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Manufacturing specification for steel butt welded pipe fittings

The procurement of steel pipe fittings is an important part of the procurement of materials or accessories in devices or piping systems. Compared with other accessories, pipe fittings are characterized by complex varieties, disordered specifications, different materials and more standards. Therefore, to ensure the smooth completion of pipe procurement, the quality of pipe procurement personnel must be continuously improved.
The following discussion is based on three parts: manufacturing process, terminology and procurement information. The introduction of relevant materials, standards and specifications is included in the third part. Other contents such as the capacity and qualification of the manufacturer and price trend are not discussed. These contents should be understood in the examination and the accumulation of information at ordinary times. Please give us some advice on what is wrong.

Manufacturing process of pipe fittings

The purchaser shall basically understand the general situation of pipe fitting manufacturing process. The manufacturing of pipe fittings mainly involves pressure processing technology, in addition to welding, heat treatment, cutting, nondestructive testing, surface treatment, etc. The pressure processing process of pipe fittings is also the forming process of pipe fittings, which is mainly completed according to the plastic deformation characteristics of metal materials. The process can be roughly divided into two ways: hot machining and cold machining, in which they can be divided into different pressure machining processes. See Table 1 for common process methods and application examples of pipe fitting pressure processing.
The pressure processing technology used for pipe fittings of a certain material shall be comprehensively considered according to its material characteristics, equipment, manufacturing technology and manufacturing cost. For example, carbon steel elbows of common specifications usually adopt the expanding hot pushing process. Low carbon steel materials (commonly used brands are 20 and A106B) still have certain rigidity and good toughness after heating to a certain temperature. It is not easy to produce defects in the deformation process of expanding bending and has a high yield; The equipment used is a special elbow pushing machine, which has been specially produced by some factories; The manufacturing technology of hot push elbow has a history of more than 30 years in China. The manufacturing equipment has been relatively perfect, the heating method has been continuously improved, and the manufacturing level of mandrel has been greatly improved; The carbon steel elbow can be produced continuously by the hot push manufacturing process, which is suitable for the characteristics of large batch of products, and can avoid the subsequent heat treatment process, reducing energy consumption and cost. For the thick wall stainless steel elbow, if it is manufactured by hot pushing process, due to the high thermal strength of its material, the requirements for mandrel material are very high, and the commonly used induction device is difficult to reach the temperature required for forming, and the order quantity of this product is small, so it is mostly manufactured by hot pressing process.
The manufacturing process reflecting the technical level of the industry should be continuously improved. Manufacturers with long-term planning considerations need to invest certain resources in technology R & D, so as to improve the manufacturing level, reduce production costs, expand market share and promote the technical progress of the industry.

Manufacturing process flow of pipe fittings

The main process flow of pipe fitting manufacturing is shown in Figure 1.

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Fig.1 main process flow chart of steel pipe fitting manufacturing

Forming process of pipe fittings

For the manufacturing process of all pipe fittings, forming is an indispensable process. The forming processes of different products are different. The forming of pipe fittings is usually completed by stamping, extrusion, bending, expanding, reducing, bulging, forging, welding, cutting and other processes. According to the forming needs of different pipe fittings, one, two or more forming processes are adopted.
Table.1 common process methods and application examples of pipe fitting pressure machining

Processing mode Processing technology Application examples
Hot working Expanding hot push It is mainly used for manufacturing carbon steel and alloy steel elbows
Hot pressing It is mainly used for tee manufacturing above DN300, thick wall elbow manufacturing, pipe cap manufacturing and single piece pressing of weld elbow
Reducing or expanding hot pressing For reducer manufacturing
Heat turnover It is mainly used for the manufacture of stub end
Hot bending For the manufacture of bends
Hot forging/hot stamping It is used for manufacturing forged pipe fittings such as socket welded pipe fittings and threaded pipe fittings
Cold working Cold push It is mainly used for manufacturing stainless steel elbow
Hydraulic bulging Used for manufacturing tee below DN400
Reducing or expanding cold pressing For reducer manufacturing
Cold roll For reducer manufacturing
Cold pressing It is mainly used for the manufacture of pipe cap and the single piece pressing of weld elbow
Cold bending Used for manufacturing small diameter stainless steel bend

The common forming processes of pipe fittings are briefly introduced below.

Hot push elbow

The hot pushing elbow forming process is a process of heating, expanding and bending the blank sleeved on the die under the push of the pushing machine by using a special elbow pushing machine, core die and heating device. See Fig. 2 for the forming diagram of hot push elbow.

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1 – die, 2 – blank, 3 – heating device, 4 – Elbow being formed
Fig.2 forming diagram of hot push elbow

The deformation characteristic of the hot push elbow is to determine the billet diameter according to the law that the volume of the metal material remains unchanged before and after plastic deformation. The billet diameter used is less than the elbow diameter. The deformation process of the billet is controlled through the core die to make the compressed metal flow at the inner arc and compensate to other parts thinned due to diameter expansion, so as to obtain an elbow with uniform wall thickness.
Hot push elbow forming process has the characteristics of beautiful appearance, uniform wall thickness and continuous operation, which is suitable for mass production. Therefore, it has become the main forming method of carbon steel and alloy steel elbow.
The heating method of the forming process is usually medium frequency or high frequency induction heating (the heating ring can be multi circle or single circle), and it can also be heated by flame heating and reverberatory furnace. The heating method depends on the requirements of the forming product and energy conditions.

