Summary of Basic Knowledge on Heat Treatment

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Heat treatment is a metal processing technique in which materials are heated, held at a certain temperature, and then cooled while still in the solid state, in order to achieve the desired structure and properties. I. Heat Treatment 1. Normalizing: A heat treatment process in which steel or steel components are heated to an appropriate temperature above the critical points AC3 or ACM, held at that temperature for a certain period of time, and then cooled in air, resulting in a pearlitic microstructure. 2. Annealing: A heat treatment process in which the hypoeutectoid steel workpiece is heated to 20–40 degrees above AC3, held at that temperature for a certain period of time, and then slowly cooled in the furnace (or by burying it in sand or lime) until its temperature drops below 500 degrees, after which it is cooled in air. 3. Solution heat treatment: The alloy is heated to the high-temperature single-phase region and held at that temperature to allow the excess phases to dissolve fully into the solid solution; thereafter, it is cooled rapidly to obtain a supersaturated solid solution. 4. Ageing: The phenomenon whereby the properties of an alloy change over time when it is left at room temperature or at a temperature slightly above room temperature after undergoing solution heat treatment or cold plastic deformation. 5. Solution treatment: This process ensures that all phases in the alloy are fully dissolved, strengthening the solid solution and improving its toughness and corrosion resistance, while also eliminating stresses and softening the material to enable further processing and shaping. 6. Aging treatment: Heating at the temperature at which the strengthening phase precipitates and maintaining that temperature allows the strengthening phase to settle out, thereby hardening the material and increasing its strength. 7. Quenching: A heat treatment process in which the steel is austenitized and then cooled at an appropriate rate, causing a transformation of the material’s structure into unstable forms such as martensite throughout or in a certain area of its cross-section. 8. Tempering: A heat treatment process in which a quenched workpiece is heated to an appropriate temperature below the critical point AC1 and held there for a certain period of time, and then cooled using a suitable method, so as to obtain the desired microstructure and properties. 9. Carbonitriding of steel: Carbonitriding is the process of simultaneously diffusing carbon and nitrogen into the surface layer of steel. *Carburitriding, also known as cyaniding, is widely used in medium-temperature gas carburitriding and low-temperature gas carburitriding (i.e., gas soft nitriding). The main purpose of medium-temperature gas carbonitriding is to improve the hardness, wear resistance, and fatigue strength of steel. Low-temperature gas carbonitriding is primarily nitridation, and its main purpose is to improve the wear resistance and anti-galling properties of steel. 10. Quenching and tempering: Generally, the heat treatment that combines quenching with high-temperature tempering is referred to as quenching and tempering. Quenching and tempering is widely used in various important structural components, especially those such as connecting rods, bolts, gears, and shafts that operate under alternating loads. After quenching and tempering, a tempered sorbite structure is obtained, whose mechanical properties are superior to those of the normalized sorbite structure with the same hardness. Its hardness depends on the high-temperature tempering temperature, as well as the tempering stability of the steel and the cross-sectional dimensions of the workpiece; it generally ranges from HB200 to HB350. 11. Brazing: A heat treatment process in which two workpieces are heated and melted together using a filler metal. II. Process Characteristics Metal heat treatment is one of the important processes in mechanical manufacturing. Compared with other processing methods, heat treatment generally does not change the shape of the workpiece or its overall chemical composition; instead, it enhances or improves the performance of the workpiece by altering its internal microstructure or the chemical composition of its surface. Its feature is improving the internal quality of the workpiece, which is generally not visible to the naked eye. To endow metal workpieces with the desired mechanical, physical, and chemical properties, in addition to the proper selection of materials and various forming processes, heat treatment is often essential. Steel is the most widely used material in the machinery industry. Its microstructure is complex, but it can be controlled through heat treatment; therefore, heat treatment of steel constitutes the main aspect of metal heat treatment. Furthermore, metals such as aluminum, copper, magnesium, titanium, and their alloys can also have their mechanical, physical, and chemical properties altered through heat treatment to achieve different performance characteristics. III. Process Procedure The heat treatment process generally includes three steps: heating, holding at a constant temperature, and