||Flange faces and outer circles at both ends||Drilling template|
2) Processing plan for medium and large caliber: First, process the flanges at both ends, and then use the end flanges as the precision benchmark to sequentially process the middle flange, guide rib, sealing face position, and flange bolt holes.
Fixtures mainly ensure the angle processing of the above two sealing surfaces. The same angle motherboard is usually used when manufacturing diagonal tire fixtures to achieve this goal. Two mother plates with the same angle as the sealing surface of the valve body are used, one of which is used as the mother plate for manufacturing and processing the fixture for the sealing surface of the valve body. Overlapping two mother plates (equal to twice the oblique angle of the valve body sealing surface) can serve as the mother plate for manufacturing and processing the clamp for the sealing surface of the gate. The fixture manufacturing using this method can ensure that the angle between the valve body sealing surface and the gate sealing surface is completely consistent. To ensure the symmetry of the two sealing surfaces and the accuracy of the crotch width size, it is necessary to improve the manufacturing accuracy of the entire length of the valve body to control the half length size. The full opening size is controlled using a dedicated opening measuring tool.
The advantage of this solution is that the process equipment is relatively simple, which is conducive to the standardization and generalization of fixtures. Its disadvantage is that the processing of the two sealing surfaces requires two clamping operations, and the positioning reference for the two clamping operations is not uniform, which inevitably leads to positioning errors. With corresponding process measures, this processing plan is easier to produce valve bodies that meet interchangeable assembly requirements. Due to independent individual fixtures, this plan is suitable for processing large and medium caliber valve bodies and is widely used in the valve industry.
|Item||Process content||Positioning reference||Fixture features|
|1||Flange end face and outer circle at one end of the vehicle||End flange outer circle||Four jaw chuck|
|2||The end face and outer circle of the other flange of the vehicle||Finished flange end face and outer circle||Stop positioning plate|
|3||End face, outer circle, and seam of flange in the vehicle||End faces and outer circles of flanges at both ends||V-shaped positioning and pressing plate at both ends|
|4||Draw a processing line for guide ribs||Middle flange stop|
|5||Planing (inserting) guide bars||Middle flange end face and stop||Stop positioning plate|
|6||Sealing surface area on both sides of the vehicle||End flange end face and end stop||Diagonal tire stop positioning plate|
|7||Drill the middle flange hole||Middle flange end face and stop||Drilling template|
|8||Drill flange holes at both ends||End flange end face and end stop||Drilling template|
Below are several machining fixtures for valve bodies:
Processing of valve cover parts
The valve cover is an important component that supports the valve stem and transmission mechanism and forms a sealed pressure chamber with the valve body. The valve cover structures matched with the flange through the valve body mainly include frame and ball cap types.
1) The structure and typical process of the frame type valve cover:
Due to its complex structure, frame type valve covers typically use castings of the same material as the valve body. Its main processing surface is rotating, often processed using turning methods. Due to the large flange of the valve cover and the small outer circle of the small end, when machining the flange, if the outer circle of the small end is positioned, the clamping point is relatively far from the machining surface, resulting in poor rigidity, which affects the machining quality and efficiency. During processing, the small end can be roughly turned first and then positioned with the small end’s inner hole and the large flange’s back. The outer circle of the large end flange can be processed with a sleeve tire: end face and upper sealing area.
The following are typical processes commonly used in mass production using general equipment:
|Item||Process content||Positioning reference||Fixture features|
|1||Rough turning small end flat surface, inner hole, flange back||Outer circle and alignment of large end flange||Four jaw chuck|
|2||Outer circle, end face, and middle hole of large flange||Small end inner hole, flange back||Sleeve mold|
|3||Fine turning of small end face and inner hole parts||Large flange end face and stop||Stop positioning plate|
|4||Draw a line to determine the hinge screw groove and pin hole||Large flange end and centerline|
|5||Milling hinge screw slots||Large flange end face and stop||Stop positioning plate|
|6||Drill pin shaft hole||Large flange end face and stop||Stop positioning plate|
|7||Drill large flange holes||Large flange end face and stop||Drilling template|
2) Process analysis and typical process of ball cap valve cover:
The upper and lower ends of the ball cap valve cover are equipped with flanges, which are respectively connected to the bracket and valve body. Due to its complex structure, the blank is made of castings with the same material as the valve body.
The main structural feature is that the main processing surfaces are rotating surfaces, but there is a significant difference in the size of the large and small flanges. The small flanges often appear rectangular, which poses certain difficulties in processing.
The following are typical processes commonly used in mass production using general equipment:
|Item||Process content||Positioning reference||Fixture features|
|1||Coarse turning small flat surface, inner hole, flange back||Outer circle and alignment of large end flange||Four jaw chuck|
|2||Outer circle, end face, and middle hole of large flange||Small end flange and end stop||Stop positioning plate|
|3||Fine turning of small end face and inner hole parts||Large flange end face and stop||Stop positioning plate|
|4||Draw a line to determine the hinge screw groove and pin hole||Large flange end and centerline|
|5||Milling hinge screw slots||Large flange end face and stop||Stop positioning plate|
|6||Drill pin shaft hole||Large flange end face and stop||Stop positioning plate|
|7||Drill small flange holes||Small end flange stop||Drilling template|
|8||Drill large flange holes||Large flange end face sealing||Drilling template|
Processing of valve core closure parts
The valve core closure component refers to the moving part that plays a closing role in the valve, which is used to cut off, regulate, or change the flow direction of the medium. There are significant differences in the structure and shape of the closing components among different types of valves, such as the gate plate of the gate valve, the disc of the globe valve, the ball of the ball valve, the disc of the butterfly valve, and the cone plug of the plug valve. All closure components have sealing surfaces with high precision and smoothness that match the sealing surface of the valve body, which are more important and difficult to process than other mechanical parts of the same shape. Among them, the difficulty of the wedge gate of the gate valve is particularly complex, and the ball processing of the ball valve is quite typical. Below is a representative introduction to the gate and ball processing technology.
(1) Processing of Wedge Gate
1) The structural characteristics of the wedge-type gate: The gate is a disc with convex blocks on the outer edge, with inclined sealing surfaces (mostly 5 °) symmetrical to the center surface at both ends. A T-shaped groove or thread is connected to the valve stem at the protruding part of its upper end. To enable the gate to open and close smoothly in the valve body, there are guide grooves or ribs on both sides of the outer edge of the gate.
