A Comprehensive Guide to Heat Treatment Solutions

What is heat treatment?

A process in which a solid metal or alloy is heated appropriately, held for a certain period and cooled at a certain cooling rate to change its organization to obtain the desired properties.

What is the purpose of heat treatment?

Heat treatment of steel not only improves its organization and properties but also improves its machinability. More importantly, it is a key process that empowers the final performance of the part. Many parts cannot be used directly after processing and molding but must be heat-treated before use.
The main role of heat treatment heat treatment of the main role of the following aspects:
(1) Change the internal organization of the workpiece
For example, steel annealing can get a lower pearlite organization, while quenching can get a high martensitic organization.
(2) Change the performance of the workpiece to facilitate cutting and machining or to meet the requirements of the use of the performance of the workpiece
For example, mild steel is too soft for cutting; the cutting process is easy to stick knife, which can be normalized to improve its hardness and cutting performance. Another example is tempered steel after tempering treatment can be obtained after strength, hardness, plasticity, and toughness are also better, that is, better overall mechanical properties to meet the general requirements of using the workpiece.
(3) Change the composition, organization, and performance of the surface layer of the workpiece
For example, the laser surface quenching of automobile cylinders can improve the hardness and wear resistance of its working surface to extend its service life. Another example is low carbon steel. After carburizing treatment, the surface layer of carbon content can reach about 1% to improve the workpiece’s surface hardness and wear resistance.
(4) Heat treatment can eliminate casting, forging, welding, and other machining processes caused by a variety of defects
For example, refine the grain, eliminate segregation, and reduce internal stress so that the organization of steel performance is more uniform. In short, the heat treatment process can strengthen the material, fully tap the potential of material properties, reduce structural quality, save materials and energy, and improve the quality of mechanical products, significantly extending the service life of machine parts. However, only some heat treatment processes can fulfill all the above roles and can only complete one or more of these roles.

The basic principle of heat treatment

The principle of heat treatment mainly includes the material in the heating, holding, and cooling process of the phase change and microstructure evolution of the law, as well as the corresponding material composition – process – organizational structure – performance – service behavior of the interrelationships and so on.

Heat treatment process

The heat treatment is to heat, hold, and cool the material or workpiece appropriately to obtain the desired organization and properties.

Figure.1 Schematic diagram of heat treatment process curve
Common heat treatment process parameters include heating temperature, heating rate, holding time, cooling rate and mode, etc., which determine the type and effect of the heat treatment process. Heat treatment process with the type of material and the organization of heat-treated parts of different performance requirements and different. Steel heat treatment of the basic process of annealing, normalizing, quenching, tempering surface heat treatment, chemical heat treatment, etc. The general textbooks or manuals introduced in the heat treatment principle are mainly for these heat treatment processes. Still, different types of materials, different heat treatment purposes, and different process requirements may correspond to different heat treatment principles. For example, the role of alloying elements in steel under different process conditions is extremely important in the heat treatment principle of alloy steel, which is not important for non-alloy steel heat treatment. For example, the heat treatment principle required for the “water toughness” of high manganese wear-resistant steels significantly differs from that associated with water quenching of quench-hardened steels.
Phase diagrams are often used to predict the organization of materials. After heat treatment, e.g., iron-carbon phase diagrams have been developed due to the demands of heat treatment practice for iron and steel materials and the results of many experimental observations. For annealing, normalizing quenching, and many other heat treatment processes, the heating temperature is mostly higher than the critical point of the steel A₁ or A₃ to obtain the austenite organization and then cooled in a certain way or speed to obtain the required organization and properties. Steel, in the cooling of the organization of the transformation law, can be used to supercooled austenite constant temperature transformation curve (often referred to as the C curve or T-T-T curve) and continuous cooling transformation curve (often referred to as the C-C-T curve) and so on to express. These curves are the thermodynamic and kinetic process of the material phase transformation law of the comprehensive expression.
The relationship between the heat treatment process and the heat treatment principle is a typical portrayal of the relationship between materials engineering and materials science, highlighting that materials science and engineering are an organic whole and inseparable. The development of materials science will inevitably have an immeasurable significant impact on optimizing materials heat treatment process design methods and principles. For example, the use of a finite element numerical simulation method can predict the change of spatial temperature field of heat-treated parts of different sizes and shapes under the given process conditions and then predict the spatial distribution of its microstructure and the corresponding performance field based on the principles of materials science, and even predict the service behavior of heat-treated parts that may occur in the process of using them, which realizes the cross-scale prediction of the material organization and performance in the process of heat treatment. It is foreseeable that numerical simulation and multi-scale cross-scale computational materials science methods, including thermodynamic calculations (e.g., CALPHAD) and kinetic calculations, will play an increasing role in guiding the optimal design of heat treatment processes and analyzing and solving technical problems encountered in heat treatment.

Common heat treatment processes and methods

Depending on the heating medium, heating temperature, and cooling method, each major category can be divided into several different heat treatment processes. Different heat treatment processes can be used for the same metal to obtain different microstructures and thus exhibit different properties.

Many heat treatment methods can be divided into three categories: overall heat treatment, surface heat treatment, and chemical heat treatment, based on the type of process, process name, and method of implementing heat treatment.

• Overall heat treatment: including annealing, normalizing, quenching, and tempering.
• Surface heat treatment: including surface quenching, carburizing, nitriding, and carbonitriding.
• Chemical heat treatment, including carburization, nitriding, and metal infiltration.

The process elements of heat treatment are temperature and time. Heat treatment consists of three stages: heating, insulation, and cooling. Therefore, to master the heat treatment principle of steel, the main task is to grasp the microstructure transformation law of steel during heating and cooling.

Overall heat treatment

Overall heat treatment is a metal heat treatment process that heats the workpiece, then cools it at an appropriate rate to obtain the required metallographic structure and change its overall mechanical properties. There are four basic processes for the overall heat treatment of steel: annealing, normalizing, quenching, and tempering.
(1) Annealing of Steel

A heat treatment process that heats the workpiece to a certain temperature and maintains insulation, followed by slow cooling.

Figure.2 Heating Range of Various Annealing and Normalizing Processes

Different annealing methods should be used based on the composition and annealing purpose of the steel. The commonly used annealing methods are:

• Complete annealing (ordinary annealing) – Method: Heat the hypoeutectoid steel to 30-50 ℃ above the A_C3 line, hold it for a certain period, and slowly cool it (cooling in the furnace).
• Spheroidizing annealing (incomplete annealing) – Method: Heat eutectoid or hypereutectoid steel to 20-30 ℃ above the A_C1 line, hold it, and slowly cool it.
• Stress Relieving Annealing (Low-Temperature Annealing) – Method: Heat the steel below the A_C1 line, keep it warm, and then slowly cool it.
• Complete annealing (ordinary annealing) – Purpose: Refine grain size and uniform structure, reduce hardness, and eliminate stress. At the same time, it is also prepared for cutting and final heat treatment. Mainly used in castings, forgings, and rolled parts of hyper eutectoid steel.
• Spheroidizing annealing (incomplete annealing) – To eliminate network or flake cementite, reduce hardness, and improve toughness. At the same time, it is also prepared for cutting and final heat treatment. Mainly used in eutectoid, hypereutectoid steel, and alloy tool steel
• Stress relieving annealing (low-temperature annealing) – Purpose: to eliminate internal stress in castings and weldments and stabilize dimensions; Main feature: No organizational changes

(2) Normalizing

Figure.3 Comparison of microstructure after annealing and normalizing

Method: Heat the steel parts to A_C3 or A_Cm above 30-50 ℃, hold for a certain period, and then take them out of the furnace and cool them in air.

Objective:

• Reduce hardness, improve plasticity, and improve cutting and pressure processing performance;
• Refine grain size, improve mechanical properties, and prepare for the next process;
• Eliminate internal stress generated by cold and hot processing.

Attention: Normalizing and ordinary annealing belong to the same heat treatment process. Both heat the steel to the austenitic state. The difference is normalizing cools in the air while annealing cools with the furnace.
Mainly applied in the following aspects:

• ① Final heat treatment of parts with low mechanical performance requirements.
• ② Improve the machinability of low-carbon steel.
• ③ As a preliminary heat treatment for medium carbon steel (it can replace partial annealing heat treatment).