cooling; sometimes it only involves two steps: heating and cooling. These processes are interconnected and must continue without interruption. Heating is one of the important processes in heat treatment. There are many heating methods for metal heat treatment; initially, charcoal and coal were used as heat sources, and later liquid and gas fuels were employed. The use of electricity makes heating easy to control and causes no environmental pollution. These heat sources can be used for direct heating, or for indirect heating via molten salts or metals, as well as floating particles. When metals are heated and the workpiece is exposed to air, oxidation and decarburization often occur (that is, the carbon content on the surface of steel parts decreases), which has a very negative impact on the surface properties of the parts after heat treatment. Therefore, metals should generally be heated in a controlled or protective atmosphere, in molten salts, or in a vacuum; they can also be protected during heating using coatings or packaging methods. Heating temperature is one of the important process parameters in heat treatment processes. Selecting and controlling the heating temperature is a key issue in ensuring the quality of heat treatment. The heating temperature varies depending on the metal material being treated and the purpose of heat treatment, but it is generally raised above the phase transition temperature to obtain a high-temperature microstructure. Furthermore, the transformation requires some time; therefore, once the surface of the metal workpiece reaches the desired heating temperature, it must be held at that temperature for a certain period to ensure that the temperatures inside and outside are equal and that the microstructure undergoes complete transformation. This period is known as the holding time. When using high-energy-density heating and surface heat treatment, the heating rate is extremely fast; generally, there is no holding time. In contrast, the holding time for chemical heat treatment is usually quite long. Cooling is also an indispensable step in the heat treatment process. The cooling methods vary depending on the process, with the main objective being to control the cooling rate. Generally, the cooling rate for annealing is the slowest, that for normalizing is faster, and that for quenching is the fastest. However, there are also different requirements depending on the steel grade; for example, air-hardening steel can be hardened using a cooling rate similar to that used in normalizing. IV. Process Classification Metal heat treatment processes can be broadly divided into three categories: overall heat treatment, surface heat treatment, and chemical heat treatment. Depending on the heating medium, heating temperature, and cooling method, each major category can be further divided into several different heat treatment processes. The same metal can exhibit different microstructures, and thus different properties, when subjected to various heat treatment processes. Steel is the most widely used metal in industry, and its microstructure is also the most complex; as a result, there are numerous types of heat treatment processes for steel. Overall heat treatment is a metal heat treatment process in which the entire workpiece is heated and then cooled at an appropriate rate to obtain the desired metallographic structure, thereby altering its overall mechanical properties. The overall heat treatment of steel mainly includes four basic processes: annealing, normalizing, quenching, and tempering. Processing methods: Annealing involves heating the workpiece to an appropriate temperature; depending on the material and size of the workpiece, a certain holding time is applied, followed by slow cooling. The purpose is to bring the internal structure of the metal to an equilibrium state, thereby achieving good mechanical and functional properties, or to prepare the structure for further quenching. Normalizing involves heating the workpiece to an appropriate temperature and then cooling it in air. The effect of normalizing is similar to that of annealing, except that the resulting microstructure is finer. It is often used to improve the machinability of materials, and sometimes as the final heat treatment for parts with less stringent requirements. Quenching involves heating a workpiece and holding it at that temperature before rapidly cooling it in a quenching medium such as water, oil, or other inorganic salts and organic aqueous solutions. After quenching, the steel parts become harder, but they also become more brittle; to eliminate this brittleness promptly, tempering is usually required. To reduce the brittleness of steel parts, the quenched steel parts are held at an appropriate temperature above room temperature but below 650°C for an extended period of time before being cooled; this process is known as tempering. Annealing, normalizing, quenching, and tempering are the “four key processes” in overall heat treatment. Among them, quenching and tempering are closely related and are often used together; neither can be omitted. “The “four fires,” depending on the heating temperature and cooling methods, give rise to various heat treatment processes. The