Hot pressed elbow

Hot pressing elbow is the first forming process used in mass production of elbow. At present, it has been replaced by hot pushing method or other forming processes in the production of elbow with common specifications, but it is still used in some elbow specifications due to small production quantity, excessive wall thickness or special requirements of products. The pipe blank equal to or greater than the outer diameter of the elbow is used for forming, and the press is used for direct pressing in the die. The Schematic diagram of forming is shown in Figure 3.

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1 – upper mold, 2 – end mold, 3 – inner core, 4 – Elbow, 5 – lower mold
Fig.3 forming diagram of stamping elbow
As shown in the figure, before pressing, place the heated pipe blank on the lower die, install the inner core and end die into the pipe blank, the upper die moves downward to start pressing, and the elbow is formed through the restraint of the outer die and the support of the inner die.
Compared with the hot pressing process, the appearance quality of hot pressing is not as good as the former; When the hot pressed elbow is formed, the outer arc is in the tensile state, and there is no excess metal in other parts to compensate, so the wall thickness at the outer arc is reduced by about 10%. However, due to the characteristics of single piece production and low cost, it is mostly used for the manufacture of small batch and thick wall elbows.

Cold push elbow

The forming process of cold push elbow is to use a special elbow forming machine to put the pipe blank into the outer die. After the upper and lower dies are closed, the pipe blank moves along the gap reserved between the inner die and the outer die under the push of the push rod to complete the forming process. See Figure 4.

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Figure.4 cold push elbow forming

Induction heating bend

The bend is usually formed by medium frequency or high frequency induction heating. The special pusher, fixture and heating device are used to make the steel pipe move forward under the push of the pusher, and the forming is completed through the process of heating, bending and cooling. The forming of elbow is shown in Figure 5.

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Fig.5 induction heating bend forming
The outer arc in bend forming is thinned because it is in the tensile state, so the side with thicker pipe wall thickness should be placed at the outer arc.

Hot pressed tee

Tee hot pressing is to flatten the pipe blank larger than the tee diameter to the size of the tee diameter, and open a hole at the part of the stretched branch pipe; The tube blank is heated and put into the forming die, and the die for drawing the branch pipe is installed in the tube blank; Under the action of pressure, the tube blank is compressed radially. In the process of radial compression, the metal flows to the branch pipe and forms a branch pipe under the tension of the die. The whole process is formed through the radial compression of pipe blank and the stretching process of branch pipe. The forming process of hot pressed tee is shown in Figure 6.

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Fig.6 forming diagram of hot pressing tee
Because the tee is pressed after heating, the tonnage of equipment required for material forming is reduced. Hot pressed tee has wide adaptability to materials and is suitable for carbon steel, alloy steel and stainless steel; Especially for tees with large diameter and thick pipe wall, this forming process is usually adopted.

Hydraulic bulging tee

The hydraulic bulging of tee is a forming process of expanding branch pipe through axial compensation of metal material. The process is to use a special hydraulic press to inject liquid into the pipe blank with the same diameter as the tee, and squeeze the pipe blank through the synchronous centering movement of the two horizontal side cylinders of the hydraulic press. The volume of the pipe blank becomes smaller after extrusion, and the liquid in the pipe blank increases with the volume of the pipe blank. When the pressure required for the expansion of the tee branch pipe is reached, The metal material flows along the inner cavity of the die and expands out of the branch pipe under the dual action of the liquid pressure in the side cylinder and the tube blank. The Schematic diagram of hydraulic tee forming is shown in figure 15-9.

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        (a) Bulging start                        (b) bulging middle                  (c) bulging end
Fig.7 forming diagram of hydraulic bulging tee
The hydraulic bulging process of tee can be formed at one time and has high production efficiency; The wall thickness of the main pipe and shoulder of the tee increased.
Due to the large equipment tonnage required for the hydraulic bulging process of seamless tee, it is mainly used for the manufacture of standard wall thickness tee less than dn400 in China. The applicable forming materials are low carbon steel and stainless steel with relatively low cold work hardening tendency, including some non-ferrous metal materials, such as copper, aluminum, titanium, etc.

Pressed reducer

The reducing forming process of reducer is to put the pipe blank with the same diameter as the large end of reducing pipe into the forming die, and press along the axial direction of the pipe blank to make the metal move along the die cavity and shrink. According to the size of reducing pipe, it can be divided into one-time pressing forming or multiple pressing forming. Fig. 8 is a Schematic diagram of reducing forming of concentric reducer.

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Fig.8 Schematic diagram of reducing forming of concentric reducer
Expanding forming is to expand the diameter of the tube blank along the inner diameter of the tube blank with an internal die. The expanding process mainly solves the problem that the reducer with large variable diameter is not easy to form through reducing. Sometimes, the expanding and reducing methods are combined according to the forming needs of materials and products.
In the process of reducing or expanding deformation pressing, cold pressing or hot pressing is determined according to different materials and reducing conditions. Generally, cold pressing shall be adopted as far as possible, but hot pressing shall be adopted for severe work hardening caused by multiple diameter changes, wall thickness deviation or alloy steel materials.