Specific use

• For low-carbon steel and low-alloy steel, normalizing can increase their hardness to improve their machinability;
• For medium carbon steel, normalizing can replace quenching and tempering treatment to prepare the structure for high-frequency quenching, reduce deformation of steel parts, and reduce processing costs;
• For high-carbon steel, normalizing can eliminate the network cementite structure and facilitate spheroidization annealing;
• For large steel forgings or steel castings with sharp changes in cross-section, normalizing can be used instead of quenching to reduce the tendency for deformation and cracking or to prepare the structure for quenching;
• For quenched and repaired parts of steel, the effect of overheating can be eliminated by normalizing so that they can be re-quenched;
• Used for cast iron castings to increase the amount of pearlite in the matrix, and improve the strength and wear resistance of castings.

(3) Quenching
Method:
Heat the steel to 30-50 ℃ above A_C3 (hypereutectoid steel) or A_Cm (eutectoid or hypereutectoid steel), hold it for a certain period to austenitize, and then rapidly cool it in a cooling medium.
Purpose:
It is to obtain a uniform and fine martensite structure, which is then tempered to improve the mechanical properties of the steel.
Attention:

• ① The key to quenching is determining the quenching temperature and cooling method.
• ② It is the most commonly used heat treatment and the key to determining product quality.

1). Selection of quenching temperature

Figure.4 Quenching heating temperature range of carbon steel

Quenching medium
The medium used for quenching and cooling of workpieces is called quenching and cooling medium (or quenching medium). The ideal quenching medium should have the condition that the workpiece can be quenched into martensite without causing too much quenching stress.
The commonly used quenching media include water, aqueous solution, mineral oil, molten salt, and alkali.
● Water
Water is a quenching medium with a strong cooling ability.
Advantages: Wide source, low price, stable composition, and not easy to deteriorate.
Disadvantages: The cooling capacity is unstable, which can easily cause deformation or cracking of the workpiece. In the “nose” region of the C curve (around 500-600 ℃), water is in the steam film stage, and cooling is not fast enough, forming “soft spots”; In the martensitic transformation temperature range (300-100 ℃), water is in the boiling stage, and cooling too fast can easily cause the martensitic transformation speed to be too fast, resulting in significant internal stress and deformation or even cracking of the workpiece. When the water temperature increases, it contains more gas or insoluble impurities (such as oil, soap, mud, etc.), significantly reducing its cooling capacity.
Application: Suitable for quenching and cooling carbon steel workpieces with small cross-sectional dimensions and simple shapes.
● Brine and alkaline water
Add salt and alkali to the water to immerse the high-temperature workpiece in the cooling medium. During the steam film stage, crystals of salt and alkali precipitate and immediately burst, damaging the steam film and shattering the oxide skin on the surface of the workpiece. This can improve the cooling capacity of the medium in the high-temperature zone, but its disadvantage is that the medium is highly corrosive.
Application: In general, the concentration of saline water is 10%, and the concentration of caustic soda aqueous solution is 10-15%. It can be used as a quenching medium for carbon steel and low alloy structural steel workpieces, and the temperature should not exceed 60 ℃. After quenching, it should be cleaned promptly and undergo rust prevention treatment.
● Oil
The cooling medium generally uses mineral oil (mineral oil), such as engine oil, transformer oil, and diesel. Engine oil generally uses No. 10, No. 20, and No. 30 engine oils. The larger the oil grade, the greater the viscosity, the higher the flash point, the lower the cooling capacity, and the corresponding increase in operating temperature.

Quenching method
● Single liquid quenching
It is a quenching operation method of immersing austenitic chemical components in a certain quenching medium and cooling them to room temperature. Single liquid quenching media include water, salt water, alkaline water, oil, and specially formulated agents.
Advantages: Simple operation, conducive to achieving mechanization and automation.
Disadvantage: The cooling rate is limited by the cooling characteristics of the medium and affects the quenching quality.
Application: Single liquid quenching is only suitable for carbon steel workpieces with simpler shapes.
● Double liquid quenching
Austenitic chemical components are first immersed in a medium with a strong cooling capacity and then taken out before the steel component reaches the temperature of the quenching medium. They are immediately immersed in another medium with weak cooling capacity, such as water followed by oil, water followed by air, etc. Double liquid quenching reduces deformation and cracking tendency, which is difficult to master and has certain limitations in application.
● Martensite-graded quenching
Austenitic chemical components are first immersed in a liquid medium (salt or alkali bath) with a temperature slightly higher or lower than the martensitic point of steel, maintained for an appropriate time, and taken out for air cooling after both the inner and outer layers of the steel components reach the medium temperature to obtain martensitic structure, also known as staged quenching.
Advantages: Graded quenching can effectively reduce phase transformation and thermal stress and reduce quenching deformation and cracking tendency due to air cooling after the grading temperature stays at the same temperature inside and outside the workpiece.
Application: Suitable for alloy steel and high alloy steel workpieces with high deformation requirements and carbon steel workpieces with small cross-sectional dimensions and complex shapes.
● Bainite isothermal quenching
It is a quenching process that austenitizes steel parts and rapidly cools them to the bainite transformation temperature range (260-400 ℃), holding them in an isothermal manner to convert austenite into bainite. It is sometimes called isothermal quenching, and the general holding time is 30-60 minutes.
● Composite quenching
Quench the workpiece below Ms to obtain 10% to 20% martensite, then isothermal it in the lower bainite temperature range. This cooling method can achieve M+B microstructure in larger cross-section workpieces. The martensite formed during pre-quenching can promote the transformation of bainite, which in turn causes the martensite to temper during isothermal annealing. Composite quenching is used for alloy tool steel workpieces, which can avoid the first type of temper brittleness and reduce the amount of residual austenite, i.e., the tendency to deform and crack.

2). Common quenching and cooling methods
Single liquid quenching method: Heat to quenching temperature and continuously cool in a quenching medium. Common carbon steel is quenched in water, while alloy steel is quenched in oil.
Double medium quenching method: Heat to quenching temperature, first cool to the area below Ms point in a medium with strong cooling capacity, then cool in another medium with weak cooling capacity. Suitable for high carbon steel parts and larger alloy steel parts.
Grading quenching method: Heat to the quenching temperature, cool in an oil and salt bath near the Ms point (2-5 minutes), and then take out the air cooling. Suitable for smaller-sized workpieces.
Isothermal quenching method: important parts with complex shapes, precise size requirements, and high requirements for hardness and toughness.
(4) Tempering
Method:
Heat the quenched steel below A_C1, maintain insulation, and cool it to room temperature using a certain cooling method.
Purpose:
Reduce or eliminate quenching stress to prevent deformation and cracking of steel parts; Stable organization and size; Obtain the required organization and performance.
Attention:

• ① Quenched steel cannot be directly used without tempering.
• ② Tempering transformation of quenched steel: decomposition of martensite and decomposition of residual austenite.

Tempering type and application:

• 1) Low-temperature tempering (150-250 ℃) aims to reduce the internal stress and brittleness of quenched steel while maintaining high hardness (56-64HRC) and wear resistance, such as molds, cutting tools, etc.
• 2) Medium temperature tempering (350-500 ℃) aims to achieve high elasticity and maintain high hardness (35-50HRC) and certain toughness of the steel. Such as springs, forging dies, etc.
• 3) The purpose of high-temperature tempering tempering (500-650 ℃) is to obtain comprehensive mechanical properties such as strength and toughness. Such as connecting rods, crankshafts, gears, etc.