Due to the different uses of valves, the surfacing materials used for the sealing surface of the gate are also different. Commonly used materials include copper, stainless steel, cobalt chromium tungsten hard alloy, etc. There are three methods for forming the sealing surface of the gate: directly turning it out from the body, turning it out after welding, and embedding the sealing ring.
The structural forms of gate plates can be further divided into two categories: elastic and nonelastic rigid. Small specifications often use forged integral blanks, while medium and large specifications often use cast hollow elastic gate plate structures.
2) Typical process of wedge gate:
|Item||Process content||Positioning reference||Characteristics of tooling|
|1||Draw centerline and guide groove, T-shaped groove line||Three scoring jacks, 90 degree bending ruler|
|2||Milling guide grooves on both sides||Align and level the lower diagonal tire according to the line marking||Double sided milling machine, single angle inclined tire|
|3||Milling the upper T-shaped groove||Guide grooves on both sides||Positioning and clamping of guide grooves on both sides|
|4||Sealing and welding base surface at one end of the vehicle||T-shaped groove end face, guide grooves on both sides||Single angle inclined tire, T-shaped groove end face centering, guide groove compression|
|5||Sealing and welding base surface at the other end of the vehicle||Overlay welding base surface and process seam||Dual angle inclined tire, T-shaped groove end face centering, guide groove compression|
|6||Overlay welding sealing surface (heat treatment)|
|7||Sealing surface at one end of the vehicle (with grinding allowance)||Guide groove, T-shaped groove||Single angle inclined tire, T-shaped groove end face centering, guide groove compression|
|8||Sealing surface at the other end of the car (with allowance for wear)||Sealing surface and inner seal||Dual angle inclined tire, T-shaped groove end face centering, guide groove compression|
|9||Milling elastic grooves||Sealing surface and inner seal||Single angle inclined tire, T-shaped groove end face centering, guide groove compression|
|10||Grinding two sealing surfaces||Sealing surface||Dual angle inclined tire, T-shaped groove end face centering, guide groove compression|
|11||Grind two sealing surfaces||Sealing surface||Grinder (vibration, planetary)|
The following are several fixtures for processing the gate:
(2) Processing of ball valve balls: The spherical surface of the ball is the main processing surface, and its basic processing sequence is: first, turn out the channel hole according to the process requirements, and then use the channel hole as the positioning reference to process the spherical surface.
In addition to the spherical surface, the sphere with a handle also has high-precision cylindrical surfaces with upper and lower axes. These cylindrical surfaces can be processed using the top hole as the positioning reference. Because the through-holes of this type of sphere are perpendicular to the journals at both ends, there are certain difficulties in machining the sphere. Therefore, necessary measures must be taken during machining to ensure the quality of the sphere.
Processing methods for spherical surfaces: There are usually two methods for processing spherical surfaces:
1) Milling method: The following figure is a spherical surface milling schematic diagram. This method involves installing a cutter head with two turning tools at the front end of the milling head, which an electric motor drives to rotate. The tips of the two knives should be in the same rotating plane. The rotation plane must be perpendicular to the axis of the milling head and parallel to the rotation axis of the workpiece. The axis of the cutter head should pass through the ball center, and the distance between the two tool tips can be determined based on the diameter of the processed ball.
Milling balls can be performed on regular lathes or vertical milling machines. When milling a ball, the speed of the cutter head is generally 900-1200 rpm, and the speed of the workpiece is 1-10 rpm.
The milling method has high efficiency and a good surface finish (generally up to Ra3.2), making it suitable for various ball diameters. The cutter head can be formed in one go with only radial feed when processing small spheres. When machining a large diameter sphere, the small cutter head must perform a semi rotational motion along the rotating tool holder to mill out a large sphere. Note that the axis of the cutter head should be perpendicular to the rotation axis of the ball and pass through the center of the ball. If the axis of the cutter head is higher or lower than the rotation axis of the ball, the processed ball will be elliptical in shape.
2) Turning method: This is the most widely used method for machining spherical surfaces. The turning ball method involves installing a turning ball device on a regular lathe or using a regular turning tool on a specialized machine to machine the spherical surface.
Single piece small batch production often involves machining spherical surfaces on ordinary lathes. The following figure shows the ball turning device. This device is directly installed on the bed guide rail, causing the rotary table to rotate. The turning tool installed on the small tool holder can perform spherical turning.
The characteristics of this car ball device are simple structure and convenient operation. It is directly fixed on the bed guide rail of the machine tool, so it has good rigidity, and stable and reliable operation, and the gap between the rack and gear can be adjusted through inclined blocks, thus avoiding vibration during the cutting process.
Two tool holders can be installed on the rotary table to reduce the number of workpiece installations and improve processing efficiency (as shown in the above figure). Install coarse turning tools on the rear tool holder and fine turning tools on the front.
The right figure shows the spring spindle used for machining spherical surfaces. When using, thread the ball blank onto the spindle with one end tightly against the positioning pad. Tighten the nut on the right end to support the ball tightly, then loosen the other end and remove the positioning pad. Install the spindle between the machine tooltips, shake the large sliding plate, and the center of the rotating disc coincides with the center of the ball vertically. After trial cutting, the position of the large sliding plate can be fixed.
A large-sized sphere with a supporting shaft, with some through holes pre-cast. This increases the difficulty of processing. To avoid intermittent cutting during the machining process, the through-hole is turned after the journal’s rough and spherical surface machining.
Then, seal the hole with a plug (made of the same material as the valve body) and perform precision machining on the spherical surface. The figure on the right shows the plug of the spherical channel hole. After precision turning off the spherical surface, sand belt grinding can be carried out on the machine tool, and the plug cover can be removed after grinding.
Spherical processing is mostly carried out with specialized machine tools when producing large quantities of spheres.
3) Grinding of spherical surfaces: The spherical surface can be ground using the outer circle of a flat grinding wheel or the inner side of a bowl-shaped grinding wheel on an ordinary lathe or a specialized machine tool.
① The outer circle of a flat grinding wheel can be ground on an ordinary lathe using the outer circle of the grinding wheel. Usually, the grinding head needs to be installed on a rotary tool holder, and the outer circle of the grinding wheel rotates and grinds to form a spherical surface.