Manufacturing process of welded pipe fittings

The welded pipe fittings mentioned here refer to the pipe fittings with welds completed by the pipe fitting manufacturer, excluding the pipe fittings made of welded pipes.
a) Weld elbow
The weld elbow is usually a double weld structure, and its weld position is two longitudinal welds along the inner and outer arc, as shown in Fig. 9 (a); There are also weld elbows manufactured by single weld process, and the weld position is a longitudinal weld along the inner arc, as shown in Fig. 9 (b).

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(a) Two longitudinal welds of inner and outer arc        (b) One longitudinal weld of inner arc

Fig.9 Schematic diagram of weld position of weld elbow
The material used for the weld elbow is steel plate. The steps for manufacturing the elbow with two longitudinal welds are as follows: firstly, set out on the steel plate, cut two pieces of material sheets with the same shape, determine whether they need to be directly cold pressed or heated and hot pressed according to the material situation, and then put the material sheets into the mold and press them through the upper and lower mold. Half of the weld elbow is pressed. Similarly, the other half is suppressed by the law. Before assembling and welding the press formed two halves of the blank, it is necessary to cut off the remaining material along the horizontal direction of the internal and external arc, and process the welding groove according to the welding process requirements, and finally assemble and weld.
b) Weld tee

The weld position of the welded tee is usually a longitudinal weld on the back of the tee, as shown in Figure 10 (a); Or two longitudinal welds horizontally symmetrical along the axial direction of the tee main pipe, as shown in Fig. 10b). The weld position of the cross is the same as that shown in FIG. 10 (b), i.e. two longitudinal welds horizontally symmetrical along the axis of the main pipe.

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(a) One longitudinal weld on the back                 (b) two longitudinal welds with horizontal symmetry
Fig. 10 Schematic diagram of weld position of welded tee
c) Weld reducer
Weld reducers are usually formed by welding after the steel plate is rolled on the rolling machine. The weld position is a longitudinal weld along the axial direction. When the width of the steel plate is limited and the weld needs to be added, the position of the weld should be arranged axially and symmetrically.

Forged Fittings 

Such pipe fittings include socket welded and threaded pipe fittings, branch pipe seats and other pipe fittings that need to be forged, such as forged tee with thick wall. See Figure 11 for forged pipe fittings.

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(a) Socket welded fittings                           (b) Threaded fittings

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(c) Branch seat                                                 (d) Reducer

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(e) Forged tee finished products             (f) Forged tee processing

Figure.11 forged pipe fittings

According to different varieties and requirements of pipe fittings, forged pipe fittings are mainly formed by die forging, free forging and cutting. Forged pipe fittings are manufactured by die forging, free forging and cutting.
a) Die forging
For small-size pipe fittings such as tee, cross and elbow with socket welding and thread, the shape is relatively complex and should be manufactured by die forging process.
The blank used for die forging shall be rolled profile, such as bar, thick wall pipe or plate. When the steel ingot is used as raw material, the steel ingot shall be rolled into bar in advance or used as blank for die forging after forging, so as to eliminate segregation, porosity and other defects in the steel ingot.
After heating, the blank is put into the die for forging (depending on the situation, die forging may be carried out after initial forging), and the pressure makes the metal flow and fill the die cavity, so as to complete all die forging work.
b) Free forging
Free forging process can be used for special shape pipe fittings or pipe fittings that are not suitable for die forging. The general shape of pipe fittings shall be forged by free forging; For tee, the branch pipe shall be forged.
c) Machining
For some pipe fittings with cylindrical shape, they can be directly cut by bar or thick wall pipe, such as double socket pipe hoop, union joint, etc. During processing, the fiber flow direction of metal materials shall be roughly parallel to the axial direction of pipe fittings.

Other important processes

The important processes of pipe fitting manufacturing also include heat treatment, welding, nondestructive testing and marking.