Surface heat treatment

Some parts can achieve high surface hardness and high core toughness through surface heat treatment.
Surface quenching
The characteristic of surface quenching is rapid heating, which causes the surface of the workpiece to rise to the quenching temperature rapidly. At the same time, the core remains below A_C1 temperature and then rapidly cools down. As a result, the surface layer of the workpiece is hardened into the martensitic structure, while the core remains the original structure, maintaining good toughness. After appropriate low-temperature tempering, the required performance can be obtained.
According to different heating methods, surface quenching methods can be divided into three types:
1. Flame heating surface quenching method
It is difficult to grasp the quenching depth of 2-6mm based on experience.
2. Induction heating surface quenching method
The skin effect of the current is utilized to rapidly heat the surface of the workpiece while the core is almost unheated.
① High-frequency induction heating

• The current frequency is (f = 200-300) KHZ
• Quenching depth 0.5-2mm

Mainly used for heating medium and small parts with shallow hardening layers, such as small shafts, small module gears, etc.
The process route for high-frequency quenching gears is:
Forging → normalizing or annealing → rough machining → quenching and tempering → precision machining → high-frequency quenching → low-temperature tempering → grinding.

Chemical heat treatment

Chemical heat treatment is the process of heating and insulating a workpiece in a specific medium, allowing one or more elements to penetrate the surface of the workpiece to change its surface composition and structure. The surface and center of the workpiece have different mechanical properties or special physical and chemical properties.
Principle: The active element to be infiltrated is heated and decomposed, called activation, then absorbed by the surface of the workpiece and then diffused into the interior of the workpiece.
Common chemical heat treatment methods include:
1) Carburization
Carburization is the process of absorbing carbon atoms on the surface of steel.

• Purpose: To achieve high hardness (HRC58-64) on the surface of the parts and high strength and toughness at the heart.
• Gas carburization method: mainly uses methane, coal gas, toluene, kerosene, etc., as carburizing agents and decomposes activated carbon atoms at high gas temperatures for carburization.
• Solid carburization method: The main solid carburizing agent is charcoal, followed by a small amount of carbonate. As shown in the figure.
• Liquid carburization method: It is not widely used due to generating toxic gases.

2) Nitridation
Even if nitrogen atoms infiltrate the surface of the workpiece.
The gas nitriding method is currently widely used. Place the workpiece in a specialized nitriding furnace and heat it to 500-600 ℃. At the same time, ammonia gas (NH₃) is introduced. When the ammonia gas is heated to 450 ℃, it decomposes into active ammonia atoms and diffuses into the surface of the workpiece, forming a nitriding layer. The elements of nitriding are temperature and time, which are used to control the thickness of the nitriding layer.
The disadvantage of nitriding treatment is that it takes a long time and generally requires alloy steel, resulting in high costs. Only used for high-speed transmission shafts in machine tools, Precision gears, etc.
The process route for general nitrided parts is:

Forging → annealing → rough machining → quenching and tempering → precision machining → stress relieving annealing → grinding → nitriding → precision grinding.

3) Carbonitriding
The process of simultaneously penetrating carbon and nitrogen into the surface of a part is called cyanidation.
According to the different processing temperatures, cyanidation can be divided into high, medium, and low temperatures.
4) Other chemical heat treatment methods

• (1) The purpose of aluminizing is to provide high oxidation resistance on the steel surface.
• (2) The purpose of chromizing is to increase the corrosion resistance of parts and improve carbon steel’s hardness and wear resistance.

Other heat treatment processes

Solid solution treatment

Operation method
Heat the alloy to a high temperature (980-1250 ℃) and maintain a constant temperature in the single-phase zone, allowing the excess phase to fully dissolve in the solid solution and then rapidly cool.
Objective

• Obtaining single-phase austenite structure;
• Improve the plasticity and toughness of steel and alloys, and prepare for precipitation hardening treatment;
• Fully dissolve various phases in the alloy, strengthen the solid solution, and improve toughness and corrosion resistance;
• Relieve stress and soften in order to continue processing or forming.

Key points of application
The solid solution temperature should be adjusted according to the alloy usage temperature, and the higher the usage environment temperature, the higher the solid solution temperature should also be; For alloys with low supersaturation, a faster cooling rate is usually chosen, while for alloys with high saturation, air cooling is usually used.

Cryogenic treatment

Operation method
Cool the quenched steel parts in a low-temperature medium (such as dry ice or liquid nitrogen) to -40 — -80 ℃ or lower, and remove them to room temperature after the uniform temperature.
Objective

• Transforming all or a large part of the residual austenite in quenched steel parts into martensite, thereby improving the hardness, strength, wear resistance, and fatigue limit of the steel parts;
• Stabilize the steel structure to stabilize the shape and size of steel components.

Key points of application
After quenching, steel parts should be immediately subjected to cold treatment, followed by low-temperature tempering to eliminate internal stress during low-temperature cooling;
Cold treatment mainly applies to tight-cutting tools, measuring tools, and tight parts made of alloy steel.

Vacuum heat treatment

Because the heating, cooling, and other operations of metal workpieces require dozens or even dozens of actions to complete. These actions are carried out inside the vacuum heat treatment furnace, which is inaccessible to operators. Therefore, there is a high requirement for the automation level of the vacuum heat treatment furnace. At the same time, some actions, such as quenching the metal workpiece after heating and insulation, require six or seven actions and must be completed within 15 seconds. Completing many actions under such agile conditions can easily cause operator tension and result in misoperation. Therefore, only high automation can accurately and timely coordinate according to the program.
Metal parts undergo vacuum heat treatment in a sealed vacuum furnace; strict vacuum sealing is well-known. Therefore, obtaining and adhering to the furnace’s original leak rate and ensuring the vacuum furnace’s working vacuum is of great significance for ensuring the quality of vacuum heat treatment of parts. So, a key issue for vacuum heat treatment furnaces is to have a reliable vacuum sealing structure. 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, which is to use airtight welding on the furnace body and to minimize or avoid the use of dynamic sealing structures on the furnace body, to minimize the opportunity for vacuum leakage. The components and accessories installed on the vacuum furnace body, such as water-cooled electrodes and thermocouple export devices, must also be designed with sealing structures.
Most heating and insulation materials can only be used in a vacuum state. The heating and insulation lining materials of vacuum heat treatment furnaces work under vacuum and high temperature, thus requiring high temperature resistance, good radiation performance, and low thermal conductivity for these materials. The requirement for anti-oxidation performance is low. Therefore, vacuum heat treatment furnaces widely use tantalum, tungsten, molybdenum, and graphite as heating and insulation materials. These materials are highly susceptible to oxidation in atmospheric conditions. Therefore, ordinary heat treatment furnaces cannot use these heating and insulation materials.

Water cooling device: The shell, cover, electric heating element, water-cooled electrode, intermediate vacuum insulation door, and other components of the vacuum heat treatment furnace work under vacuum and heating conditions. Working under extremely unfavorable conditions, it is necessary to ensure that the structure of each component is not deformed or damaged and that the vacuum sealing ring is not overheated or burned. Therefore, each component should be equipped with water cooling devices according to different situations to ensure the vacuum heat treatment furnace’s normal operation and sufficient service life.

Formulation of heat treatment process plan

The formulation of the heat treatment process plan refers to determining the heat treatment process and process method based on the part drawing and technical requirements, as well as the manufacturing process route of the part.
1) Arrangement of heat treatment process positions
In the manufacturing process of parts, the arrangement of heat treatment processes can be roughly classified into four types:

• The heat treatment processes arranged before mechanical processing include normalizing, annealing, and artificial aging.
• The heat treatment processes arranged after rough machining and before semi precision machining include tempering, aging, and annealing.
• The heat treatment processes arranged after semi-precision machining and before precision machining includes carburization, carbonitriding, multi-element co infiltration, quenching and tempering, surface induction quenching, stress relieving annealing, etc.
• The heat treatment processes arranged after precision machining include nitriding, carburization, contact resistance heating fire, laser fire, metal infiltration, and deposition of carbides and nitrides.