② Grinding the spherical surface with a bowl shaped grinding wheel is suitable for small diameter spherical surface grinding. Generally used for spherical surfaces with a diameter of less than 100 millimeters. During grinding, the grinding head can be installed on the middle slide of an ordinary lathe or a specialized machine tool. When grinding, the axis of the bowl-shaped grinding wheel must pass through the ball center. If the axis of the grinding wheel does not pass through the center of the ball, the spherical ground surface will be elliptical. The following figure shows the situation of grinding a spherical surface with a bowl-shaped grinding wheel.
When using the inner side of the grinding wheel for grinding, the grinding wheel and the spherical surface are in circular contact. When grinding stainless steel balls, the working surface of the grinding wheel is easily blocked by chips and loses its grinding effect, often burning the ball surface. To avoid the above phenomenon, in addition to using a structured grinding wheel, several grooves are made in the working part of the bowl-shaped grinding wheel, or sand strips are used to make the grinding wheel. To ensure the quality of grinding the spherical surface, a large amount of emulsion is required for lubrication and cooling during grinding. At the same time, the grinding wheel must be trimmed promptly. After each trimming, the remaining sand particles on the surface of the grinding wheel must be brushed clean with a brush to avoid scratching the spherical surface with residual sand.
Grinding process for valve sealing surface
Grinding is a commonly used finishing method and plays a significant role in valve manufacturing. The metal sealing surface of valves is mostly achieved through grinding to meet its smoothness and flatness requirements, which plays a decisive role in the sealing quality of valves.
Grinding of valve sealing surface
(1) The basic principle of grinding: During grinding, the grinding tool is attached to the surface of the workpiece, and the grinding tool undergoes complex grinding movements along the attached surface. The abrasive is injected between the grinding tool and the surface of the workpiece, and some of the abrasive particles in the abrasive slide or roll between the two surfaces. In contrast, the other part of the abrasive particles is embedded or fixed in the surface layer of the grinding tool. When the grinding tool moves relative to the workpiece, the abrasive particles cut a very thin layer of metal off the surface of the workpiece. The convex peaks on the workpiece are first ground off, and then the surface gradually reaches the required geometric flatness.
Due to the sliding and rolling of some abrasive particles between the grinding tool and the surface of the workpiece, the surface of the grinding tool is also worn by the abrasive, and the geometric accuracy of the grinding tool itself directly affects the geometric accuracy of the workpiece. Therefore, in addition to requiring the material wear resistance and uniform organization of the grinding tool, the wear of the grinding tool should also be uniform to ensure its flatness accuracy as long as possible. Grinding is the mechanical processing of metals by abrasives and the chemical action. The grease in the grinding agent can form an oxide film on the processed surface, accelerating the grinding process.
(2) Grinding motion: To ensure uniform grinding of all points on the workpiece surface and uniform wear of the grinding tool, the relative sliding distance between each point on the workpiece surface and the grinding tool should not be the same when the grinding tool is in relative motion with the workpiece. The direction of relative motion between the grinding tool and the workpiece should constantly change. The continuous change in the direction of motion ensures that each abrasive particle does not repeat its trajectory on the surface of the workpiece but should have a mesh like trajectory (as shown in the figure below) to avoid causing obvious wear marks and reducing the smoothness of the workpiece. Although the grinding motion is complex and the direction of motion changes, the grinding motion is always carried out along the bonding surface between the grinding tool and the workpiece. When the grinding tool moves, it cannot leave the fitting surface, and there should be no other mandatory guidance. The geometric accuracy of the workpiece is mainly affected by the geometric accuracy of the grinding tool and the grinding motion.
(3) Grinding speed: The faster the grinding movement, the higher the grinding efficiency. The excessive grinding speed can cause the workpiece to generate heat, reducing its dimensional and geometric accuracy.
When high grinding accuracy is required, the grinding speed is generally 30 meters per minute. The grinding speed of the valve sealing surface is related to the sealing surface material. The grinding speed of copper and cast iron sealing surfaces is about 10-45 meters per minute, hardened steel and hard alloy sealing surfaces are about 25-80 meters per minute, and austenitic stainless steel sealing surfaces are about 10-25 meters per minute.
(4) Grinding pressure: The grinding efficiency increases with the increase of grinding pressure, usually using counterweights, springs, or hydraulic devices to apply grinding pressure. The grinding pressure is generally 1-4 kg/cm2. When grinding the sealing surfaces of cast iron, copper, and austenitic stainless steel materials, the grinding pressure is 1-3 kg/cm2. Hardened steel and hard alloy sealing surfaces are 1.5-4 kg/cm2. The larger value can be taken for rough grinding, and the smaller value can be taken for fine grinding.
(5) Grinding allowance: Grinding is a finishing process with a very small grinding allowance, which depends on the machining accuracy and surface finish of the previous process. On the premise of ensuring the removal of machining marks from the previous process and correcting geometric shape errors of the workpiece, the smaller the grinding allowance, the better.
The plane allowance of the sealing surface after precision turning is 0.01-0.05mm. The sealed surface, after grinding, can be directly ground, with a minimum grinding allowance of 0.006-0.015 mm. When manual grinding or material hardness is high, take the small value; when mechanical grinding or material hardness is low, take the large value.
(6) Grinding tool materials: There are two requirements for grinding tool materials: firstly, they should be easy to embed abrasive particles, and secondly, they should be able to maintain the geometric shape accuracy of the grinding tool for a relatively long time. The grinding tool material should have good wear resistance, and its structure should also be uniform. Uniformly organized materials also exhibit uniform wear, which is beneficial for the geometric shape accuracy of the grinding tool.
Gray cast iron grinding tools are suitable for grinding sealing surfaces of various metal materials, which can achieve good grinding quality and high production efficiency. The commonly used gray cast iron grades are HT250 and HT300. There are shallow grooves on the working surface of the grinding tool, which can accommodate a large amount of abrasive during grinding. When the abrasive is scarce, the abrasive in the groove can be automatically added to the grinding surface, thereby improving the grinding efficiency. Due to the geometric error generated during use, which affects the geometric accuracy of the workpiece, the grinding tool should be frequently trimmed.