process that combines quenching and high-temperature tempering to achieve certain strength and toughness is called quenching and tempering. After certain alloys are quenched to form a supersaturated solid solution, they are held at room temperature or an appropriately higher temperature for an extended period of time in order to enhance the alloy’s hardness, strength, or electrical and magnetic properties. Such a heat treatment process is called aging treatment. The method of effectively and closely combining pressure processing deformation with heat treatment to enable the workpiece to achieve an excellent balance between strength and toughness is known as deformation heat treatment ； Heat treatment carried out in a negative-pressure atmosphere or vacuum is known as vacuum heat treatment. It not only prevents oxidation and decarburization of workpieces, maintains a smooth surface on the treated parts, and improves their properties, but also allows for the introduction of carburizing agents to perform chemical heat treatment. Surface heat treatment is a metal heat treatment process that involves heating only the surface layer of a workpiece in order to modify its mechanical properties at that surface layer. In order to heat only the surface layer of the workpiece without allowing excessive heat to penetrate into its interior, the heat source used must have a high energy density, that is, it must deliver a large amount of thermal energy per unit area of the workpiece, enabling the surface layer or specific areas to reach high temperatures in a short period of time or instantaneously. The main methods of surface heat treatment include flame quenching and induction heating treatment, with common heat sources including oxygen-acetylene or oxygen-propane flames, induced currents, lasers, and electron beams. Chemical heat treatment is a metal heat treatment process that alters the chemical composition, structure, and properties of the surface layer of a workpiece. The difference between chemical heat treatment and surface heat treatment is that the former alters the chemical composition of the workpiece’s surface layer. Chemical heat treatment involves heating a workpiece in a medium containing carbon, salts, or other alloying elements (gas, liquid, solid), and maintaining this temperature for an extended period of time, thereby allowing elements such as carbon, nitrogen, boron, and chromium to penetrate into the surface layer of the workpiece. After the infiltration of elements, other heat treatment processes such as quenching and tempering may also be required. The main methods of chemical heat treatment include carburizing, nitriding, and metal infiltration. Heat treatment is one of the important processes in the manufacturing of mechanical parts and molds. Generally speaking, it can ensure and improve various properties of the workpiece, such as wear resistance and corrosion resistance. It can also improve the microstructure and stress state of the blank, facilitating various cold and hot processing operations. For example: white cast iron can be transformed into malleable cast iron through prolonged annealing, thereby increasing its plasticity ； Gears that undergo the proper heat treatment process can have a service life that is several times or even dozens of times longer than those that are not heat-treated ； Furthermore, inexpensive carbon steel can acquire certain properties of more expensive alloy steels by having certain alloying elements added to it, and can thus replace certain heat-resistant steels and stainless steels ； Molds and dies almost all require heat treatment before they can be used. Supplementary methods I. Types of annealing Annealing is a heat treatment process in which the workpiece is heated to an appropriate temperature, held there for a certain period of time, and then slowly cooled. There are many types of annealing processes for steel. Based on the heating temperature, they can be divided into two main categories: one is annealing at temperatures above the critical temperature (Ac1 or Ac3), also known as phase transformation recrystallization annealing, which includes full annealing, partial annealing, spheroidizing annealing, and diffusion annealing (homogenization annealing) ； Another category is annealing below the critical temperature, including recrystallization annealing and stress-relief annealing, etc. Based on the cooling method, annealing can be divided into isothermal annealing and continuous cooling annealing. 1. Complete annealing and isothermal annealing: Complete annealing, also known as recrystallization annealing and commonly referred to simply as annealing, is a heat treatment process in which the steel part or bar is heated to 20–30°C above Ac3, held at that temperature for a sufficient length of time to ensure complete austenitization of the structure, and then cooled slowly to obtain a structure that is close to equilibrium. This type of annealing is mainly used for cast, forged pieces, and hot-rolled profiles of various carbon steels and alloy steels with hypoeutectoid compositions; it is sometimes also used for welded structures. It is generally used as the final heat treatment for parts that are not subject to heavy processing, or as a preliminary heat treatment for certain parts. 