Heat treatment

The properties of metal materials can be restored, improved and improved by using the correct heat treatment process for different materials and forming methods.
For the same material, the heat treatment requirements specified in different product standards may be different, and the heat treatment processes adopted by each manufacturer are also different. The principle of which heat treatment method shall be adopted shall be clearly specified in the product standard, and shall be implemented in accordance with the product standard; If it is not clearly specified in the product standard, the appropriate process can be selected according to the heat treatment provisions of the material in relevant standards or specifications. In order to ensure the quality of heat treatment, no matter which method is adopted to determine the heat treatment process, the manufacturer shall conduct process test according to the proposed process conditions and parameters before formulating the formal heat treatment process. The samples used shall be taken from the formed products or samples prepared by the same forming process as the products. The process test includes the tests of hardness, mechanical properties and metallographic structure (intergranular corrosion test shall also be included for stainless steel) to verify whether the proposed heat treatment process meets the requirements of relevant standards for product performance.
According to the forming method and different materials, the commonly used heat treatment processes of pipe fittings include normalizing, annealing, normalizing and tempering, solid solution heat treatment, etc.
a) Normalizing heat treatment
Compared with annealing, because the cooling mode of normalizing is out of furnace air cooling, the time required is short and the relative efficiency is high; Because the cooling speed is fast, the grain of the material is fine, which will make the strength, toughness and plasticity of the material higher. Therefore, normalizing heat treatment process is widely used for cold formed or hot formed low carbon steel and carbon manganese steel pipe fittings.
b) Annealing heat treatment
Annealing heat treatment process is also widely used in the heat treatment of pipe fittings. Due to different treatment purposes, stress relief annealing, complete annealing and recrystallization annealing are often used in the annealing of pipe fittings.
Stress relief annealing is mostly used for post weld heat treatment of welded pipe fittings to eliminate residual stress caused by welding and hydrogen in the weld and prevent deformation of pipe fittings.
Complete annealing is a heat treatment method for low carbon steel and alloy steel pipe parts. Complete annealing can eliminate the residual stress caused by deformation, eliminate segregation, refine grain, uniform structure and restore the normal properties of materials.
Recrystallization annealing is a heat treatment method for cold formed low carbon steel pipe parts,   Through recrystallization heat treatment, the deformed metal grains are transformed into equiaxed grains, and the residual stress and work hardening caused by cold deformation are eliminated.
c) Normalizing and reheating heat treatment
The heat treatment method of normalizing and tempering is usually used for the formed chromium molybdenum alloy steel pipe parts. This kind of material has strong hardenability, certain stress exists after normalizing, and the strength and hardness are also high. Tempering should be used to improve the excessive strength and hardness, improve the toughness of the material, eliminate stress and prevent possible deformation and cracking of pipe fittings.
d) Solution heat treatment
Solid solution heat treatment shall be adopted for the formed austenitic stainless steel, and the common cooling method is water cooling. Through solid solution treatment, the martensite and processing stress produced in the forming process are eliminated, and the microstructure is single-phase austenite, so as to obtain high plasticity and corrosion resistance.
e) Others
In addition to the above commonly used heat treatment methods, other suitable heat treatment processes such as isothermal annealing, quenching and tempering can be adopted for different steel types. Sometimes, austenitic stainless steel containing Ti and Nb stabilizing elements need to be stabilized after solution heat treatment.


Pipe fittings with welds include two cases. One is pipe fittings made of welded pipes. For pipe fitting manufacturers, the forming process of welded pipes is basically the same as that of seamless pipes, and the forming process of pipe fittings does not include welding process; The other is the welding process required by the pipe fitting manufacturer to complete the pipe fitting forming, such as the elbow assembled and welded after single piece pressing, the tee welded into pipe blank after steel plate drum, etc.
The common welding methods of pipe fittings are manual arc welding, gas shielded welding and automatic welding.
The manufacturer shall prepare the welding procedure specification to guide the welding work, and conduct the welding procedure qualification according to the corresponding specification requirements to verify the correctness of the welding procedure specification and evaluate the welding ability of the welder.
Welders engaged in pipe fitting welding shall pass the examination of the quality and technical supervision department and obtain corresponding qualification certificates before engaging in the welding of relevant steel grades (according to the regulations of some industries, the welded pipe fittings used in some industries shall obtain the welder examination and welding procedure qualification specified by the industry, For example, the welding of marine pipe fittings shall obtain the welder examination and welding procedure qualification of the corresponding classification society).

Nondestructive testing

Nondestructive testing is an important process to inspect the possible defects in the processing of materials and pipe fittings. The requirements for nondestructive testing are specified in most pipe fitting product standards, but the requirements are inconsistent. In addition to meeting the requirements of product standards and ordering requirements for nondestructive testing, some manufacturers with strict quality control also formulate nondestructive testing requirements according to materials, processing technology and internal quality control regulations to ensure the quality of ex factory products.
In actual work, the qualification level of nondestructive testing of pipe fittings shall be determined according to the ordering requirements or clear provisions of standards. Since the surface of the pipe fittings is basically in the state of the original pipe, plate or forging, for the nondestructive testing (MT, PT) of the surface quality of the pipe fittings, if there is no clear grade requirement, it can be regarded as grade II. However, no matter how the qualification grade is specified, the defects that are difficult to determine the depth such as interlayer and crack shall be regarded as unqualified. For the non-destructive testing (RT, UT, etc.) of the internal quality of pipe fittings, if there is no clear grade requirement, the radiographic testing shall be level II (such as weld testing), and the ultrasonic testing shall be level I.
In order to prevent possible defects of products during heat treatment, the final NDT of pipe fittings shall be carried out after heat treatment.
JB/T 4730 specification is usually used in nondestructive testing of pipe fittings manufacturers in China. Personnel engaged in nondestructive testing shall obtain corresponding qualifications according to relevant regulations.


Marking is an indispensable part of products and the basis for realizing traceability requirements. Generally, the content and method of marking are specified in the product standard. The marking content of pipe fittings generally includes the manufacturer’s trademark or name, material grade, specification and other contents required by the order. Marking methods include permanent marks, such as steel seal, engraving, electric etching, etc; Non permanent signs, such as spray printing, labels, etc.


At present, most of the terms of pipe fittings have standard names or conventional names. Related to pipe fitting procurement, the following terms are commonly used:

General terms

  • Butt welding fitting/BW – refers to the pipe fitting in which the pipe and pipe fitting are connected by butt weld of welding groove.
  • Socket welding fitting/SW – refers to the pipe fitting in which the pipe is inserted into the socket of the pipe fitting for fillet welding.
  • Threaded fitting/THD – refers to the pipe fitting connected by matching the internal thread and external thread of the pipe and the pipe fitting.