Nowadays, some valve manufacturers also use hard steel flat surfaces for grinding or pressing and pasting sandpaper or cloth strips on the grinding plate for finishing, which improves production efficiency and conditions and has also achieved good results in use.
(7) Grinding agent: The grinding agent is a mixture of abrasive and grinding fluid, and the correct selection of grinding agent can improve the efficiency and quality of grinding.
① Abrasives: There are several commonly used abrasives:
- Alumina Al2O3: also known as corundum, has two types: artificial and natural. Colors include brown, white, and light purple. Aluminum oxide has high hardness, low price, and is widely used. Generally used for grinding workpieces made of cast iron, copper, steel, and stainless steel.
- Silicon carbide SiC: It has a higher hardness than aluminum oxide and is available in green and black. Green silicon carbide is suitable for grinding hard alloys, while black silicon carbide is used for grinding brittle and soft material workpieces, such as cast iron, brass, etc. Silicon carbide is also widely used.
- Chromium oxide Cr2O3: dark green, is a kind of abrasive with high hardness and fine. Chromium oxide is often used for finishing hardened steel and is generally used for polishing sealing surfaces.
The particle size of an abrasive is called particle size, and the number of mesh holes per inch of length represents the particle size number obtained by the screening method. For example, 20 # particle size refers to 20 mesh holes per inch of length. Due to the difficulty in manufacturing too fine screens, the sedimentation method is generally used to separate abrasive particles. At this time, the particle size is represented by the size of the abrasive particles, such as W20 particle size, which represents the size of micro powder abrasive particles ranging from 14 to 20 microns.
The size of abrasive particles significantly impacts grinding efficiency and surface finish after grinding. During rough grinding, the requirement for surface smoothness of the workpiece is low. To improve grinding efficiency, coarse-grained abrasives should be selected. During precision grinding, the grinding allowance is small, and the requirement for surface smoothness of the workpiece is high. Fine grained abrasives can be used. When rough grinding the sealing surface, the particle size of the abrasive is generally 120 # -240 #; During precision research, it is W40-14.
② Grinding fluid: Grinding fluid is used to mix abrasives and plays a role in lubrication and cooling during grinding. Some grinding fluids (such as strong oxidizing stearic acid, oleic acid, industrial glycerol, etc.) also have chemical effects, which attach to the surface of the workpiece and quickly form an oxide film on the processed surface. During grinding, the oxide film on the protrusion of the workpiece is first removed, and the exposed metal surface is oxidized again. The newly formed oxide film is easily removed again, and as this continues, the protrusion gradually flattens out. The oxide film on the concave surface of the workpiece prevents the metal from continuing to oxidize due to not being worn off. This chemical action of the grinding fluid improves the efficiency of grinding.
Factories can use purchased grinding paste. A small amount of grinding paste is placed in a container and mixed evenly with diluents (water, glycerol, kerosene, etc.) before use.
Manual grinding of valve sealing surface
Wet grinding is generally used for manual grinding. During the wet grinding process, it is necessary to frequently add thin grinding agents to flush dull abrasive particles off the working surface and continuously add new abrasive particles to achieve high grinding efficiency. For sealing surfaces with particularly high requirements for precision and smoothness, sometimes a flat plate of sanding cloth (paper) is also used for dry grinding.
(1) Grinding of valve body sealing plane: The valve body sealing plane is located in the inner cavity of the valve body, which is difficult to grind. Usually, a disc shaped grinding tool with a square hole is used, placed on the sealing surface of the inner cavity, and then a long handle with a square head is used to drive the grinding plate for grinding motion. There are cylindrical protrusions or guide washers on the grinding plate to prevent uneven grinding caused by the local departure of the grinding tool from the annular sealing surface during the grinding process.
The grinding pressure should be uniform, with a higher pressure during rough grinding and a lower pressure during fine grinding. Care should be taken not to partially detach the sealing surface of the grinding tool due to pressure application. After grinding for some time, check the unevenness of the workpiece. When contact marks are evenly displayed on the annular sealing surface, and the ratio of the minimum radial contact width to the sealing surface width (i.e., the fit between the sealing surface and the inspection flat plate) reaches the specified value in the process, the unevenness can be considered qualified.
(2) Grinding of the sealing surface of the gate: The sealing surface of the gate and valve seat can be manually ground using a grinding plate. Before starting work, evenly apply a layer of abrasive on a clean flat plate. After fitting the workpiece onto the plate, you can rotate it with your hand and perform linear or zigzag movements. Due to the continuous changes in the direction of the grinding motion, the abrasive particles continuously play a role in new directions, thus improving grinding efficiency.
(3) Grinding of conical sealing surfaces: Manufacturing and repairing conical sealing surfaces is relatively difficult, but due to the large sealing force formed by the conical surface and good sealing performance, conical sealing is commonly used on high-pressure small diameter globe valves, plug valves, and butterfly valves.
The conical sealing surface requires a grinding rod or sleeve with a taper. The taper of the grinding rod and grinding sleeve should be consistent with the taper of the valve body sealing surface of the valve disc sealing surface, respectively. Spiral shaped shallow grooves are also required on the grinding rod and cone surface of the grinding plug body and plug to accumulate excess grinding agent. When grinding the stop valve body, due to the short sealing cone and poor stability, a guide plate is usually added at the flange stop in the valve body to keep the grinding rod vertical and stable.
The taper of the grinding rod of the grinding plug body and the grinding sleeve of the grinding plug should be consistent. Otherwise, leakage will easily occur between the conical sealing surfaces after grinding. Some factories grind the plug body first and then directly match the plug body with the valve body when grinding the plug.
(4) Grinding of spherical surfaces: The grinding of spherical surfaces often uses cast iron bowl-shaped grinding discs with shallow grooves on the surface, coated with grinding paste, and manually swung and pushed onto the slowly rotating spherical surface. Many manufacturers also use the method of fixing sand strips on bowl shaped metal plates instead of grinding plates for spherical grinding (see figure above), which has achieved good results.
Mechanical grinding of valve sealing surface
Most valve factories use mechanical equipment to grind the sealing surface, which has high efficiency and stable quality. Most grinders are self-made specialized equipment with various types and varying grinding effects. When selecting a valve grinder, it is crucial to consider the complex grinding trajectory and reasonable motion, followed by the grinding efficiency.