2. Spheroidizing annealing: Spheroidizing annealing is mainly used for hypereutectoid carbon steels and alloy tool steels (such as the steel grades used in the manufacture of cutting tools, measuring instruments, and molds). Its main purpose is to reduce hardness, improve machinability, and prepare for subsequent quenching. 3. Stress-relief annealing: Stress-relief annealing is also known as low-temperature annealing (or high-temperature tempering). This type of annealing is used primarily to eliminate residual stresses in castings, forgings, welded parts, hot-rolled products, and cold-drawn parts. If these stresses are not eliminated, they will cause the steel component to deform or develop cracks after a certain period of time, or during subsequent machining processes. 4. Partial annealing is a heat treatment process in which steel is heated to a temperature between Ac1~Ac3 (for hypoeutectoid steel) or Ac1~ACcm (for hyper-eutectoid steel), and then cooled slowly after holding at that temperature to obtain a structure that is close to equilibrium. II. During quenching, the most commonly used cooling media are brine, water, and oil. Workpieces quenched in salt water can easily achieve high hardness and a smooth surface, and are less prone to the formation of soft spots that do not harden. However, they are prone to severe deformation, and even cracking. Using oil as a quenching medium is only suitable for quenching certain alloy steels with high stability of supercooled austenite, or small-sized carbon steel workpieces. III. Purposes of steel tempering 1. To reduce brittleness and eliminate or minimize internal stresses. After quenching, steel parts have high levels of internal stress and brittleness; if not tempered in a timely manner, these parts may deform or even crack. 2. To achieve the mechanical properties required by the workpiece, quenching results in a high hardness but also high brittleness of the workpiece. In order to meet the different performance requirements of various workpieces, the hardness can be adjusted and brittleness reduced through appropriate tempering, thereby obtaining the desired toughness and ductility. 3. Stabilize the dimensions of the workpiece. 4. For certain alloy steels that are difficult to soften through annealing, high-temperature tempering is often used after quenching (or normalizing) to cause the carbides in the steel to aggregate appropriately, thereby reducing its hardness and facilitating machining. Additional concept 1: Annealing refers to a heat treatment process in which a metal material is heated to an appropriate temperature, held there for a certain period of time, and then cooled slowly. Common annealing processes include: recrystallization annealing, stress-relief annealing, spheroidizing annealing, full annealing, etc. The purpose of annealing is mainly to reduce the hardness of metal materials and increase their plasticity, thereby facilitating machining or forming; to reduce residual stresses; to improve the homogeneity of the structure and composition; or to prepare the structure for subsequent heat treatment. 2. Normalizing: refers to a heat treatment process in which steel or steel components are heated to or above the upper critical temperature of the steel, held at that temperature for 30–50°C for an appropriate period of time, and then cooled in still air. The purpose of normalizing is mainly to improve the mechanical properties of low-carbon steel, enhance its machinability, refine the grain structure, eliminate structural defects, and prepare the material structure for subsequent heat treatment. 3. Quenching: It refers to a heat treatment process in which the steel part is heated to a temperature above Ac3 or Ac1 (the lower critical temperature of the steel), held at that temperature for a certain period of time, and then cooled at an appropriate rate to obtain a martensite (or bainite) structure. Common quenching processes include single-medium quenching, double-medium quenching, staged martensite quenching, bainite isothermal quenching, surface quenching, and local quenching. The purpose of quenching is to enable the steel part to acquire the desired martensitic structure, increase the hardness, strength, and wear resistance of the workpiece, and prepare the structure for subsequent heat treatment. 4. Tempering: Refers to the heat treatment process in which, after a steel part has been quenched, it is heated to a temperature below Ac1, held at that temperature for a certain period of time, and then cooled back to room temperature. Common tempering processes include: low-temperature tempering, medium-temperature tempering, high-temperature tempering, and multiple temperings. Purpose of tempering: It is mainly to eliminate the stresses generated in steel components during quenching. This ensures that the steel components possess high hardness and wear resistance, as well as the required plasticity and toughness. 5. Quenching and tempering: refers to the combined heat treatment process of quenching and high-temperature tempering of steel materials or steel parts. Steel used for quenching and tempering is called quenched and tempered steel. It generally refers to medium-carbon structural steel and medium-carbon alloy structural steel. 