What is heat treatment

What is heat treatment?

Heat treatment is a metal hot working process in which materials are heated, insulated and cooled in the solid state to obtain the expected microstructure and properties.

Characteristics of metal heat treatment process

Metal heat treatment is one of the important processes in mechanical manufacturing. Compared with other processing processes, heat treatment generally does not change the shape and overall chemical composition of the workpiece, but endows or improves the service performance of the workpiece by changing the microstructure inside the workpiece or the chemical composition on the surface of the workpiece. Its characteristic is to improve the internal quality of the workpiece, which is generally not visible to the naked eye. In order to make the metal workpiece have the required mechanical, physical and chemical properties, in addition to the reasonable selection of materials and various forming processes, heat treatment process is often essential. Steel is the most widely used material in the mechanical industry. The microstructure of steel is complex and can be controlled by heat treatment. Therefore, the heat treatment of steel is the main content of metal heat treatment. In addition, aluminum, copper, magnesium, titanium and their alloys can also change their mechanical, physical and chemical properties through heat treatment to obtain different service properties.

Process of heat treatment process

Heat treatment process generally includes three processes: heating, heat preservation and cooling. Sometimes there are only two processes: heating and cooling. These processes are interconnected and uninterrupted.
Heating is one of the important processes of heat treatment. There are many heating methods for metal heat treatment. Charcoal and coal were first used as heat sources, and liquid and gas fuels were recently used. The application of electricity makes heating easy to control and no environmental pollution. These heat sources can be used for direct heating or indirect heating through molten salt or metal or even floating particles.
When the metal is heated, the workpiece is exposed to the air, and oxidation and decarburization often occur (that is, the carbon content on the surface of steel parts is reduced), which has a very adverse impact on the surface properties of parts after heat treatment. Therefore, metals should usually be heated in controlled atmosphere or protective atmosphere, molten salt and vacuum, or protected by coating or packaging.
Heating temperature is one of the important process parameters of heat treatment process. Selecting and controlling heating temperature is the main problem to ensure the quality of heat treatment. The heating temperature varies with the treated metal materials and the purpose of heat treatment, but it is generally heated above the phase transformation temperature to obtain high-temperature microstructure. In addition, the transformation takes a certain time. Therefore, when the metal workpiece surface reaches the required heating temperature, it must be maintained at this temperature for a certain time to make the internal and external temperatures consistent and complete the transformation of microstructure. This period of time is called holding time. When high-energy density heating and surface heat treatment are adopted, the heating speed is very fast, and generally there is no holding time, while the holding time of chemical heat treatment is often longer.
Cooling is also an indispensable step in the process of heat treatment. The cooling method varies with different processes, mainly controlling the cooling rate. Generally, the cooling speed of annealing is the slowest, that of normalizing is faster, and that of quenching is faster. However, there are different requirements due to different steel types. For example, air hardening steel can be hardened at the same cooling rate as normalizing.