Pipe fittings of mixed connection type – refers to pipe fittings with different connection types at different ends of a pipe fitting, such as butt welding at one end and thread at the other end.
Nominal pipe size (NPS) – the nominal internal diameter of the pipe. It is a rounded value that has been standardized, serialized and generalized, representing the specification of the pipe. Generally, DN is the code of nominal size in China.
Structural dimension refers to the dimension from the center to the end face (such as elbow, tee and cross) and from the end face to the end face (such as reducer) of some pipe fittings.

Terms in butt welded pipe fittings

Elbow – a pipe fitting used to change the direction of the pipe at the turn of the pipe. In butt welded pipe fittings, according to the radius of curvature of the elbow, it is usually divided into: short radius elbow (SR), that is, the radius of curvature of the elbow is about equal to the nominal diameter of the pipe fitting, that is, R ≈ 1D or 1D elbow; Long radius elbow (LR) the radius of curvature of the elbow is about 1.5 times the nominal diameter of the pipe fitting, i.e. R ≈ 1.5D or 1.5D elbow.
According to the angle of elbow, it is usually divided into 45 ° elbow, 90 ° elbow and 180 ° elbow. Generally speaking, the diameters of both ends of the elbow are the same, that is, equal diameter elbow. Sometimes, reducing elbow is used, that is, the diameters of both ends of the elbow are not equal.
Bend – a pipe fitting with a radius of curvature greater than or equal to three times the outer diameter of the pipe fitting, i.e. R ≥ 3D. Different from the calculation basis of the radius of curvature of the elbow, the radius of curvature of the elbow is usually calculated according to the outer diameter D of the pipe. Elbows generally have straight pipe sections with a certain length at both ends, and the length of the straight pipe section is usually specified by the standard or given by the design.
Tee – a fitting used at a branch of a pipe. According to the size of main pipe and branch pipe, it is usually divided into same diameter tee (the diameter of main pipe and branch pipe of tee is equal) and reducing tee (the diameter of branch pipe of tee is less than that of main pipe). The tee generally refers to the tee where the main pipe and branch pipe intersect at 90 °. In addition, there are also Y-type tees (terminal) or inclined tees, that is, tees where the main pipe of the tee intersects the center line of the branch pipe at an angle of less than 90 °, such as 45 ° tees. Y-type tees are usually used to transport solid particles or pipeline branches with serious erosion and corrosion. In addition, there is a Y-shaped tee (Wye) or trouser tee, that is, the included angle of the two branch pipes of the tee is 90 °.
Cross – a pipe fitting used to divide the pipe into four ways at the same time. Other descriptions are basically the same as those of tee.
Reducer – a pipe fitting used for reducing the diameter of a pipe. It includes two varieties, concentric reducer/CR, and the center line of the large end coincides with the center line of the small end; Eccentric reducer (eccentric reducer/ER), the outer wall of one side of the big end and the small end is on the same straight line.
Cap – a fitting used to close the end of a pipe. Oval pipe caps are commonly used for pipes. The nominal size of the pipe cap is generally not greater than DN600.
Lap joint stub end – usually used for fitting with loose flange on stainless steel pipe. One end of the flanging nipple is a welding groove type, which is connected with the pipe welding; The other end is used with loose flange and connected in the form of flange.

Terminology in socket welded and threaded fittings

Elbow (elbow) – the function is the same as that of butt welded elbow. The same as butt welding elbow, there are also 45 ° and 90 °, but not long radius and short radius.
Tee – same function as butt welded tee. Similarly, it can also be divided into same diameter and different diameter, positive tee and 45 ° inclined tee.
Cross – same function as butt welded cross. Similarly, it also has the same diameter and different diameter.
Cap – same function as butt welded cap. However, most of them are in the form of threaded connection, which is used as the terminal for liquid drainage and venting for secondary protection.
Coupling – socket welded couplings are divided into two types: double socket and single socket (the thread is called double interface and single interface). The double socket pipe hoop can be divided into the same diameter and different diameter, and the different diameter double socket pipe hoop can be divided into concentric and eccentric; Double socket pipe hoop is used for the connection between pipes that are not suitable for butt welding connection. The function of single socket pipe hoop (also known as reinforcing nozzle) is similar to that of tee. It is often used for the branch connection of a small-diameter branch pipe from a large-diameter main pipe. One end is connected with the fillet weld of the main pipe, and the other end is connected with socket weld or thread. According to the size of the main pipe, the single socket pipe hoop is divided into flat bottom type and arc bottom type. Generally, the main pipe with the size below DN100 uses arc bottom type, and the main pipe larger than DN100 uses flat bottom type. The purpose of adopting arc bottom type is to make its bottom shape similar to the outer arc of the main pipe, so as to ensure a good welding quality during welding.
Plug/plugging – similar to threaded pipe cap, it is a pipe fitting with external thread, commonly used in three types: square head, round head and hexagonal head.
Union – a pipe fitting usually connected by three parts and threads. It usually has two types: socket welding and thread. It is often used together with threaded nipples to realize detachable connection. Because in normal pipelines, it is impossible to disassemble only by other threaded fittings, and it must be equipped with unions.
Threaded nipple (nipple) – used to realize the connection between pipes and fittings or between fittings and fittings. There are usually single head thread and double head thread.