2) Preparation of Part Manufacturing Process Route
Due to the varying complexity of component structure, material selection, and technical requirements, the manufacturing process route is very complex, as shown in the following figure. It can be roughly divided into three categories:

• (1) The process route for ordinary parts that require general performance: raw materials (or cast forged blanks or semi-finished products) → normalizing or annealing → mechanical cutting, rough machining → finished parts. Its normalizing or annealing involves both preliminary and final heat treatment; It not only improves the processability of mechanical cutting processing but also endows the parts with the necessary performance.
• (2) The process route for ordinary parts with high-performance requirements: raw materials (cast forged blanks or semi-finished products) → preliminary heat treatment (normalizing or annealing) → mechanical cutting rough machining → final heat treatment (quenching or tempering) → mechanical cutting precision machining → finished parts. For parts with low hardness requirements and when machining is not difficult, the position of mechanical cutting and final heat treatment can be switched; that is, rough machining and finishing machining can be carried out after preliminary and final heat treatment. This can reduce parts turnover between the heat treatment and mechanical cutting workshops.
• (3) Precision parts that require high performance have not only high-performance requirements but also have strict requirements for dimensional machining accuracy, shape and position accuracy, as well as surface roughness; the process route is as follows: raw materials (or casting and forging blanks, or semi-finished products) → preparation heat treatment (normalizing or annealing) → mechanical cutting rough machining → quenching and tempering heat treatment → mechanical cutting semi-precision machining → overall heat treatment (quenching and tempering) or carburizing, quenching, tempering → precision machining → stabilization treatment or nitriding, metal infiltration → finished parts. For some high-precision parts, such as high-precision measuring tools and precision gears, one or more precision machining and stabilization treatments are required after semi-precision machining.

Based on the arrangement of the heat treatment process location and the determination of the part processing process route mentioned above, the following contents should also be noted:

• The arrangement of the heat treatment process and mechanical processing process should be reasonable. For example, parts’ quenching and tempering treatment is generally placed before mechanical finishing. The final heat treatment is placed between rough machining and grinding for parts with high hardness requirements.
• We are adopting advanced technology and simplified heat treatment processes. Try to combine different processes as much as possible while ensuring the organization and performance of the parts, for example, forging waste heat and correcting deformation during the bainitic steel parts’ quenching and air-cooling process.
• Shorten the production cycle as much as possible. For example, to reduce the amount of deformation correction, increase the structural strength of the thin part before heat treatment, or reserve machining allowance for subsequent processing during rough machining. For parts with complex structures prone to deformation, reinforcing ribs should be added before heat treatment and cut off after heat treatment.
• Sometimes, to improve product quality and extend service life, it is necessary to add heat treatment processes. For example, normalizing of tooling before spheroidizing annealing, Multiple tempering after quenching, etc.

For complex and easily deformable parts, multiple reciprocating cycles such as heat treatment and mechanical processing can be carried out to achieve higher accuracy requirements.
3) Determination of heat treatment process method
The arrangement of heat treatment processes and the formulation of manufacturing process routes are generally determined by the comprehensive process (coordinating cold and hot processing) department. It is impossible to specify the heat treatment process very specifically, often only indicating the name of the process (such as normalizing, annealing, quenching, carburizing, nitriding, etc.). As for which process method to use (such as which type of annealing, which type of carburizing, etc.), it needs to be specifically determined by the heat treatment process designer. Only after determining the process method can it further determine its process parameters (such as heating speed, heating temperature, cooling medium, cooling transfer temperature, etc.).
Principles for determining process methods:

• Carefully analyze the importance of the processed parts (general parts, important parts, key parts), the difficulty level of the heat treatment process methods specified in the drawings, and the feasibility of implementation inside and outside the enterprise. Starting from meeting the process methods specified in the part drawings as much as possible, it is necessary to communicate and coordinate with the product design department promptly if it is not feasible. Ultimately, the product design department will decide whether to choose or make changes and provide feedback to the process design department in the form of documents according to the approval procedures specified by the enterprise.
• The specific process method should be determined based on the conditions possessed by the enterprise, and the feasibility of outsourcing heat treatment should only be considered when the technical conditions of the product and its parts cannot be changed.
• When determining the process method, only parts with low importance can be treated as parts with high importance, and the opposite is not true. Only high-quality materials can be used to replace high-quality and ordinary materials, and the opposite is not allowed. It shall be handled according to the approval procedures stipulated by the enterprise, and no one shall use it without authorization.
• When there are no clear technical requirements for heat treatment in technical documents such as product and component drawings, the designer must search for reliable technical standards and relevant information and determine the process method with evidence without unquestioningly determining.
• Overall heat treatment, surface heat treatment, and chemical heat treatment shall not replace each other. When replacement is necessary, it should be handled according to the approval procedures stipulated by the enterprise, and no one is allowed to replace it without authorization.
• When determining the specific process method, whether it is high or low. For example, high carbon steel and high carbon alloy steel annealing require spheroidization annealing and ordinary annealing processes are not allowed to handle it. Vacuum furnace quenching heating shall not be replaced by ordinary furnace quenching heating; Carbonitriding shall not be replaced by carburizing; Nitriding shall not be used as a substitute for nitrocarburization. If replacement is required, it should be handled according to the approval procedures stipulated by the enterprise, and no one is allowed to replace it without authorization.
• When determining the process method, consideration should be given to the unification of technical progressiveness and technical economy, economy and safety, safety and production efficiency, and the relationship between the determination principles should be handled well.

Common types and defect forms of heat treatment

The phenomenon of steel parts losing their original working ability due to cracks, deformation, wear, corrosion, and other reasons during the heat treatment process is called failure or defect. The purpose of failure analysis is to analyze the causes from both external and internal factors, to take effective prevention and solutions to prevent the recurrence of failure.
The quality defects generated can be divided into two aspects. One type is congenital disabilities, such as unreasonable structural design of parts, defects in raw materials, or blanks, which are generated or expanded into heat treatment defects during the heat treatment process. This is something that heat treatment workers cannot solve, and it only requires designers to understand the consequences caused by poor design, select the correct materials, and formulate reasonable technical requirements. Avoid sharp changes in the cross-section, printing markings, and adopting sharp corner transitions. It is also necessary to pay attention to defects in raw materials, such as fluctuations and unevenness in chemical composition, excessive impurity content, severe segregation, non-metallic inclusions, looseness, banded structure, creases, hairlines, white spots, microcracks, oxidation decarburization, and scratches, which are not caused by heat treatment. These defects should be strictly controlled. At the same time, it is required that raw material inspection personnel carefully inspect and strive to avoid the production of materials with quality problems.
In addition, defects caused by casting, forging, welding, and mechanical processing, such as cracks, poor organization, and appearance defects, may also cause congenital heat treatment defects. Another type is acquired factors, namely heat treatment factors and non-standard processing and use factors, such as unreasonable formulation of heat treatment processes for parts, improper operation, unsuitable equipment, environmental conditions, improper subsequent mechanical processing processes, and early failure of parts during use. Therefore, in heat treatment production, special attention should be paid to controlling the heat treatment process of parts, achieving prevention first, reducing deterioration, and eliminating waste, thoroughly analyzing the six major factors that affect product quality: human, machine, material, method, environment, and inspection, and take effective measures at the lowest cost to produce high-quality products. The types and causes of common heat treatment defects are summarized in Tables 1 and 2 below for the convenience of understanding and systematic classification.
Table.1 Types and Defect Forms of Heat Treatment

Category	Influence factor
Non heat treatment reason or congenital reason	Unreasonable design of parts		The section size of the part changes greatly , There are edges and corners , Scratches or printing marks on the surface , Improper selection of materials , Excessive load on parts
	Defect of material itself		The decarburization layer is too thick , Non metallic inclusion exceeds the requirements , Tissue segregation , Uneven distribution of carbide , impurity (P 、 S) Excessive content , Surface folding , Surface microcrack , White spot
Heat treatment factors and factors of non-standard processing and use	The established heat treatment process is unreasonable		overheated , Low quenching temperature , Uneven heating , Quenching complete cooling , Uneven cooling , Secondary quenching , Fish scale fracture , carburization , Oxidative decarbonization , Poor spheroidizing annealing , Untimely tempering
	Improper subsequent machining		Grinding crack , Grinding burn , Grinding quenching , EDM crack , Improper pickling
	Use defects of parts		Improper installation, excessive stress concentration, high working environment temperature, improper welding repair, excessive use without timely replacement

The quality of heat treatment for steel parts is closely related to the operator. Heating errors can lead to oxidation, decarburization, overheating, tempering cracks, and tempering brittleness of the parts; Cooling errors can cause quenching cracks, quenching strains, tempering cracks, quenching soft spots, annealing brittleness, tempering brittleness, and cold treatment cracks; Subsequent processing errors can cause grinding burns, grinding cracks, acid pickling brittleness, and immersion brittleness on the surface of the parts, which should be highly valued. The causes of heat treatment defects can be analyzed from several aspects, such as phase transformation (structural changes) of metal materials, the effect of thermal stress, element precipitation, and chemical reactions with the outside world.