Classification of heat treatment process

Metal heat treatment process can be divided into three categories: overall heat treatment, surface heat treatment and chemical heat treatment. According to the different heating medium, heating temperature and cooling method, each category can be divided into several different heat treatment processes. Different structures can be obtained by different heat treatment processes for the same metal, so it has different properties. Steel is the most widely used metal in industry, and the microstructure of steel is also the most complex, so there are many kinds of steel heat treatment processes.
Integral heat treatment is a metal heat treatment process that heats the workpiece as a whole and then cools it at an appropriate speed to obtain the required metallographic structure and change its overall mechanical properties. There are four basic processes for overall heat treatment of iron and steel: annealing, normalizing, quenching and tempering.
Process means:
Annealing is to heat the workpiece to an appropriate temperature, adopt different holding time according to the material and workpiece size, and then cool it slowly. The purpose is to make the internal structure of the metal reach or close to the equilibrium state, obtain good process performance and service performance, or prepare the structure for further quenching.
Normalizing is to heat the workpiece to a suitable temperature and then cool it in the air. The effect of normalizing is similar to annealing, but the obtained structure is finer. It is often used to improve the cutting performance of materials, and sometimes used for some parts with low requirements as the final heat treatment.
Quenching is to quickly cool the workpiece in water, oil or other inorganic salts, organic aqueous solutions and other quenching media after heating and thermal insulation. After quenching, the steel becomes hard but brittle at the same time. In order to eliminate brittleness in time, it is generally necessary to temper in time.
In order to reduce the brittleness of steel parts, the quenched steel parts are kept warm for a long time at an appropriate temperature higher than room temperature and lower than 650 ℃, and then cooled. This process is called tempering. Annealing, normalizing, quenching and tempering are the “four fires” in the overall heat treatment. Among them, quenching and tempering are closely related and often used together. “Four fires” evolve different heat treatment processes with different heating temperatures and cooling methods. In order to obtain certain strength and toughness, the process of combining quenching and high temperature tempering is called quenching and tempering. After some alloys are quenched to form supersaturated solid solutions, they are kept at room temperature or slightly higher appropriate temperature for a long time to improve the hardness, strength or electrical magnetism of the alloys. Such heat treatment process is called aging treatment.
The method of effectively and closely combining pressure machining deformation and heat treatment to make the workpiece obtain a good combination of strength and toughness is called thermomechanical treatment; The heat treatment in negative pressure atmosphere or vacuum is called vacuum heat treatment. It can not only prevent the workpiece from oxidation and decarburization, maintain the surface of the workpiece after treatment and improve the performance of the workpiece, but also carry out chemical heat treatment with infiltrating agent.
Surface heat treatment is a metal heat treatment process that only heats the surface of the workpiece to change its mechanical properties. In order to heat only the surface layer of the workpiece and not transfer too much heat into the interior of the workpiece, the heat source used must have high energy density, that is, large heat energy is given to the workpiece per unit area, so that the surface layer or part of the workpiece can reach high temperature in a short time or instantaneously. The main methods of surface heat treatment include flame quenching and induction heating heat treatment. The common heat sources include oxyacetylene or oxypropane flame, induced current, laser and electron beam.
Chemical heat treatment is a metal heat treatment process by changing the surface chemical composition, microstructure and properties of the workpiece. The difference between chemical heat treatment and surface heat treatment is that the former changes the chemical composition of the surface of the workpiece. Chemical heat treatment is to heat the workpiece in the medium containing carbon, salt or other alloy elements (gas, liquid and solid) for a long time, so as to infiltrate carbon, nitrogen, boron, chromium and other elements into the surface of the workpiece. After infiltration of elements, other heat treatment processes, such as quenching and tempering, are sometimes carried out. The main methods of chemical heat treatment are carburizing, nitriding and metalizing.
Heat treatment is one of the important processes in the manufacturing process of mechanical parts and tools and dies. Generally speaking, it can ensure and improve various properties of the workpiece, such as wear resistance, corrosion resistance and so on. It can also improve the microstructure and stress state of the blank, so as to facilitate all kinds of cold and hot processing.
For example, malleable cast iron can be obtained by annealing white cast iron for a long time to improve plasticity; With the correct heat treatment process, the service life of the gear can be doubled or dozens of times longer than that of the gear without heat treatment; In addition, cheap carbon steel has some expensive alloy steel properties by infiltrating some alloy elements, which can replace some heat-resistant steel and stainless steel; Almost all tools and dies need heat treatment before they can be used.

Type of annealing

Annealing is a heat treatment process in which the workpiece is heated to an appropriate temperature, maintained for a certain time, and then cooled slowly.
There are many kinds of annealing processes for steel, which can be divided into two categories according to the heating temperature: one is annealing above the critical temperature (AC1 or AC3), also known as phase transformation recrystallization annealing, including complete annealing, incomplete annealing, spheroidizing annealing and diffusion annealing (homogenization annealing); The other is annealing below the critical temperature, including recrystallization annealing and stress relief annealing. According to the cooling method, annealing can be divided into isothermal annealing and continuous cooling annealing.
1. Complete annealing and isothermal annealing
Complete annealing is also called weighing crystallization annealing, which is generally referred to as annealing. It is a heat treatment process that heats steel parts or steel to 20 ~ 30 ℃ above AC3 for a long enough time to make the structure fully austenitized and then cool slowly, so as to obtain a nearly balanced structure. This annealing is mainly used for casting, forging and hot rolled sections of various carbon and alloy steels with hypoeutectoid composition, and sometimes for welded structures. Generally, it is often used as the final heat treatment of some non heavy parts, or as the pre heat treatment of some workpieces.
2. Spheroidizing annealing
Spheroidizing annealing is mainly used for hypereutectoid carbon steel and alloy tool steel (such as steel used for manufacturing cutting tools, measuring tools and molds). Its main purpose is to reduce hardness, improve machinability and prepare for future quenching.
3. Stress relief annealing
Stress relief annealing, also known as low temperature annealing (or high temperature tempering), is mainly used to eliminate the residual stress of castings, forgings, weldments, hot rolled parts, cold drawn parts, etc. If these stresses are not eliminated, deformation or cracks will occur in the steel after a certain time or in the subsequent machining process.
4. Incomplete annealing is a heat treatment process in which the steel is heated to AC1 – AC3 (hypoeutectoid steel) or AC1 – ACCM (hypereutectoid steel) and cooled slowly after heat preservation to obtain a nearly equilibrium structure.