6. Carburizing: Carburizing refers to the process of allowing carbon atoms to penetrate into the surface layer of steel. It also enables the workpiece made of low-carbon steel to have a surface layer similar to that of high-carbon steel; through quenching and low-temperature tempering, this surface layer acquires high hardness and wear resistance, while the core part of the workpiece retains the toughness and plasticity of low-carbon steel. The vacuum method requires a dozen or even dozens of operations to carry out tasks such as heating and cooling metal workpieces. These operations take place inside the vacuum heat treatment furnace, where operators cannot access it; therefore, a high degree of automation is required for such vacuum heat treatment furnaces. At the same time, for some operations, such as the quenching process of metal workpieces after heating and holding, six or seven actions are required to be completed within 15 seconds. Such agile conditions for performing many actions can easily cause stress among operators, leading to mistakes. Therefore, only a high level of automation can ensure accurate and timely coordination according to procedures. Vacuum heat treatment of metal parts is carried out in a sealed vacuum furnace, and the importance of strict vacuum sealing is well known. Therefore, achieving and maintaining the furnace’s predetermined leak rate to ensure the operating vacuum level of the vacuum furnace is of great significance for guaranteeing the quality of the vacuum heat treatment of parts. Therefore, 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 fundamental principle must be followed in the structural design of such furnaces: the furnace body should be welded hermetically, with as few openings as possible, or none at all; moreover, dynamic sealing mechanisms should be used as little as possible or avoided altogether, in order to minimize the chances of vacuum leakage. Components and accessories installed on the vacuum furnace, such as water-cooled electrodes and thermocouple outlet devices, must also be designed with a sealed structure. Most heating and insulation materials can only be used in a vacuum environment. The heating and insulation linings of vacuum heat treatment furnaces operate under vacuum and high temperatures; therefore, these materials are required to have high temperature resistance, good radiation resistance, and a low thermal conductivity. The requirements for antioxidant properties are not high. Therefore, vacuum heat treatment furnaces widely use materials such as tantalum, tungsten, molybdenum, and graphite for heating and insulation purposes. These materials oxidize very easily under atmospheric conditions; therefore, ordinary heat treatment furnaces cannot use these heating and insulation materials. Water cooling system: Components such as the furnace shell, furnace lid, electric heating elements, water-cooled electrodes, and intermediate vacuum insulation doors of the vacuum heat treatment furnace all operate under vacuum and at elevated temperatures. When working under such extremely unfavorable conditions, it is necessary to ensure that the structure of all components does not deform or get damaged, and that the vacuum seals do not overheat or burn out. Therefore, water cooling systems should be installed in various components depending on the specific conditions, to ensure that the vacuum heat treatment furnace can operate properly and have a sufficient service life. Low voltage and high current are used: Within a vacuum chamber, when the degree of vacuum ranges from a few torr to 10^-4 torr, a glowing discharge occurs in the conductive elements present in the chamber at higher voltages. In a vacuum heat treatment furnace, severe arc discharge can burn out the electric heating elements and insulation layers, leading to serious accidents and losses. Therefore, the operating voltage of the electric heating elements in vacuum heat treatment furnaces generally does not exceed 80 to 100 volts. At the same time, effective measures should be taken in the structural design of the electric heating elements, such as avoiding components with sharp edges and ensuring that the distance between electrodes is not too small, in order to prevent the occurrence of glow discharge or arc discharge. Tempering: Depending on the required properties of the workpiece and the corresponding tempering temperature, tempering can be divided into the following types: (1) Low-temperature tempering (150–250 degrees). The microstructure obtained through low-temperature tempering is tempered martensite. Its purpose is to reduce the internal stresses and brittleness of quenched steel, while maintaining its high hardness and wear resistance, thereby preventing it from cracking or suffering premature damage during use. It is mainly used in various high-carbon cutting tools, measuring instruments, cold stamping dies, rolling bearings, and carburized parts, etc. The hardness after tempering is generally HRC 58–64. (II) Medium-temperature tempering (250–500 degrees): The microstructure obtained through medium-temperature tempering is