Terms of other pipe fittings

Outlet – similar to the function of tee, it is often used for branch connection of pipes from large-diameter main pipe to small-diameter branch pipe. The connection type is that the bottom of the branch pipe seat is welded with the main pipe, and the branch pipe has three connection modes: butt welding, socket welding and thread.
Swaged – similar to reducer, used for pipe reducing connection. There can be different connection types at the two ends of the reducing pipe. The two ends can be butt welded, socket welded or threaded connection, or there can be different connection types at each end. Therefore, there are 9 different connection types.
Instrument nozzle – used to connect the pipeline with the instrument installed on the pipeline. One end is connected with the pipeline by welding, and the other end is connected with the instrument by thread. According to different instruments, they are usually divided into pressure gauge nozzle (cone internal thread RC), bimetallic thermometer nozzle (column thread G) and thermocouple nozzle (column thread G).

Purchase information

Pipe fitting is an integral part of pipeline accessories and a common part in pipeline engineering. At present, most pipe fittings have been standardized and serialized, so they are usually purchased according to standard parts.
It is the responsibility of the purchaser to specify the procurement requirements in the pipe fitting purchase order. The incomplete or unclear content of procurement requirements may lead to procurement errors, so the procurement personnel shall be aware of the content that must be required by procurement, so as to ensure that the quality of purchased products meets the design and use requirements. When the material list provided by the designer cannot meet the requirements of procurement, it shall communicate with the material engineer of the design department in time. The procurement requirements of pipe fittings generally include (but are not limited to) the following contents:
Pipe fitting name, specification, material brand, structure, standard, quantity, additional requirements and supplementary provisions, etc. Now they are introduced as follows:


The name of pipe fittings shall be in accordance with the name specified in the national standard or industrial standard of pipe fittings. It should be noted that the purchase order should be marked with the correct name without any disagreement. The correct way is that the name of the material should be consistent with the name specified in the standard. There are two main points to pay attention to. First, the full name should be marked. For example, the elbow should be marked with long radius or short radius, 90 ° or 45 °, otherwise there may be procurement errors; Second, there should be no incomprehensible name in the name. For example, the name marked in the purchase order is pipe joint. In this case, it is impossible to understand what kind of pipe joint it is and what kind of pipe fitting it is.


The specification of pipe fittings must be indicated in the order of pipe fittings, which is an important basis for quotation, material preparation, production and inspection of pipe fittings manufacturers.
The diameter of pipe fittings is usually expressed in nominal size DN (English unit is expressed in NPS), such as DN100; However, sometimes it is directly marked with outer diameter dimension (mm) or inner diameter dimension (mm). There are different ways to express the wall thickness. See Table 2 for the commonly used methods.
Table.2 representation of pipe fitting wall thickness

Type of pipe fittings Wall thickness representation
Pipe Schedule No Pipe weight Nozzle wall thickness Pressure class
Butt welded pipe fittings Sch10Sch160Sch5sSch80s Including STD, XS and XXS mm Such as PN
Socket welded pipe fittings Sch80Sch160 XXS 300060009000
Threaded pipe fittings Sch80Sch160 XXS 200030006000
Other pipe fittings Sch10Sch160Sch5sSch80s Including STD, XS and XXS mm Such as PN or 2000, 3000, 6000, 9000
Note: Sch5s ~ Sch80s are only used for stainless steel pipe fittings.

In the table, Sch is the abbreviation of Schedule number. The value after Sch indicates the classification of pipe wall thickness. In some pipe standards, Sch is the wall thickness value determined by the standard and can be found directly. For example, the wall thickness value specified in the pipe Schedule can be found in ASME B36.10 and B36.19.
STD, XS and XXS are abbreviations of standard weight pipe, thickened weight pipe and extra thickened weight pipe respectively. In some pipe standards, corresponding to different pipe diameters, the wall thickness values of STD, XS and XXS have been listed in the standards, which can be found directly, such as ASME B36.10 and B36.19.
PN is the symbol of pressure grade, usually in MPa. In petrochemical process pipeline, it is rarely purchased with pressure grade.
2000, 3000, 6000 and 9000 are grade codes, which were originally pound codes, but now they are dimensionless, that is, grade codes of pipe fittings without units. In procurement, they are usually used for socket welding and threaded pipe fittings designed according to ASME code.
Another point to note in the specification is that if there are two outer diameter series specified in the pipe fitting standard, the outer diameter series number shall also be marked, such as GB/T 12459 standard. If the outer diameter of Series II is adopted, the outer diameter series number II must be marked (the outer diameter of series I can be omitted).
For bends used for oil and gas transmission, the requirements for bending radius shall also be included in the specification.