The cracks generated during the quenching process are the result of the transformation of undercooled austenite into martensite, the occurrence of structural transformation, the increase of specific volume, and the combined effect of thermal stress and structural stress; The grinding cracks generated during grinding are caused by poor cooling, and the surface heating temperature of the part is higher than the decomposition temperature of martensite (martensite transforms into martensite or sorbite), resulting in secondary quenching; Quenching soft spots are caused by uneven cooling during the cooling process, resulting in the appearance of non martensitic structures (pearlite) on the surface; Quenching deformation, especially changes in shape, are caused by phase transformation stress and thermal stress during the cooling process, i.e. stress; The commonly mentioned temper brittleness and annealing brittleness are defects caused by the precipitation of carbides at grain boundaries; And the brittleness of immersion plating is the result of the interaction of hydrogen atoms in the initial ecology; Finally, it should be mentioned that defects such as oxidation and decarburization on the surface of the parts are the result of chemical reactions with oxygen during the heating process, in contact with the heating medium or air during the cooling process.

Factors and Solutions for Heat Treatment Deformation

1) Reasons for deformation
The main reason for the deformation of steel is the presence of internal or external applied stress in the steel. Internal stress is caused by uneven temperature distribution or phase transformation, and residual stress is also one of the reasons. The deformation caused by external stress is mainly caused by the “collapse” of the workpiece due to its weight. In special cases, it should also be considered to collide with the heated workpiece or the depression caused by clamping tools. Deformation includes two types: elastic deformation and plastic deformation. The size change is mainly based on organizational transformation, exhibiting the same expansion and contraction. However, when there are pores or complex-shaped workpieces on the workpiece, it will lead to additional deformation. If a large amount of martensite is formed during quenching, expansion will occur, and if a large amount of residual austenite is produced, corresponding shrinkage will occur. In addition, shrinkage generally occurs during tempering, while alloy steel with a secondary hardening phenomenon expands. If subjected to deep cooling treatment, further expansion occurs due to the martensitization of residual austenite. The specific volume of these structures increases with the increase of carbon content, so an increase in carbon content also increases the size change.
2) The main occurrence period of quenching deformation

• Heating process: During heating, the workpiece undergoes deformation due to the gradual release of internal stress.
• Insulation process: Mainly due to self-weight collapse deformation, i.e., collapse bending.
• Cooling process: deformation due to uneven cooling and structural transformation.

3) Heating and deformation
When heating large workpieces, residual stress or uneven heating can cause deformation. The residual stress mainly comes from the machining process. When these stresses exist, as the temperature increases, the yield strength of the steel gradually decreases. Even if the heating is uniform, even slight stresses can cause deformation.
Generally, the residual stress at the outer edge of the workpiece is relatively high. When the temperature rises from the outside, the deformation at the outer edge is relatively large. The deformation caused by residual stress includes two types: elastic deformation and plastic deformation.
The thermal stress and strain generated during heating are both causes of deformation. The faster the heating speed, the larger the workpiece size, and the greater the cross-sectional change, the greater the heating deformation. Thermal stress depends on the degree of uneven temperature distribution and temperature gradient, which are the reasons for differences in thermal expansion. If the thermal stress is higher than the high-temperature yield point of the material, it causes plastic deformation, manifested as “deformation”.
The phase change stress mainly originates from the nonisochronous nature of phase change, which occurs when a part of the material undergoes phase change while the other parts have not yet undergone phase change. Plastic deformation can occur when the material’s microstructure transforms into austenite and undergoes volume shrinkage during heating. No stress is generated if all parts of the material undergo the same structural transformation simultaneously. For this reason, slow heating can appropriately reduce heating deformation, and it is best to use preheating.
In addition, due to the frequent occurrence of “collapse” deformation due to self-weight during heating, the higher the heating temperature and the longer the heating time, the more severe the “collapse” phenomenon.
4) Cooling and deformation
When the cooling is uneven, thermal stress is generated, leading to deformation. Due to differences in cooling rates between the outer and inner edges of the workpiece, this thermal stress is inevitable. Under quenching, the thermal and structural stress are superimposed, resulting in more complex deformation. In addition, the non-uniformity and decarburization of the organization can also lead to differences in phase transition points and varying expansion amounts of the phase transition.
In short, “deformation” is caused by both phase transformation and thermal stress, but not all stress is consumed in deformation. Still, a portion exists as residual stress in the workpiece, which is the reason for aging deformation and aging cracks.

Figure.5 Deformation caused by thermal stress

Figure.6 Deformation caused by phase change stress

Figure.7 Deformation behavior caused by quenching and cooling

Figure.8 Different quenching methods exhibit different deformations

The deformation caused by cooling manifests in the following forms:

• In the early stage of rapid cooling, one side of the workpiece is concave and then becomes convex, resulting in the rapidly cooling side being convex. This situation belongs to the deformation caused by thermal stress greater than that caused by phase change.
• The deformation caused by thermal stress tends to spheroidize the steel material (see Figure 5), while the deformation caused by phase change stress tends to be wound around the wire axis (see Figure 6). Therefore, the deformation caused by quenching and cooling is manifested as a combination of the two (Figure 7), and according to the different quenching methods, different deformations are shown in Figure 8.
• When only quenching the inner hole, the inner hole shrinks. When the entire annular workpiece is heated and quenched, its outer diameter always increases, while the inner diameter expands and shrinks according to different sizes. Generally, when the inner diameter is large, the inner hole expands, and when the inner diameter is small, the inner hole shrinks.

5) Cold treatment and deformation
The cold treatment promotes martensitic transformation at lower temperatures, resulting in less deformation than quenching cooling. However, at this time, the stress generated is greater, and the superposition of residual stress, phase transformation stress, and thermal stress can easily lead to cracking.
6) Tempering and deformation
During the tempering process, the deformation of the workpiece tends to decrease due to the homogenization, reduction, or even disappearance of internal stress, as well as changes in the microstructure. However, once deformation occurs, it is also difficult to correct. Methods such as pressure tempering or shot peening hardening are often used to correct this deformation.
7) Repeated quenching and deformation
Usually, repeated quenching without intermediate annealing of the workpiece after one quenching will increase deformation. The deformation caused by repeated quenching tends to accumulate into a spherical shape after repeated quenching, which is prone to cracking. However, the shape is relatively stable and no longer deformation prone. Therefore, intermediate annealing should be added before repeated quenching, and the number of repeated quenching should be less than or equal to 2 times (excluding the first quenching).
8) Residual stress and deformation
During the heating process, at around 450 ℃, the steel transforms from an elastic body to a plastic body, making it prone to upward plastic deformation. At the same time, residual stress will also disappear due to recrystallization at temperatures around this temperature. Therefore, during rapid heating, due to the temperature difference between the inner and outer parts of the workpiece, the outer part reaches 450 ℃ and becomes a plastic zone, which undergoes deformation due to residual stress at lower internal temperatures. After cooling, this area is where the deformation occurs. Due to the difficulty in achieving uniform and slow heating in actual production processes, it is very important to perform stress relief annealing before quenching. In addition to heat stress relief, vibration stress relief is also effective for large parts.
9) Adopting high-precision heat treatment equipment
Adopting high-precision heat treatment equipment to accurately control temperature differences and reduce deformation.