During quenching, the most commonly used cooling media are brine, water and oil.

The workpiece quenched by brine is easy to obtain high hardness and smooth surface, and it is not easy to produce soft spots that cannot be hardened, but it is easy to cause serious deformation and even cracking of the workpiece. Using oil as quenching medium is only suitable for quenching of some alloy steel or small-size carbon steel workpieces with high stability of undercooled austenite.

Purpose of steel tempering

• 1. Reduce brittleness and eliminate or reduce internal stress. There is great internal stress and brittleness after quenching. If not tempered in time, the steel will often deform or even crack.
• 2. Obtain the mechanical properties required by the workpiece. After quenching, the workpiece has high hardness and high brittleness. In order to meet the requirements of different properties of various workpieces, the hardness can be adjusted through appropriate tempering to reduce brittleness and obtain the required toughness and plasticity.
• 3. Stable workpiece size
• 4. For some alloy steels that are difficult to soften after annealing, high temperature tempering is often used after quenching (or normalizing), so as to properly aggregate carbides in the steel and reduce the hardness for machining.

Supplementary concept

• 1. Annealing: refers to the heat treatment process in which metal materials are heated to an appropriate temperature, maintained for a certain period of time, and then cooled slowly. Common annealing processes include recrystallization annealing, stress relief annealing, spheroidizing annealing, complete annealing, etc. The purpose of annealing is mainly to reduce the hardness and improve the plasticity of metal materials, so as to facilitate cutting or pressure processing, reduce residual stress, improve the homogenization of structure and composition, or prepare the structure for subsequent heat treatment.
• 2. Normalizing: refers to the heat treatment process of heating steel or steel parts to or above (the upper critical point temperature of steel), maintaining 30 ~ 50 ℃ for an appropriate time, and cooling in still air. The purpose of normalizing is mainly to improve the mechanical properties of low carbon steel, improve machinability, refine grains, eliminate structural defects, and make structural preparations for subsequent heat treatment.
• 3. Quenching: refers to the heat treatment process of heating steel parts to a temperature above AC3 or AC1 (lower critical point temperature of steel) for a certain time, and then obtaining martensite (or bainite) structure at an appropriate cooling rate. Common quenching processes include single medium quenching, double medium quenching, martensite step quenching, bainite isothermal quenching, surface quenching and local quenching. The purpose of quenching is to make the steel parts obtain the required martensite structure, improve the hardness, strength and wear resistance of the workpiece, and make structural preparations for the subsequent heat treatment.
• 4. Tempering: refers to the heat treatment process in which the steel parts are hardened, then heated to a temperature below AC1, held for a certain time, and then cooled to room temperature. Common tempering processes include low temperature tempering, medium temperature tempering, high temperature tempering and multiple tempering.
• The purpose of tempering: it is mainly to eliminate the stress generated by steel parts during quenching, so that the steel parts have high hardness and wear resistance, as well as the required plasticity and toughness.
• 5. Quenching and tempering: refers to the composite heat treatment process of quenching and high temperature tempering of steel or steel parts. The steel used for quenching and tempering treatment is called quenched and tempered steel. It generally refers to medium carbon structural steel and medium carbon alloy structural steel.
• 6. Carburizing: carburizing is the process of penetrating carbon atoms into the surface layer of steel. It also makes the low-carbon steel workpiece have the surface layer of high-carbon steel, and then after quenching and low-temperature tempering, the surface layer of the workpiece has high hardness and wear resistance, while the central part of the workpiece still maintains the toughness and plasticity of low-carbon steel.