(1) Pendulum type grinding machine: Pendulum type grinding machine is designed for grinding the sealing surface inside the valve body and is often modified using old vertical drilling machines. It can also be used to grind the sealing surface of gate plates, valve seats, and valve discs. It has the advantages of reasonable grinding trajectory and wide applicability, making it particularly suitable for use with small and medium-sized valves.
The spindle of the vertical drill drives the rotation of the eccentric sleeve, causing the grinding disc to rotate while also engaging in yaw motion. Therefore, the trajectory of each abrasive particle on the grinding disc is in a grid pattern. Therefore, the grinding of the workpiece and the wear of the grinding tool are relatively uniform, and the efficiency is also high.
The grinding pressure is obtained by hanging heavy objects on the handle of the vertical drill, which can be adjusted by changing the weight.
(2) Planetary grinder
The planetary grinder is suitable for grinding the external sealing surfaces of valve discs, valve seats, and gate plates.
The structure of this grinder is simple and convenient to use. It can grind multiple workpieces simultaneously, resulting in high efficiency. The workpiece’s shape accuracy and surface finish after grinding are of good quality.
Its working principle is that the motor drives the grinding plate to rotate through a worm gear reducer. Due to the unequal linear velocities at various points along the diameter direction of the grinding disc, the circular ring with an outer gear ring placed on the grinding disc is forced to rotate around a fixed point by the central roller, resulting in a relatively complex grinding trajectory. When several circular workpieces are placed inside the ring simultaneously, the grinding trajectory becomes more complex due to mutual collision and interference between the workpieces.
Grinding pressure is generally obtained by the self weight of the workpiece. The weight of the wedge gate valve disc is unevenly distributed on the circumference of the sealing surface, and a counterweight tool can be used.
(3) Vibration Grinder:
The vibration grinder is suitable for grinding small and medium-sized stop valves’ sealing surfaces and checking valve discs. The equipment has a simple structure, reliable operation, and can grind dozens of valve disc parts simultaneously, so it has high production efficiency. Due to the grinding disc’s high vibration frequency and small amplitude, the direction of relative motion between the workpiece and the grinding disc is constantly changing. Therefore, not only is the geometric accuracy and glossiness of the workpiece after grinding good, but the wear of the grinding disc is also relatively uniform.
When the motor fixed on the grinding plate rotates, the vibration of the grinding plate installed on several sets of springs is caused by the swinging effect of the eccentric wheel on the motor shaft. The workpiece freely placed on the grinding plate experiences a short relative sliding due to its inertia and the vibration of the grinding plate. The relative sliding of workpieces on the grinding plate is irregular, and collisions between workpieces often occur, making the grinding motion more complex. Therefore, the grinding of workpieces and the wear of the grinding plate are relatively uniform.
Common quality problems and their correction methods in grinding
Grinding is a finishing method with minimal cutting volume, and there are generally few grinding waste products. A common grinding quality problem is that it is easy to cause geometric shape errors of the ground surface due to uneven grinding. For example, when grinding the sealing surface of the valve body, there is often a protrusion in the middle of the sealing surface and a lower inner and outer edge. At this point, two methods can be used to correct:
① Using a specially designed grinding disc with a slight protrusion in the middle to increase the grinding and cutting effect on the protrusion in the middle of the sealing surface, thereby correcting the phenomenon of the high sealing surface center and the low two edges.
② Another method is to clean the grinding tool and sealing surface, apply abrasive locally to the protrusions of the sealing surface, and then grind it. This also increases the grinding and cutting effect on the middle protrusion of the sealing surface and can also correct the phenomenon of high center and low two edges of the sealing surface.
After finishing the sealing surface, it should also be polished with felt and chromium oxide abrasive to improve the smoothness of the sealing surface and remove individual abrasive particles embedded on the surface. Otherwise, sealing leakage is still prone to occur during valve performance testing.
The assembly process of valves
The valve assembly is the final stage of the manufacturing process. Valve assembly combines various parts and components of a valve according to specified technical conditions, making it a product.
Parts are the most basic unit of the valve assembly, and several parts comprise the valve’s components (such as valve cover, valve disc components, etc.). The assembly process of combining several parts into a component is called a component assembly. Combining several parts and components into a valve is called total assembly.
Assembly work has a significant impact on product quality. Even if the design is correct and the parts are qualified, the valve will only meet the specified requirements if the assembly is properly, and there may be sealing leakage. Therefore, special attention should be paid to using reasonable assembly methods to ensure the quality of the final product of the valve.
The assembly process specified in document form during production is called the assembly process specification.
Common assembly methods for valves
There are three commonly used assembly methods for valves, namely the complete exchange method, repair method, and selection method.
(1) Complete Interchange Method: When the valve is assembled using the complete interchange method, each part of the valve can be assembled without any modification or selection, and the assembled product can meet the specified technical requirements. At this point, the valve parts must be processed completely according to the design requirements to meet the dimensional accuracy requirements and geometric tolerances.
The advantages of the complete exchange method are: the assembly work is simple and economical, workers do not need a high level of technical proficiency, the production efficiency of the assembly process is high, and it is easy to organize assembly lines and professional production. However, when using fully interchangeable assembly, there is a high requirement for the machining accuracy of the parts. Suitable for valves with relatively simple structures such as globe valves and check valves and valves with medium and small diameters.
(2) Selection method: The valve is assembled using the selection method, and the parts can be processed according to economic accuracy. A certain size with adjustment and compensation functions is selected to achieve the specified assembly accuracy during the assembly. The principle of the selection method is the same as the repair method, but there are differences in the method of changing the size of the compensation ring. The former uses the method of selecting accessories to change the size of the compensation ring. In contrast, the latter uses the method of trimming accessories to change the size of the compensation ring. For example, the top core and adjusting gasket of the double gate wedge gate valve and the adjusting gasket between the two bodies of the split ball valve are selected as compensation components in the dimension chain related to assembly accuracy. The required assembly accuracy is achieved by adjusting the thickness and size of the gasket. To ensure that fixed compensation components can be selected in different situations, it is necessary to pre-make a set of washers and shaft sleeve compensation components with different thicknesses and sizes for selection during assembly.