Factors that cause deformation during heat treatment

1) Carbon content and its impact on heat treatment changes
The increase in yield strength of high carbon steel results in less deformation than that of medium carbon steel. In most cases, the deformation of T7A steel is the smallest for carbon steel. When the mass fraction of carbon is greater than 0.7%, it tends to shrink, but when it is less than 0.7%, both the inner and outer diameters tend to expand.
Carbon steel has a relatively low yield strength, so carbon steel parts with inner holes (or cavities) tend to deform greatly, and the inner holes (or cavities) swell. Due to its high strength, low Ms point, and high residual austenite content, alloy steel undergoes relatively small quenching deformation, mainly manifested as thermal stress type deformation, and its internal holes (or cavities) tend to shrink therefore when quenched under the same conditions as medium carbon steel, high carbon steel and high alloy steel workpieces tend to experience mainly internal hole shrinkage.
2) The effect of alloy elements on heat treatment deformation
The influence of alloy elements on the deformation of workpieces during heat treatment is mainly reflected in their effects on the Ms point and hardenability of the steel. Most alloy elements, such as manganese, chromium, silicon, nickel, molybdenum, boron, etc., reduce the Ms point of the steel, increase the amount of residual austenite, reduce the specific volume change and structural stress of the steel during quenching, thus reducing the quenching deformation of the workpiece. The alloying elements significantly improve the hardenability of steel, thereby increasing the steel’s volume deformation and structural stress, leading to an increase in the tendency of the workpiece to undergo heat treatment deformation. In addition, the critical quenching cooling rate is reduced due to the improved hardenability of steel by alloying elements. In actual production, mild quenching media can be used for quenching, thereby reducing thermal stress and reducing the heat treatment deformation of the workpiece. Silicon has little effect on the Ms point and only reduces the deformation of the sample; Tungsten and vanadium have little effect on the hardenability and Ms point and have little effect on the deformation of the workpiece during heat treatment. Therefore, the so-called micro-deformed steel in industry contains many alloy elements, such as silicon, tungsten, and vanadium.
3) The influence of original structure and stress state on heat treatment deformation
The original structure of the workpiece before quenching, such as the shape, size, quantity, and distribution of carbides, segregation of alloy elements, and the direction of fibers formed by forging and rolling, all have a certain impact on the heat treatment deformation of the workpiece. Spherical pearlite has a larger volume and higher strength than sheet pearlite, so the quenching deformation of the workpiece after pre-spheroidization treatment is relatively small. For some high carbon alloy tool steels, such as 9Mn2V, CrWMn, and GCr15 steel, the spheroidization level has a significant impact on the correction of heat treatment deformation cracking and deformation after quenching, and it is usually advisable to use a 2.5-5 level spheroidized structure. Quenching and tempering treatment not only reduces the absolute value of workpiece deformation but also makes the quenching deformation of the workpiece more regular, which is beneficial for controlling deformation.
The distribution of strip-shaped carbides significantly impacts the heat treatment deformation of workpieces. After quenching, the workpiece expands parallel to the direction of the carbide strip and contracts in the direction perpendicular to the carbide strip. The coarser the carbide particles, the greater the expansion in the direction of the strip. For ledeburite steels such as Cr12-type steel and high-speed steel, the morphology and distribution of carbides have a particularly significant impact on quenching deformation.
In short, the more uniform the original structure of the workpiece, the smaller the heat treatment deformation, the more regular the deformation, and the easier it is to control.
4) The stress state of the workpiece itself before quenching has a significant impact on deformation
Especially for workpieces with complex shapes that have undergone high feed rate machining, if the residual stress is not eliminated, it significantly impacts quenching deformation.
5) The influence of workpiece geometry on heat treatment deformation
For workpieces with complex geometric shapes and asymmetric cross-sectional shapes, such as shafts with keyways, keyway broaches, tower-shaped workpieces, etc., one side dissipates heat quickly during quenching and cooling. In contrast, the other side dissipates heat slowly, resulting in uneven cooling. If the deformation caused by uneven cooling above Ms is dominant, the side with faster cooling is concave. If the deformation caused by uneven cooling below Ms is dominant, the side with faster cooling is convex. Increasing the isothermal time increases the bainite transformation, making residual austenite more stable, reducing the amount of martensite transformation in air cooling, and significantly reducing the deformation of the workpiece.
6) Effect of process parameters on heat treatment deformation
Whether it is conventional heat treatment or special heat treatment, heat treatment deformation may occur. When analyzing the influence of heat treatment process parameters on heat treatment deformation, the most important thing is to analyze the influence of heating and cooling processes. The main parameters of the heating process are the uniformity of heating, heating temperature, and heating speed. The main parameters of the cooling process are the cooling uniformity and cooling speed. The effect of uneven cooling on quenching deformation is the same as that caused by the asymmetric cross-sectional shape of the workpiece. This section mainly discusses the influence of other process parameters.

Solution for heat treatment deformation

• Reverse bending method: According to the heat treatment deformation law of shaft parts, stress can be pre-applied before quenching, that is, reverse bending in the bending direction of the parts to compensate for the bending deformation generated after quenching, which can reduce the straightening workload. Suitable for parts with significantly uneven cross-section and severe deformation.
• tatic quenching method: It is required that the temperature of the quenching coolant be uniform and in a static state just after being stirred before quenching. Clamp the parts with pliers and quench them into the coolant. This method can reduce the deformation much less than hanging and quenching with lead wire.
• Uniform and symmetrical part design: The cross-sectional shape of the part should be designed as evenly and symmetrically as possible, and if necessary, process grooves can be opened. For example, there are two symmetrical grooves on the boring bar, but only one is used, and the other is designed to reduce heat treatment deformation.
• Using a dedicated quenching fixture: If the section of the part is symmetrical, a dedicated fixture can be inserted after being discharged from the furnace, and then the cooling liquid can be quenched in a vertical direction. Due to the limitations of the fixture on part deformation, it is generally possible to control it within the reserved margin range.
• Heating using an embedded salt bath furnace: The plug-in furnace heats the parts quickly on one side and is prone to bending deformation. However, the embedded salt bath furnace has a more uniform temperature, is energy-saving, and can also use a flowing particle furnace.
• Vertical lifting and horizontal placement: For storage of long parts before and after quenching, attention should be paid to not bending the parts due to their weight, and it is best to use a rack for vertical lifting. During long-distance transportation, multiple plastic air cushion bags can be used, automatically balancing the parts and providing shock absorption.
• Stress relief before quenching: used for important parts prone to deformation, such as precision long wire rods. Annealing or normalizing before quenching to refine grains and homogenize the structure, reducing internal stress. Strictly control the heating temperature during quenching and heating.

A machining solution for heat treatment deformation

Taking carburizing and quenching gear shafts as an example, the machining allowance before and after carburizing is large according to the standard. Direct teeth grinding is not economical and inconvenient, and the hardness is high. What method is there for direct machining?

Using non-metallic adhesive cutting tools can complete a large amount of one-time machining of hardened workpieces. Ultra-hard cutting tools can cut off the hardened layer with a large amount of surplus, replacing a large amount of rough grinding processing. This can save the traditional processing method of annealing followed by secondary quenching, save processing costs and corresponding process costs, and greatly improve production efficiency.

Epower Metal’s Heat Treatment Solution (Example explanation)

Gear Heat Treatment Solution

Gear working conditions
Important parts used to transmit power, change direction or speed, the force situation is complex.
Common forms of gear failure

• Gear contact surface wear or plastic deformation of the tooth surface (surface hardness is insufficient).
• Gear face spalling (fatigue damage, pitting).
• Broken teeth (low strength or overload).