Vacuum heat treatment method

Because the heating, cooling and other operations of metal workpieces need more than ten or even dozens of actions to complete. These actions are carried out in the vacuum heat treatment furnace, which is inaccessible to the operators. Therefore, the requirements for the automation degree of the vacuum heat treatment electric furnace are high. At the same time, some actions, such as the quenching process of metal workpiece after heating and insulation, must be completed in six or seven actions within 15 seconds. Such agile conditions to complete many actions, it is easy to cause operator tension and constitute misoperation. Therefore, only high automation can coordinate accurately and timely according to procedures.
The vacuum heat treatment of metal parts is carried out in a closed vacuum furnace, and the strict vacuum sealing is well known. Therefore, it is of great significance to obtain and adhere to the original air leakage rate of the furnace and ensure the working vacuum of the vacuum furnace to ensure the quality of vacuum heat treatment of parts. Therefore, a key problem of vacuum heat treatment furnace is to have a reliable vacuum sealing structure. In order to ensure the vacuum performance of the vacuum furnace, a basic principle must be followed in the structural design of the vacuum heat treatment furnace, that is, the furnace body shall adopt air tight welding, at the same time, the furnace body shall be less or no holes, and the dynamic sealing structure shall be less or avoided, so as to minimize the opportunity of vacuum leakage. The components and accessories installed on the vacuum furnace body, such as water-cooled electrode and thermocouple export device, must also be designed with sealing structure.
Most heating and insulation materials can only be used in vacuum. The heating and thermal insulation lining materials of vacuum heat treatment furnace work under vacuum and high temperature. Therefore, the requirements of high temperature resistance, good radiation results and low thermal conductivity are put forward for these materials. The requirements for oxidation resistance are not high. Therefore, tantalum, tungsten, molybdenum and graphite are widely used as heating and thermal insulation materials in vacuum heat treatment furnace. These materials are easily oxidized in the atmosphere, so these heating and thermal insulation materials cannot be used in ordinary heat treatment furnaces.
Water cooling device: the furnace shell, furnace cover, electric heating element, water cooling electrode, intermediate vacuum insulation door and other components of the vacuum heat treatment furnace work under vacuum and heating. When working under such extremely unfavorable conditions, it must be ensured that the structure of each component is not deformed and damaged, and the vacuum sealing ring is not overheated and burned. Therefore, water cooling devices shall be set for each component according to different conditions to ensure that the vacuum heat treatment furnace can operate normally and have sufficient service life.
Low voltage and high current: in the vacuum container, the energized conductor in the vacuum container will produce glow discharge under high voltage. In the vacuum heat treatment furnace, serious arc discharge will burn the electric heating elements and thermal insulation layer, resulting in major accidents and losses. Therefore, the working voltage of the electric heating element of the vacuum heat treatment furnace generally does not exceed 80-100 volts. At the same time, effective measures should be taken in the structural design of electric heating elements, such as avoiding sharp parts as far as possible, and the spacing between electrodes should not be too small to prevent glow discharge or arc discharge.
tempering
According to different performance requirements of workpiece and tempering temperature, tempering can be divided into the following types:
(1) Low temperature tempering (150-250 ℃)
The microstructure obtained by low temperature tempering is tempered martensite. The purpose is to reduce the quenched internal stress and brittleness on the premise of maintaining the high hardness and wear resistance of quenched steel, so as to avoid cracking or premature damage during use. It is mainly used for various high carbon cutting tools, measuring tools, cold stamping molds, rolling bearings and carburized parts. The hardness after tempering is generally HRC58-64.
(2) Medium temperature tempering (250-500 ℃)
The microstructure obtained by medium temperature tempering is tempered troostite. The purpose is to obtain high yield strength, elastic limit and high toughness. Therefore, it is mainly used for the treatment of various springs and hot working dies. The hardness after tempering is generally HRC35-50.
(3) High temperature tempering (500-650 ℃)
The microstructure obtained by high temperature tempering is tempered sorbite. Traditionally, the heat treatment combining quenching and high temperature tempering is called quenching and tempering treatment. Its purpose is to obtain comprehensive mechanical properties with good strength, hardness, plasticity and toughness. Therefore, it is widely used in important structural parts of automobiles, tractors and machine tools, such as connecting rods, bolts, gears and shafts. The hardness after tempering is generally hb200-330.