tempered martensite. Its purpose is to achieve high yield strength, elastic limit, and high toughness. Therefore, it is mainly used for treating various springs and hot-working dies, with a hardness of generally HRC35–50 after tempering. (III) High-temperature tempering (500–650 degrees): The microstructure obtained through high-temperature tempering is tempered sorbite. *The heat treatment that typically combines quenching with high-temperature tempering is called quenching and tempering. Its purpose is to achieve good overall mechanical properties, including strength, hardness, plasticity, and toughness. Therefore, they are widely used as important structural components in automobiles, tractors, machine tools, etc., such as connecting rods, bolts, gears, and shafts. The hardness after tempering is generally HB200–330. Deformation prevention: The causes of deformation in precision and complex molds are often complex. However, by understanding the patterns of deformation, analyzing its underlying reasons, and employing various methods to prevent such deformation, it is possible to reduce and control it. Generally, the thermal treatment deformation of precision and complex molds can be prevented by the following methods. (1) Select materials reasonably. For precision and complex molds, micro-deformation die steels with good properties (such as air-quenched steel) should be selected. For die steels with severe carbide segregation, proper forging followed by quenching and tempering treatment is necessary; for larger molds or those that cannot be forged, solution treatment combined with grain refinement can be employed. (2) The mold structure design should be reasonable; the thickness differences should not be too large, and the shape should be symmetrical. For molds that undergo significant deformation, it is necessary to understand the patterns of such deformation and leave appropriate machining allowances. For large, precision-based, and complex molds, a modular structure can be employed. (3) Precision and complex molds require pre-heat treatment to eliminate residual stresses generated during machining. (4) Select the heating temperature appropriately and control the heating rate; for precision and complex molds, slow heating, preheating, and other methods of uniform heating can be employed to reduce deformation during heat treatment. (5) On the premise of ensuring the hardness of the mold, it is advisable to adopt pre-cooling, step cooling quenching, or warm quenching processes as much as possible. (6) For precision and complex molds, vacuum heating quenching and cryogenic treatment after quenching should be employed as much as possible when conditions permit. (7) For some precision and complex molds, pre-heat treatment, aging heat treatment, and quenching-and-nitriding heat treatment can be employed to control the accuracy of the molds. (8) When repairing defects such as sand holes, pores, and wear on molds, select repair equipment with minimal heat influence, such as cold welding machines, to prevent deformation during the repair process. Furthermore, proper heat treatment procedures (such as hole plugging, hole sealing, mechanical fixation, appropriate heating methods, correct selection of the cooling direction for the mold, and its movement direction within the cooling medium) as well as reasonable tempering processes are also effective measures to reduce deformation in precision and complex molds. Surface quenching and tempering heat treatment is usually carried out using induction heating or flame heating. The main technical parameters are surface hardness, local hardness, and effective hardened layer depth. Hardness testing can be carried out using a Vickers hardness tester, as well as a Rockwell or surface Rockwell hardness tester. The selection of the testing force (scale) is related to the depth of the effective hardened layer and the surface hardness of the workpiece. Three types of hardness testers are involved here. I. The Vickers hardness tester is an important tool for measuring the surface hardness of heat-treated workpieces. It allows the use of testing forces ranging from 0.5 to 100 kg, and it can measure surface hardening layers as thin as 0.05 mm. It offers the highest precision, enabling the detection of even minor differences in the surface hardness of heat-treated workpieces. Furthermore, the depth of the effectively hardened layer also needs to be measured using a Vickers hardness tester; therefore, it is necessary for organizations that carry out surface heat treatment or use a large number of parts that have undergone such treatment to have a Vickers hardness tester on hand. II. The surface Rockwell hardness tester is also very suitable for testing the hardness of surface-hardened workpieces, and it offers three scales to choose from. It is possible to test various surface-hardened workpieces with an effective hardening depth of over 0.1 mm. Although the accuracy of surface Rockwell hardness testers is not as high as that of Vickers hardness testers, they are sufficient as testing tools for quality management and acceptance inspection in heat treatment plants. Moreover, it features simple operation, ease of use, low cost, fast measurement, and