(3) Repair method: The valve is assembled using the repair method, and the parts can be processed according to economic accuracy. During assembly, a certain size with adjustment and compensation functions is repaired to achieve the specified assembly purpose. For example, most manufacturers adopt the repair method process due to the high processing cost of achieving interchangeability requirements for the gate and valve body of wedge gate valves. When controlling the opening size of the gate sealing surface during final grinding, the method of “matching the plates” with the opening size of the valve body sealing surface is used to achieve the final sealing requirements. Although this method adds the “plate matching” process, it greatly simplifies the dimensional accuracy requirements of the previous processing process. The skilled personnel in the “plate matching” process will not affect production efficiency overall.
Assembly process of valves
Valves are generally assembled on a fixed site, and the assembly and assembly of parts and components are carried out in the assembly workshop. All necessary parts and components are transported to the assembly site. Usually, component assembly and general assembly are carried out by several groups of workers simultaneously, which shortens the assembly cycle, facilitates the use of specialized assembly tools, and has relatively low requirements for the technical level of workers. Some foreign manufacturers or high-tech valves also adopt the mode of assembling hanging lines or assembling turntables.
(1) Preparation before assembly: Before assembly, valve parts must remove burrs formed by mechanical processing and welding residue and clean and cut fillers and gaskets.
(2) Cleaning of valve parts: As a fluid pipeline control device, the inner chamber of the valve must be clean. Especially for valves used in nuclear power, medicine, and food industries, the valve chamber’s cleanliness requirements are stricter to ensure the purity of the medium and avoid medium pollution. Before assembly, valve parts should be cleaned to remove chips, residual lubricating oil, coolant, burrs, welding slag, and other dirt from the parts.
Valves are cleaned by spraying with alkaline water or hot water (kerosene can also be used for brushing) or cleaning in an ultrasonic cleaning machine. After grinding and polishing, the parts need to be finally cleaned. The final cleaning usually involves brushing the sealing surface with gasoline, blowing it dry with compressed air, and wiping it clean with a cloth.
(3) Preparation of fillers and gaskets: Graphite fillers are widely used for their advantages, such as corrosion resistance, good sealing performance, and low friction coefficient. Fillers and gaskets prevent media leakage between the valve stem, valve cover, and flange joint surface. These accessories should be prepared for cutting and receiving before valve assembly.
(4) Valve assembly: Valves are usually assembled using the valve body as the reference part in the sequence and method specified by the process. Before assembly, parts and components should be inspected to prevent parts that have not been deburred or cleaned from entering the final assembly. The parts should be handled carefully during the assembly process to avoid collisions and scratches on the machined surface. The moving parts of the valve (such as valve stem, bearings, etc.) should be coated with industrial grease.
Bolts mostly connect the valve cover and the flange in the valve body. When tightening the bolts, they should be tightened symmetrically, staggered, multiple times, and evenly; otherwise, the joint surface of the valve body and cover will leak due to uneven circumferential force. The handle used during tightening should be a short time to avoid excessive pre tightening force that may affect the strength of the bolt. Valves with strict requirements for pre tightening force should use a torque wrench to tighten the bolts according to the specified torque requirements.
After the final assembly is completed, the control mechanism should be rotated to check whether the movement of the valve opening and closing parts are flexible and whether there is any jamming phenomenon. Whether the installation direction of valve covers, brackets, and other components meets the requirements of the drawings and the valve can only be tested after passing all inspections.
Testing and Inspection of Valves
What is valve testing?
The test of the valve is carried out after the final assembly is completed, which is the most important and final process to control the quality of the valve and to check whether the product meets the design requirements and quality standards. During testing, defects in the material, blank, heat treatment, mechanical processing, and valve assembly can generally be exposed.
There are many items for valve performance testing, including strength and sealing tests, flow characteristics, pressure characteristics, simulated life, fire resistance tests, high and low-temperature tests, driving device tests, sensitivity tests, etc. Nuclear power valves also need to undergo seismic tests, hot and cold cycling tests, and end-loading tests to the requirements of ANSI B16.41.
Conducting all project tests on each valve is necessary and impossible during valve manufacturing. When developing new products or having special requirements, comprehensive testing belongs to the type of test of the product. In normal production, only the items specified in the technical conditions of the valve are tested one by one, such as the strength test and sealing test, commonly referred to as the pressure test of the valve, before leaving the factory.
(1) Test medium: The test medium for valves is generally water, air, or other inert gases. The strength test usually uses water as the medium, commonly called the “hydraulic strength test”. When conducting strength tests with gas, if the valve ruptures, it can easily cause personal injury, so safety protection measures must be strengthened. In valve standards, it is usually specified that the gas pressure for the “airtightness test” is 0.4-0.7Mpa.
(2) Test pressure: The pressure for valve strength testing is generally specified to be 1.5 times the nominal pressure PN; The valve sealing test pressure is generally specified to be 1.1 times the nominal pressure PN. There will be specific numerical provisions in the technical conditions for valves with special requirements.
(3) Duration of test pressure: During valve testing, the pressure should gradually increase to the specified value and not increase sharply or suddenly. After reaching the specified pressure, the pressure should be maintained for a certain period, at which point the pressure in the system should remain unchanged. During the duration of the test pressure, the valve for the strength test did not leak; If there is no leakage or the leakage rate is within the allowable standard range of the valve for the sealing test, it can be considered qualified in its strength or sealing test. The test time can be extended if there is any doubt during the experiment.
The duration of test pressure for general industrial valves shall comply with the provisions of GB/T13927 or JB/T9092. The pressure duration of valves for special purposes such as vacuum and low temperature should be executed according to relevant technical conditions when users have special requirements.
(4) Leakage rate: The leakage rate refers to the amount of leakage per unit time of the valve. There shall be no leakage during the strength test of the valve, and leakage is generally not allowed during the sealing test. Valves that allow leakage during sealing tests, gate valves for low-pressure water, and swing check valves have a slightly higher allowable leakage rate, specified in technical standards.
(5) Leak detection method
1) Gas test leak detection method:
When conducting a sealing test with gas, the simplest leak detection method is to apply a layer of soap solution to the surface of the tested valve. If there is a leak, soap bubbles will appear. This method can quickly identify whether the valve is leaking and where it is leaking and determine the amount of leakage based on the number of bubbles. Due to the varying sizes of generated bubbles, the leakage amount obtained by this method needs to be more accurate.