Prevention of deformation

The deformation causes of precision and complex molds are often complex, but as long as we master the deformation law, analyze the causes, and adopt different methods to prevent the deformation of molds, we can reduce and control it. Generally speaking, the following methods can be taken to prevent the heat treatment deformation of precision and complex molds.

• (1) Reasonable material selection. For precision and complex dies, micro deformation die steel with good material (such as air cooling steel) shall be selected. Die steel with serious carbide segregation shall be reasonably forged and quenched and tempered heat treatment. For large and unable to forge die steel, solid solution double refinement heat treatment can be carried out.
• (2) The mold structure design shall be reasonable, the thickness shall not be too different, and the shape shall be symmetrical. For molds with large deformation, the deformation law shall be mastered, and the machining allowance shall be reserved. For large, precision and complex molds, the combined structure can be adopted.
• (3) Precision and complex dies should be pre heat treated to eliminate the residual stress produced in the machining process.
• (4) Reasonably select the heating temperature and control the heating speed. For precision and complex molds, slow heating, preheating and other balanced heating methods can be adopted to reduce the heat treatment deformation of molds.
• (5) On the premise of ensuring the hardness of the die, precooling, staged cooling quenching or warm quenching process shall be adopted as far as possible.
• (6) For precision and complex molds, vacuum heating quenching and cryogenic treatment after quenching shall be adopted as far as possible if conditions permit.
• (7) For some precision and complex molds, pre heat treatment, aging heat treatment and quenching, tempering and nitriding heat treatment can be used to control the accuracy of molds.
• (8) When repairing mold defects such as sand holes, pores and wear, repair equipment with small thermal impact such as cold welding machine shall be selected to avoid deformation in the repair process.

In addition, correct heat treatment process operation (such as hole plugging, hole binding, mechanical fixation, appropriate heating method, correct selection of mold cooling direction and movement direction in cooling medium, etc.) and reasonable tempering heat treatment process are also effective measures to reduce the deformation of precision and complex molds.
Surface quenching and tempering heat treatment is usually carried out by induction heating or flame heating. The main technical parameters are surface hardness, local hardness and effective hardened layer depth. Vickers hardness tester, Rockwell or surface Rockwell hardness tester can be used for hardness testing. The selection of test force (scale) is related to the effective hardened layer depth and workpiece surface hardness. There are three kinds of durometer involved here.

• 1. Vickers hardness tester is an important means to test the surface hardness of heat-treated workpieces. It can use the test force of 0.5-100kg to test the surface hardened layer as thin as 0.05mm. Its accuracy is the highest, and it can distinguish the small difference in the surface hardness of heat-treated workpieces. In addition, the effective hardened layer depth should also be tested by Vickers hardness tester. Therefore, it is necessary to equip a Vickers hardness tester for units that carry out surface heat treatment or use a large number of surface heat treated workpieces.
• 2. The surface Rockwell hardness tester is also very suitable for testing the hardness of surface quenched workpieces. There are three scales for the surface Rockwell hardness tester. It can test various surface hardened workpieces with an effective hardening depth of more than 0.1mm. Although the accuracy of surface Rockwell hardness tester is not as high as that of Vickers hardness tester, it can meet the requirements as a testing means for quality management and qualification inspection of heat treatment plant. Moreover, it also has the characteristics of simple operation, convenient use, low price, rapid measurement and direct reading of hardness value. The surface Rockwell hardness tester can be used for rapid and nondestructive testing of batches of surface heat-treated workpieces one by one. This is of great significance for metal processing and machinery manufacturing factories.
• 3. When the surface heat treatment hardened layer is thick, Rockwell hardness tester can also be used. HRA scale can be used when the thickness of heat treatment hardened layer is 0.4-0.8mm, and HRC scale can be used when the thickness of hardened layer exceeds 0.8mm.

Vickers, Rockwell and surface Rockwell hardness values can be easily converted into standards, drawings or hardness values required by users. The corresponding conversion tables have been given in international standard ISO, American Standard ASTM and Chinese standard GB/T.

Source: China Flanges Manufacturer – Yaang Pipe Industry (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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