Material brand

Material brand is a particularly important content in material procurement and must be clearly marked in the purchase order.
Due to different material systems in various countries, the requirements for material grades marked in pipe fittings are also different. For example, the provisions of material brand in domestic pipe fitting standards are basically consistent with those of raw materials. For example, the material brand in pipe fitting standards is 20, and the material brand in corresponding raw material standards is also 20.
It should be clear that the material grade of pipe fittings in ASME code, which is widely used in pipe fittings procurement, is different from that of raw materials in ASME code. If the tee of ASME B16.9 code is purchased, the material brand is marked as A234 WPB or only WPB. However, if the raw materials for tee manufacturing are also ASME code materials, the raw materials marked with WPB can not be found in all material specifications. In fact, in ASTM A234 specification, the meaning of WPB code refers to the grade B material of forged and rolled pipe fittings. The chemical composition and mechanical property indexes of WPB are listed in A234 specification. The raw materials used by pipe fitting manufacturers can only meet the chemical composition and mechanical property indexes specified in the specification. If the raw material of tee manufactured by the pipe fitting manufacturer also adopts the material of ASME code, the general corresponding material specification is: A106 GR.B seamless pipe can be used for seamless pipe fittings, A515 GR.60, A515 GR.65 or A516 GR.60, A516 GR.65 steel plates can be used for welded pipe fittings, and other grades of raw materials can also be used, The chemical and mechanical properties of WPB must meet the requirements. However, if the raw material specifications for pipe fittings are specified in the material list issued by the Design Institute, the pipe fittings shall be manufactured according to the specified raw materials. If it is specified that the raw material used for WPB tee shall be A516 GR.70, the manufacturer shall use A516 GR.70 steel plate as the raw material to manufacture tee.
It is important to understand the requirements of different specifications for materials. This situation has made a joke. A third-party inspector found that the material of pipe fittings specified in the order was WPB during product acceptance, so he did not confirm the raw material quality certificate of A106 GR.B provided by the manufacturer. If the manufacturer had to provide the raw material quality certificate marked WPB, the manufacturer certainly could not provide it, and several explanations could not pass, which made people cry and laugh.
Because there are many materials involved in pipe fittings, material substitution sometimes occurs. The usual practice is that the manufacturer submits a written substitution application to the purchaser, and the purchaser shall communicate with the design unit in time for approval. The principle of using substitute materials shall be that the chemical composition shall be similar, the mechanical properties shall not be lower than the indexes of raw design materials, and meet the requirements of welding process.
There is also a case of double marked materials, such as 304/304L, which means that the chemical composition should meet the requirements of ultra-low carbon, that is, the requirements of 304L, and the mechanical properties should meet the indexes of 304 material (some items of the mechanical properties indexes of 304 material are higher than 304L material). Now there are also double marked raw materials in the market. This material combines the material level and facilitates the selection of materials. It should be noted that if the design selects double standard materials and the pipe fittings provided by the manufacturer are made of single standard materials, the chemical composition and mechanical property indexes of single standard materials must meet the requirements of double standard materials, which is not a material substitution; Conversely, if the design selects single standard materials (such as 304) and the pipe fittings provided by the manufacturer are made of double standard materials (304/304L), this situation also does not belong to material substitution.


The pipe fitting structure here refers to seamless or welded (the bending radius of elbow or elbow is usually given in the specification). Generally, this is specified in the pipe fitting standard, and there is no need to specify otherwise in the order. For example, the pipe fittings of GB/ T 12459 and SH3408 standards are seamless; For the pipe fittings of GB/T 13401 and SH3409 standards, the standard provisions are welds, and there is no need to specify otherwise in the purchase order. However, when purchasing according to ASME B16.9, it shall be proposed whether seamless pipe fittings or welded pipe fittings are required, because B16.9 does not specify the structure of pipe fittings. If it is not clear in the purchase order, the manufacturer can provide seamless or welded pipe fittings.
If seamless pipe fittings are specified in the order and the pipe fittings with welds are provided by the manufacturer, a deviation application shall be handled in advance and approved; However, if the welded pipe fittings are specified and the seamless pipe fittings are provided by the manufacturer, it is not necessary to apply for deviation and can be supplied directly.


There are many standards for pipe fittings, and most kinds of pipe fittings have standards. For existing standard pipe fittings, the standard number of pipe fittings shall be marked to avoid that the pipe fittings cannot be used due to different standards. See Table 3 for commonly used pipe fitting standards in China and the United States.
Table.3 common Chinese and American pipe fitting standards

Standard code Standard name
GB/T 12459 Steel butt-welding seamless pipe fittings
GB/T 13401 Steel plant butt-welding pipe fittings
GB/T 14383 Forged steel socket welding pipe fittings
GB/T 14626 Forged steel threaded pipe fittings
GB/T 19326 Forged branch outlet fittings-socket welding, threaded and butt-welding ends
SH 3065 Specification for tube furnace sharp bend for petrochemical industry
SH 3408 Steel butt-welded seamless fittings
SH 3409 Steel plant butt-welding fittings
SH 3410 Forged steel socket-welded fittings
SY/T 0510 Steel butt-welded pipe fittings
SY/T 5257 Steel bend for oil and gas transmission
HG/T 3651 Titanium butt-welding seamless pipe fittings
HG/T 21631(HGJ 528) Steel plate butt-welding fittings
HG/T 21632(HGJ 529) Forged outlet -socket welding, threaded and butt-welding
HG/T 21634(HGJ 10) Forged steel socket-welded fittings
HG/T 21635(HGJ 514) Carbon steel, low-alloy steel butt-welded seamless fittings
GD 87-1101 Typical design manual of pipe parts for steam power plant
Typical design manual of pipe parts for steam power plant2000
DL/T 515 Bends for power plant
DL/T 695 Steel pipe butt welding fittings for power plant
JB 4726 Carbon and low-alloy steel forgings for pressure vessels
JB 4727 Low-alloy steel forgings for low temperature pressure vessels
JB 4726 Stainless steel forgings for pressure vessels
ASME B16.9 Factory-Made wrought steel buttwelding fittings
ASME B16.11 Forged fittings socket-welding and threaded
ASTM A105/AIO5M Standard Specification for Carbon Steel Forgings for Piping Applications
ASTM A182/AI82M Standard Specification for Forged or Rolled Alloy Steel Pipe Flanges, Forged Fittings, and Valves and Parts for High-Temperature Service
ASTM A234/A234M Standard Specification for Piping Fittings of Wrought Carbon Steel and Alloy Steel for Moderate and High Temperature Service
ASTM A403/A403M Standard Specification for Wrought Austenitic Stainless Steel Piping Fittings
ASTM A420/A420M Standard Specification for Piping Fittings of Wrought Carbon Steel and Alloy Steel for Low-Temperature Service
ASTM A815/A815M Standard Specification for Wrought Ferritic, Ferritic/Austenitic, and Martensitic Stainless Steel Piping Fittings
MSS SP-43  Wrought stainless steel butt-welding fittings
MSS SP-79 Socket-welding reducer inserts
MSS SP-83 Class 3000 steel pipe unions socket-welding and threaded
MSS SP-95 Swagednipples and bull plugs
MSS SP-97 Integrally reinforced forged branch outlet fittings-socket welding, threaded and buttwelding ends
Note: in the ASME code, the contents of the original ASME B16.28 (all dimensions and tolerances of butt welded short radius elbow and 180 degree elbow) have been incorporated into the ASME B16.9-2001 code, so the standard number B16.28 has been cancelled and will not be used anymore.