Another leak detection method is the immersion method. The immersion method is to immerse the valve in water, and bubbles will be generated if there is a leak. At this point, a simple device can collect the leaked gas and measure a more accurate leakage amount. This method is relatively simple and feasible, and it is easy to determine the leakage location for taking repair measures. Many valve manufacturers use flipped immersion pressure testing tables to achieve this function.
2) Liquid leak detection method:
When conducting strength tests with liquids, direct visual observation can be used. If leakage is found on the inspected surface, water droplets or water flow will appear. In this way, not only can the leakage location be found, but the leakage amount can also be determined based on water droplets. During the strength test, not only are water droplets not allowed to appear, but there should also be no wet “sweating” phenomenon.
When conducting a sealing test with liquid, nondripping water droplets appear on the edge of the valve sealing surface. After being wiped dry, they no longer appear within the specified pressure duration, which can be considered as no leakage. On the contrary, if water droplets appear again within the specified test pressure duration after being wiped dry, regardless of the number and size of the water droplets, it is considered that the valve has leakage. The leakage rate can be calculated by measuring its leakage amount, and the valve can be verified to be qualified against the standard.
Test methods for valves
(1) Strength test:
Valves are pressure vessels that need to meet the requirement of withstanding medium pressure without leakage. Therefore, the valve body, valve cover, and its connecting parts should not have defects such as cracks, shrinkage, air holes, and slag that affect strength. In addition to strict inspection of the appearance and internal quality of the blank, the valve manufacturer conducts strength tests individually to ensure the valve’s performance.
The test is usually conducted at room temperature at 1.5 times the nominal pressure PN. During the test, the valve closure component was in an open state, and one end of the valve was closed with a blind plate. Media was injected from the other end, and the test pressure was applied. Check the external surfaces of the valve body, valve cover, and connecting parts. If no leakage is found within the specified test duration, it is considered that the strength test of the valve is qualified.
To ensure the reliability of the test, the strength test should be conducted before the valve is painted. The air in the inner chamber should be completely discharged when using water as the medium.
The filler test is usually conducted simultaneously with the strength test to observe whether there is any leakage at the filler. The upper sealing test is usually conducted together with the strength test. During the test, raise the valve stem to the limit position so that the upper conical surface of the valve stem is in close contact with the upper sealing surface of the valve cover. After loosening the packing, check its sealing performance.
Suppose leakage is found in the valve body casting during the test. In that case, repair welding can be carried out according to the technical specifications within the allowable range of technical conditions. Still, the strength test must be conducted again after the repair welding.
(2) Sealing test:
All shut-off valves should have a closed sealing performance, so sealing tests should be conducted on each valve before leaving the factory. Valves with seals should also undergo a sealing test.
The test is usually conducted at room temperature at 1.1 times the nominal pressure PN. If some valves have special requirements for the sealing test pressure, the technical requirements shall be followed.
Due to two sealing pairs, gate, and ball valves require bidirectional sealing tests. During the test, first, open the valve, block one end of the flange, and introduce pressure from the other end. When the pressure rises to the specified value, close the valve and then gradually release the pressure at the blocked end for inspection. The other end should also repeat the above test (as shown in the figure above). Another test method for gate valves is to maintain the test pressure inside the body cavity and simultaneously check the bidirectional sealing of the valve from both ends of the passage. However, some foreign standards do not allow this test method, as it may be difficult to detect penetrating defects in the core of the cast gate.
During the stop valve test, after the valve disc is closed, the medium is introduced from the inlet end, the test pressure is applied, and inspection is carried out at the outlet end (see the figure on the right). However, there are also high-pressure shut-off valves that adopt a design of upper inlet and lower outlet. The medium should also be introduced from the inlet end during the test, and the test pressure should be applied.
When testing a check valve, the pressure should be introduced from the outlet end and checked at the inlet end.
During the sealing test, the closing force of the manual valve is determined by the nominal pressure and nominal diameter. Valves are usually only allowed to be closed with normal physical strength without using other auxiliary devices. In the past, the old standard (JB790-65) stipulated that when the diameter of the handwheel φ≥φ, Two people are allowed to close at 320 millimeters. Some standards stipulate that the closing force of manual valves cannot exceed 360N.
Valves with a driving device should be tested with the driving device in use, and when equipped with a manual device, their sealing should also be tested manually.
The sealing test of the valve should be carried out after the final assembly, as the test not only needs to verify the sealing performance of the valve closure parts but also the sealing performance of the packing and flange gasket.
In addition, after the valve sealing test is qualified, a pressure opening action test should be conducted to test the opening performance of the valve opening and closing components under pressure conditions.
(3) Low temperature sealing test:
With the development of modern technology, liquid oxygen, liquid nitrogen, liquid hydrogen, and liquefied natural gas have been widely used, resulting in an increasing demand for low-temperature valves.
After passing the strength and sealing tests at room temperature, low-temperature valves are subjected to sealing tests at low temperatures to verify their sealing performance under low-temperature conditions.
There are two commonly used methods for sealing tests of low-temperature valves in the Japanese valve industry: immersion and cold insulation. The recommended test method in the Chinese standard JB/T7749 “Technical Conditions for Low Temperature Valves” is the impregnation method, mainly introduced here.
Immersion method: The method and device for conducting low-temperature sealing tests on valves are shown in the right figure. The test temperature is -196 ℃, the coolant is liquid nitrogen, and the test medium is nitrogen or helium.
The specific test operations are clearly defined in the corresponding technical specifications.
The valve is considered qualified if the leakage amount does not exceed the allowable value within the specified pressure duration.
The test temperature is measured using a temperature sensor. Sensors are attached to 4-6 temperature measurement points. The wires of each temperature measurement point in the valve chamber are led out from the valve packing.
The cooling medium for low-temperature testing can be selected according to the following table:
|Test temperature ℃||-46||-101||-120||-160||-196|
|Cooling medium||Alcohol + dry ice||Ether + liquid nitrogen||Alcohol + liquid nitrogen||Ethylene pentane + liquid nitrogen||Liquid nitrogen|
(4) Vacuum sealing test:
The vacuum sealing test is a highly sensitive sealing test method. Valves used in the aerospace and nuclear industry and with high sealing requirements are generally subject to vacuum sealing tests. Vacuum testing is usually carried out after the strength and sealing test of the valve at room temperature are qualified. To ensure the accuracy of the test, the tested valve should have high cleanliness and a finely machined sealing surface.