Technical requirements for gears
High hardness, contact fatigue, wear resistance of the tooth face.
Gear root and gear with high strength and toughness.
Steel for gears: low and medium carbon
Light duty gears: 45# steel, tempered or normalized
Medium load gears: 45# steel, 40Cr, tempered, surface hardened in wear parts
Heavy duty gears: 20Cr, 20CrMnTi, carburizing and quenching.
High precision gears: 38CrMoAlA, quenched and nitrided.
Gear heat treatment process
1) Carburized gear
Material: 20CrMnTi, 20Cr, 30CrMnTiA and so on.
Service occasions: high speed and heavy load (automobile gears)
Process Route.
Material → forging → normalizing → cutting → carburizing → quenching → low-temperature tempering → finish machining
Master the purpose of each heat treatment process, technology, organization.
(1) Normalizing
Refinement of grain; adjust the hardness, easy to cut and process heating to A_c3 + 30 – 50 °C, air cooling, the organization of fine pearlite + (a small amount of ferrite)
(2) Carburizing
Increase surface carbon content
920-930°C, keep warm for 3-9h.
(3) Quenching after carburizing
Obtain martensite, improve surface hardness
Direct quenching or one time quenching
Surface: high carbon M+A_r+carbide
Transition layer: M+A_r
Heart: low carbon M+F (small amount)
(4) Tempering
Eliminate quenching stress, low temperature 200°C, M→M_tempering

Bearing Heat Treatment Solution

Working conditions of bearings
High load, alternating stress, high rotational speed, certain impact.
Failure forms of bearings

• Contact fatigue failure;
• Plastic deformation;
• Wear and tear.

Performance requirements for bearings: (inner and outer rings and rolling elements)

• Must have high hardness and wear resistance;
• Must have high contact fatigue strength;
• Must have sufficient toughness and corrosion resistance;
• Dimensional stability is required.

Process route for manufacturing bearings with GCr15 bearing steel
Forging → normalizing → spheroidizing annealing → machining → quenching+cold treatment (-60 ~ -80 °C; 1h) → low-temperature tempering → grinding processing → stabilization treatment (120-150 °C; 5-10h)

Spring heat treatment solution

Working conditions of springs
Storing energy and reducing vibration, mainly subject to tension, pressure, torsion, alternating loads.
torsion and alternating loads.
Failure of springs
Fatigue fracture, permanent deformation.
Performance requirements of springs: high strength limits, elasticity limits, permanent deformation

• High strength limit, elastic limit, fatigue limit, molding and processing performance (plastic forming, permanent deformation).
• properties (plastic molding, heat treatment properties).

Common materials for springs
65, 65Mn, 60Si2Mn and other medium carbon steel and medium carbon alloy steel.
Spring heat treatment process
(A) cold forming springs (small springs) stress relief annealing
Strengthened by the steel wire (lead quenched and cold drawn, cold drawn, quenched + tempered steel wire) cold rolled into a spring, only need to be de-stressed annealed (heating temperature of 250 – 300°C), in order to eliminate deformation process or quenching in the formation of residual stresses, to stabilize the size.
Quenching + medium temperature tempering (or isothermal quenching)
(B) Hot forming springs
Made of hot rolled steel wire or steel plate (e.g. automobile leaf springs).
Quenched – to improve strength
Medium temperature tempering-eliminate stress and increase elastic limit.
Quenching temperature above A_c3, tempering temperature 350 – 450°C.
Organization:Tempered flexural or isothermal quenching is used to obtain bainitic organization.
(C) Precipitation hardening treatment
For 17-7PH (0Cr17Ni7Al) precipitation hardening steel, in the cold drawing or solution treatment of molded springs. The main processes are as follows:
(1) solid solution treatment: austenitization.
To obtain uniform and consistent austenitic organization
(2) molding process
(3) adjust the heat treatment (or quenching)
Heating to austenitizing slightly lower temperature, cooling, to obtain low carbon martensite + residual austenite organization
(4) deep cooling treatment: so that the quenched state of the residual austenite continue to transform into low carbon martensite (as required to determine)
(5) precipitation hardening treatment
In 480-550 °C insulation for about 1 hour, so that the carbide precipitation diffuse strengthening.
(D) Spring other strengthening treatment
(1) deformation heat treatment
For 60Si2Mn, 55Si2Mn and other medium carbon steel has a high deformation strengthening effect, so these springs are suitable for hot forming + quenching + tempering
(2) chemical heat treatment
For the ferrite state can be carried out under the elemental diffusion penetration of the spring material, can not change the overall strength, toughness and elasticity of the spring limit case, chemical penetration to improve the surface hardness and fatigue strength of the spring. Typical process hot forming + quenching + tempering – chemical seepage treatment.
(3) shot peening
Make the spring parts surface to obtain compressive stress, improve the service life of the spring 5-10 times.

Tool Heat Treatment Solution

Tools generally include cutting tools, molds, and gauges.
Commonly used steel including: composition characteristics: high carbon tool steel, low alloy tool steel, high alloy tool steel (high-speed steel, etc.)
Cutting tool heat treatment
Performance requirements

• High hardness (2HRC60): mainly depends on the carbon content.
• High wear resistance:. Rely on high hardness and precipitation of fine uniform hard carbide to achieve.
• Red hardness: that is, the ability to maintain high hardness at high temperatures enough toughness: to prevent brittle breakage and edge chipping.

Steel for Cutting Tools
Carbon tool steel: T7-T12 (0.7-1.2%C)
Carbon tool steel heat treatment and organization
Heat treatment: normalizing + ball annealing + quenching + low temperature tempering
Ball annealing purpose

• ① Reduce the hardness, easy to process;
• ② For quenching to prepare the organization.

Organization in service condition.
M_temper + granular carbide + A_R (small amount).

Heat treatment solution for shafts

➢Stresses – Bending and torsional stresses are unevenly distributed in the shaft cross-section, with maximum stresses at the surface and zero stresses at the center.
➢Performance – such parts do not need the entire cross-section quenched when tempering, as long as to ensure that the surface hardened layer for its radius of 1/4 can be.
➢ Selection of materials – mostly 40 # steel, 45 # steel, important shafts with alloy tempered steel manufacturing ( 40Cr, 35CrMo, 42Mn2V, etc. )
● Light duty spindle machining process
Material: 45# steel
Process
Preparation → Forging → Normalizing or annealing → Rough machining → Tempering → Finishing
Medium-duty spindle machining process
Material: 40Cr steel
Process: Overall tempering, surface quenching of journal and taper hole
Preparation→Forging→Normalizing or annealing→Rough machining→Tempering→Finishing→Surface quenching and low-temperature tempering→Grinding

Heat treatment of the “five differences” in detail

Difference between quenching heating temperature and quenching temperature

The field usually says the quenching heating temperature of 800 ℃, or in 800 ℃ quenching; have you considered what this means? The quenching heating temperature is the quenching temperature set to 800 ° C and heated at this temperature; it can also be said to be the heating temperature of 800 ° C. The latter so-called 800 ° C quenching, the quenching temperature of 800 ° C, that is, the temperature of the part in the quenching fluid is 800 ° C. For this reason, it is estimated that the temperature from the heating furnace and then getting the quenching tank for a small period has dropped and should be increased according to the part of the drop in the heating temperature.
In other words, the set quenching heating temperature should be 800°C + α. For example, if the temperature drops by 100°C, the quenching heating temperature is 800°C + 100°C = 900°C, and the temperature of the quenching solution is exactly 800°C, which can be interpreted as 800°C quenching. It can be seen that the quenching heating temperature of 800 ℃ and quenching temperature of 800 ℃, the meaning of the two is not quite the same.
Therefore, to prevent misunderstanding, please do not use the term quenching heating temperature; it is best to distinguish between the austenitization temperature (T_a) and quenching temperature (T_q). Austenitizing temperature is the maximum heating temperature for quenching, the quenching temperature is the temperature at which the quenching solution is put into the quenching solution, and the two must be separated. Recently, the Institute and other organizations have not used the term quenching heating temperature, but the austenitization temperature of the name of the Quarter is also a worldwide trend. However, the term quenching temperature still needs to be used more; it is important that T_a and T_q are used separately.
From a variety of experiments can be seen T_a and T_q between the temperature difference of about 100 ℃. Therefore, as long as it is heated at the austenitizing temperature, it is unnecessary to hustle it into oil or water after removing it from the furnace. Quenching with agility can be a trick! This method of operation is called delayed quenching.