All standards will be revised, so it is generally unnecessary to mark the year of the standard in the purchase order. According to the standardization management regulations, the latest standards shall be adopted for the standards without the year number, while the standards with the year number can only use the standards of the specified version, but the latest standards may not be applicable.


The number of fittings is usually expressed in pieces.

Additional requirements

Additional requirements are those that exceed the requirements in the text of the pipe fitting product standard, or special requirements for the process and management of the device, such as inspection and test requirements. If so, it shall be clearly specified in the order or technical agreement.
For example, normalizing treatment and 10% magnetic particle inspection are required for carbon steel seamless elbows manufactured by hot pushing method with material code of 20 ordered according to GB/T 12459. These contents are not strictly specified in the standard. If the above heat treatment and additional inspection requirements are required, they shall be clearly specified in the purchase order or technical agreement; Because these requirements may be special requirements of the device, and the manufacturer’s quotation basis, manufacturing process and inspection test for pipe fittings with special requirements are also different.

Supplementary provisions

Generally, the following shall also be included in the order:

  • a) Delivery time;
  • b) Delivery batch (batch delivery or batch delivery, or how many goods of certain varieties should be delivered first);
  • c) Packaging requirements (depending on the situation, excessive packaging will increase procurement costs and waste resources);
  • d) Mode of transport;
  • e) Place of delivery;
  • f) Acceptance method;
  • g) Payment provisions (payment terms shall be specified, such as advance payment, or how many days after the goods arrive and pass the acceptance, etc. Generally, compared with petrochemical enterprises, the scale of pipe fittings manufacturers is small, and the price is relatively low in the competition. According to the statement of “mutually beneficial relationship with suppliers”, one of the eight principles of quality management, as the purchaser, it shall take the initiative to pay within the time limit specified in the contract and maintain a good cooperative relationship of equality and mutual benefit with suppliers, This is also a way to mobilize the enthusiasm of suppliers).

In the order (or agreement), the following contents can be selected as required:

  • a) Delivery documents – documents required to be provided by the supplier (the number of copies of documents shall be specified or whether electronic documents are required), including documents related to products, such as quality certificate, packing list, raw material certificate and other required reports (such as inspection and test report, depending on the situation, usually the inspection and test results have been included in the product quality certificate); Other required documents, such as progress plan, inspection plan, etc., shall be determined as the case may be.
  • b) Raw material requirements – such as requirements for the origin of raw materials, or raw materials with a certain brand, etc.
  • c) Quality assurance period – the safe use period of the product. It is generally stipulated by years.

Attachment: common English abbreviations
English abbreviations often appear in pipe procurement. Being familiar with these abbreviations will be helpful to understand the material list. See Table 3 for common abbreviations.
Table.3 abbreviations commonly used in pipe fitting procurement

English abbreviations Representative content English abbreviations Representative content
BW Butt welding connection SS Stainless steel
SW Socket weld connection AS Alloy steel
THD Threaded connection BE Groove end
EL or LE LR elbow PE Flat end
ES or SE Short radius elbow TE Threaded end
RC or CR Concentric reducer BBE Groove at both ends
RE or ER Eccentric reducer BLE/PSE Large end groove, small end flat head
T Tee BLE/TSE Large end groove, small end thread
TS Equal tee PBE Flat ends
TR Reducer tee PLE/BSE Groove, small end, flat end
C Pipe cap PLE/TSE Large end flat head, small end thread
OUTLET Olets TBE Thread at both ends
NPS Nominal size in Imperial System TLE/BSE Large end thread, small end groove
THK Wall thickness TLE/PSE Large end thread, small end flat head
P Pressure PC Piece
NPT 60 ° tapered pipe thread ASME American Society of Mechanical Engineers
Rc 55 ° tapered pipe thread ASTM American Society for testing and materials
FS Internal thread API American Petroleum Institute
MS External thread ANSI American National Standardization Association
CS Carbon steel MSS American pipe fitting Manufacturers Association

Source: Network Arrangement – China Pipe Fitting Manufacturer – Yaang Pipe Industry (

(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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