During the experiment, draw the tested valve to the specified vacuum degree in the open state, then close the tested valve and stop the vacuum pump from releasing air. Start the detection until the pressure increase △ P within the specified time is measured, and then calculate the leakage rate of the valve.
As is well known, all metal materials experience deflation in a vacuum. The leakage rate measured in the experiment results from two factors: gas leakage and material deflation. Therefore, the sensitivity of the static pressure boosting method is often influenced by the material release rate and calculation accuracy.
A more accurate vacuum sealing test method is helium mass spectrometry leak detection. The method uses a vacuum pump to pump the tested valve to the specified vacuum degree and then apply a mixture of helium containing gas (helium cover or helium injection) outside the tested part of the valve. If there is a leak, nitrogen will enter the tested part of the valve, and the nitrogen mass spectrometer leak detector in the system can display it, based on which the leak rate can be calculated.
The testing and calculation of vacuum valve leakage rate can be seen in JB/T6446 “Vacuum Valves” standard, which applies to valves with high vacuum requirements. The vacuum level of the steam turbine extraction system in thermal power plants is generally not required to be high, and the vacuum test of its valves is required to maintain the pressure for several hours under the required vacuum level, subject to no pressure increase on the meter.
Testing equipment for valves
The pressure source used during valve testing is mainly supplied by high-pressure and medium pressure pumps and gas compressors, which can meet the strength and sealing tests requirements at room temperature. During the high-temperature steam test, a test boiler supplies high-temperature steam with certain pressure and temperature requirements.
Since the reform and opening up, with the development of the valve industry and the improvement of the technological level, the testing equipment in the valve industry has also made great progress. Various mechanical, hydraulic, and electromechanical integrated testing equipment has been continuously promoted and applied, with the emergence of clamp pressure, flip-over, immersion, and internal pressure balance testing equipment. This has transformed the pressure testing process of valves from manual labor, such as loading and unloading blind plates and fastening bolts, to simple mechanical operations and significantly improved the methods and accuracy of leak detection.
The Development Direction of the Valve Manufacturing Process
After decades of efforts in the valve industry, the level of valve manufacturing technology in China has greatly improved. Casting production has evolved from a primitive state of one person, a shovel, and a pile of soil (sand) to various casting processes, molding materials, and the promotion and application of resin sand box less molding automatic production lines. Machining production has also developed from one person, knife, and workpiece to promoting and applying modular machine tools, machining assembly lines, CNC machine tools, and machining centers.
In rough manufacturing, the application of precision casting and die forging is becoming increasingly widespread, and the mechanization level of casting and forging has also been greatly improved. Many factories have adopted mechanization in shaping, core making, sand removal, and cleaning. Some valve factories in our country use precision casting, die forging, stamping, rolling, and heading methods to manufacture parts such as valve bodies, valve covers, and valve stems, greatly reducing material consumption and machining hours and achieving good results. Expanding the application range of chip free and chip free machining processes and reducing metal cutting volume is a trend in valve manufacturing processes.
The surfacing welding of sealing surfaces generally adopts new processes such as submerged arc welding, argon arc welding, plasma spray welding, etc. The promotion and application of new welding materials improve production efficiency and ensure welding quality.
Modular machine tools and automatic production lines were widely used during the planned economy period in China, adapting to the planned economy model of a product division and specialized centralized production at that time. However, due to the poor adjustability, variability, and maneuverability of specialized combination machine tools and automatic lines, they are limited to processing a few specifications and large batches of products. In addition, the defect of long adjustment time for tooling and cutting tools results in low equipment utilization. Foreign countries have gradually developed from specialized combination machine tools to using multiple varieties and adjustable CNC machines and machining centers for valve processing with smaller batches and higher requirements. Hard alloy cutting tools with mechanical clamping and nonregrinding are used on CNC machine tools and machining centers. The hard alloy cutting tools are also coated with titanium carbide or nitride, which increases the tool life by more than three times.
At present, the production mode of most valve factories belongs to multi-variety, single-piece, and small batch rotation production. It may not be appropriate to use efficient specialized equipment such as specialized modular machine tools and automatic production lines in this situation. Using CNC machine tools or economic CNC machine tools with digital display devices added to ordinary machine tools to process valves is a flexible mode that improves production efficiency, ensures processing quality, and adapts to multiple varieties and small batch production methods.
China’s valve industry has partially used electric, pneumatic, and hydraulic fast fixtures to reduce auxiliary time. The hydraulic fixture powered by a gas-liquid pressure boosting mechanism is suitable for promotion and application due to its simple structure, small volume, and low cost. In addition, universal adjustment fixtures, standard fixtures, and combination fixtures are also widely used.
Manual labor has long completed the valve assembly with simple tools, making it the most difficult process to achieve mechanization and automation. In recent years, China has done much work in developing valve test benches, cleaning machinery, assembly machinery, etc., reducing the labor intensity of workers and improving assembly quality and efficiency. The application of assembly benches, assembly production lines, and assembly automation lines in the foreign valve manufacturing industry is relatively widespread, and robotic arms and robots are also adopted.
The painting process is an important link that affects valves’ appearance quality and rust prevention performance. Many valve factories in China have established automatic lines for infrared spray painting and electrostatic powder (plastic) spraying, a new development in valve painting.
In short, although China has made significant progress in valve casting, forging, welding, heat treatment, and machining, there is still a certain gap between us and the advanced level of foreign countries due to market competition, interest driven, short-term behavior, and the pursuit of immediate benefits, especially in terms of assembly methods, test conditions, non-destructive testing, and basic theoretical research. Internationally renowned valve companies have testing equipment for high temperature and high pressure, low temperature and deep cooling, vacuum, lifespan, flow resistance, fire resistance, and complete non-destructive testing methods. The improvement in these aspects still needs the continuous efforts of our colleagues in the valve industry to improve jointly.
Source: China Valves Manufacturer – Yaang Pipe Industry Co., Limited (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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