Difference between heating time and holding time

In heat-treating operations, the heating temperature is strictly defined. Still, the holding time is very hastily defined, i.e., the holding time for quenching has been recognized for a long time as “30 minutes for a one-inch (2.54 mm) square”. This is what the catalogs and textbooks say. Therefore, it is common sense to hold large parts longer and small parts shorter. Is it a good idea to follow this statement?
Large parts are heated up for a long time, small parts are heated up for a short time, and the holding time after the heat treatment temperature (e.g., quenching temperature) is reached is no different between large and small parts and should be a certain amount. Naturally, this is the time after the temperature has been reached inside and outside the treated part. To check that the correct quenching temperature has been reached, it is usually based on the indication of a thermometer (temperature control gauge). However, it must be noted that this is the thermocouple end temperature, not the temperature of the part being treated. The larger the part, and the more the part is loaded, the greater the difference between the indicated temperature of the thermometer and the actual temperature of the part, i.e., the heating lag time.
That said, quenching is rapidly cooling and hardening an object that has been austenitized. When the steel phase changes to austenite, alloy structural steels (pearlitic systems) do so only instantaneously (the time required to change to austenite is zero). As soon as it changes to austenite, the first quenching stage is complete, so why bother holding it for 30 minutes in a one-inch square? It is appropriate to estimate that the heart of a juice-treated part has already been transformed into austenite before quenching. The holding time is preferably zero (the same as for high-frequency quenching).
In contrast, in tool steel (carbide system), the matrix phase into austenite, the austenite has about 50-70% of the initial precipitation carbide needs to be solidified. Quenching is not hardened, so there must be some holding time. However, even then, “30 minutes for a one-inch square” is too long; at best, about 10 minutes is enough. Of course, according to the type, shape, quantity, and distribution of carbides, the holding time should be more or less variable, in short:

• Heating time = f (the size of the treated part)
• Quenching holding time = f (steel)
• Which: alloy structural steel (pearlescent system) = 0
• Tool steel (carbide system) = approx. 10 minutes

Differences in cooling methods and cooling effects

In heat treatment, the method of cooling is important. The speed of cooling can soften or harden the steel. The cooling medium, called the coolant, includes various media such as air, oil, and water.
Generally, the cooling rate of air is slow, followed by oil, and the cooling rate is water. However, the cooling effect caused by the coolant is not absolute and can vary depending on the size of the part being processed. The cooling rate of the coolant is inherent, while the cooling effect of the coolant on the treated part varies depending on the size of the part. The cooling method and the cooling effect of heat treatment are two different things. The cooling method looks at the cooling from the viewpoint of the coolant, while the cooling effect looks at the cooling from the viewpoint of the treated part. Even if the cooling method is the same, the effect is different. In heat treatment, what is important is not how to cool but how to get a good cooling effect. The two should be distinct.
Normalizing is defined as cooling in the atmosphere (air cooling), but the cooling rate differs for small and large pieces, i.e., the cooling effect is different. Therefore, from the surface, cooling is the form of normalizing, but the substance is very different. Due to technical inappropriateness, small cooling pieces may become air quenching; on the contrary, large cooling and annealing may become. Therefore, the small pieces for normalizing must use the pit cold or covered cooling. Conversely, large pieces such as cooling without fans and other air cooling will not get the effect of normalizing; only the form of cooling will not get the sudden effect of heat treatment.
Quenching, and this is the same. Although the same oil cooling, large and small pieces of the cooling effect are different; therefore, the degree of quenching and hardening also change. Even if the small pieces with oil quenching with water quenching can be quenched through, the large pieces with oil quenching only like the degree of normalizing the cooling effect of the degree of quenching, quenching, not.
This consideration can be understood: although the water cooled quickly, the oil-cooled slowly, and the air cooled more slowly, this fast and slow is not absolute; it is related to the size of the treated parts. Therefore, it must be noted that the cooling method and cooling effect are different.
The Japanese industrial standard steel manual stipulates normalizing air cooling, annealing furnace cooling, quenching water cooling of ordinary steel, quenching oil cooling of high-quality steel, etc., are for the standard size (diameter 25mm). The cooling method must be changed appropriately according to the size of the treated parts. Identifying the difference between the cooling method and the cooling effect is important.
Differences in cooling and hardening energies of quenching liquids
There are two types of quench liquids: cooling energy and hardening energy. The so-called cooling energy is the ability of the quenchant itself. Cooling capacity is to make the steel from the austenitization temperature cool down when the cooling rate; hardening energy is the ability to make the steel hardened; in the cooling energy test, there is a cooling curvature method (silver ball, expansion ceremony), and cooling time method, etc., which are compared with the rapid cooling degree H value. Actual quenching by measuring the quenching hardness for hardening energy assessment is included in the cooling system cooling integrated capacity.
Usually, to judge the performance of the quenching fluid, cooling energy is a general standard and gets used, but cooling energy is big, and hardening energy is not necessarily big. That is to say, cold as fast, quenching hardness is not necessarily high; such instances are many. The technician needs to know the cooling speed of the quenching liquid and to what extent it can be hardened in this cooling tank. Even if the same steel with the same oil quenching, due to the quenching workshop tank hardening energy, is different, the quenching effect is also different.
The cooling energy of the quenching fluid is the quenching fluid itself has the characteristics of the hardening energy due to the hardenability of steel, the amount of quenching fluid, stirring speed, the size, shape, and surface condition of the treatment of different parts, it is best to deal with parts of the actual degree of hardening of the quenching site to determine the performance of the quenching tank, which is a direct determination of the simple method, but this method is often difficult to conduct a comparative study.
For steel quenching hardness, the external influencing factor is the cooling energy of the quenching tank, and the internal factor is the hardenability of the steel itself, especially by the s-curve constraints. Typically, quenching is done to prevent the precipitation of pearlite and bainite from austenitized steel and to make it fully martensitized. When determining the hardness of quenching by the cooling energy of the quenchant, depending on the steel grade, is it better to decide whether it is better to use pearlite (P) hardenability or bainite (B) hardenability? This is very important. General carbon steel with P-type, alloy steel with B type, and special tool steel with P + B type. Therefore, the cooling rate required to complete hardening, i.e., hardening, varies from steel to steel. There are cases where the cooling rate near the P-nose on the S-curve is required to be fast, cases where the cooling rate near the B-nose on the S-curve is required to be fast, or cases where both the P-nose and the B-nose are required to be fast. Therefore, in all of the above cases, the cooling rate in the 700 to 250°C temperature range is necessary as a common cooling rate. (In the Japanese industrial standard, an 800-400°C cooling rate is used for oil quenching).
To evaluate the quench tank in the cooling system at the quenching site, it is best to decide on the hardness of the quench after quenching under actual conditions.

Differences in quenching surface hardness and depth of penetration

In the case of quenching, there are two kinds of surface hardness and hardening depth. Surface hardness is the surface of the quenching hardness, equivalent to the hardness of small parts (diameter of 10mm or less) when quenching; it depends on the amount of broken and henge, almost unaffected by the alloying elements, which is the playful situation of structural steel. The quenching hardness of tool steel is affected by tungsten, vanadium, and other alloying elements, and its hardness is greater than HRC60, which is certain.
For structural steel, the surface hardness=f (C%) can be approximately quantitatively calculated using the following formula:

• Maximum quenching hardness (HRC)=30+50xC% (9O% martensite)
• Minimum quenched hardness (HRC)=24+40xC% (50% martensite)
• For example, structural steel (S45C) (with a maximum carbon content of 0.45%):
• Maximum Quenched Hardness (HRC)=53
• Minimum Quenched Hardness (HRC)=42

On the contrary, the quenching depth is constrained by the carbon content and special elements of the steel, as well as the austenite grain size, that is, the quenching depth=f (C%, special elements, grain size).
The depth of hardenability is academically known as hardenability, usually represented by D_f. Boron has the strongest effect on hardenability, followed by the gradual reduction of Mn, Mo, and Cr. The effects of these elements are not additive but rather a multiplicative relationship. That is (hardenability multiple of B) x (hardenability multiple of Mn) x (law permeability multiple of Mo) x.
Hardenability is a weapon to solve the mass effect. Steel with added boron is called boron steel, while steel with specified hardenability is called H steel. It is generally believed that steel for heat treatment should be selected based on hardenability (H) and price (P), and steel that is cheap and can meet the required hardenability is the appropriate material.