the ability to directly read the hardness value. The surface Rockwell hardness tester can be used to conduct rapid, non-destructive individual inspections of batches of surface-treated workpieces. This is of great significance for metal processing and machinery manufacturing plants. III. When the hardened layer resulting from surface heat treatment is thick, a Rockwell hardness tester can also be used. When the thickness of the heat-treated hardened layer is between 0.4 and 0.8 mm, the HRA scale can be used; when the hardened layer thickness exceeds 0.8 mm, the HRC scale can be employed. The three hardness values of Vickers, Rockwell, and surface Rockwell can be easily converted into one another, to match the standard, drawing, or hardness value required by the user. The corresponding conversion tables are provided in the international standard ISO, the American standard ASTM, and the Chinese standard GB/T. Local quenching: For parts requiring high hardness in specific areas, local quenching heat treatment can be performed using methods such as induction heating. For such parts, the location of the local quenching heat treatment and the desired local hardness value must typically be indicated on the drawings. The hardness testing of parts must be carried out within the designated area. Hardness testing instruments can be Rockwell hardness testers, which are used to measure HRC hardness values; if the hardened layer resulting from heat treatment is relatively thin, a surface Rockwell hardness tester can be used to measure HRN hardness values. Chemical heat treatment involves infiltrating the surface of a workpiece with atoms of one or several chemical elements, thereby altering the chemical composition, structure, and properties of that surface. After quenching and low-temperature tempering, the surface of the workpiece exhibits high hardness, wear resistance, and contact fatigue strength, while the core of the workpiece possesses high strength and toughness. Based on what has been said above, it is very important to monitor and record the temperature during the heat treatment process; poor temperature control can have a significant impact on the products. Therefore, temperature monitoring is very important, and the trend of temperature changes throughout the process is also crucial. As a result, it is necessary to record these temperature changes during heat treatment, which facilitates data analysis later on and helps identify the periods during which the temperature did not meet the required levels. This plays a very important role in improving future heat treatments. Operating Procedures: 1. Clean the operation area, and check whether the power supply, measuring instruments, and various switches are functioning properly, as well as whether the water supply is unobstructed. 2. Operators must wear appropriate personal protective equipment; otherwise, there is a risk of danger. 3. Turn on the universal power control switch, and raise or lower the temperature in stages in accordance with the technical requirements of the equipment, thereby extending its lifespan and ensuring its good condition. 4. Pay attention to the temperature of the heat treatment furnace and the speed control of the conveyor belt; be able to determine the temperature requirements for different materials, ensure the hardness of the workpieces as well as their surface flatness and oxide layer quality, and take proper safety measures. 5. Pay attention to the temperature of the tempering furnace and the speed control of the conveyor belt; turn on the exhaust system to ensure that the workpieces meet the quality requirements after tempering. 6. One should stick to one’s post at work. 7. Necessary fire-fighting equipment must be provided, and one should be familiar with its usage and maintenance methods. 8. When shutting down the machine, ensure that all control switches are in the off position before turning off the universal switch. Overheating: The overheated microstructure after quenching can be observed on the rough surfaces of bearing components in idler accessories. However, to accurately determine the degree of overheating, the microstructure must be observed. If coarse acicular martensite appears in the quenched microstructure of GCr15 steel, it is a over-quenched microstructure. The cause may be overall overheating resulting from too high quenching heating temperature or too long heating and holding time ； It may also be due to severe band-like carbides in the original tissue, which leads to the formation of coarse local martensite needles in the low-carbon regions between these bands, thereby causing local overheating. The amount of retained austenite in the overheated tissue increases, resulting in a decrease in dimensional stability. Due to overheating of the quenched structure, the crystals in the steel become large, which leads to a decrease in the toughness of the parts, reduced impact resistance, and a shorter lifespan for the bearings. Severe overheating can even cause quenching cracks. Under-heating: If the quenching temperature is too low or cooling is inadequate, a troostite structure that exceeds the specified limits will form in the microstructure; this is known as under-heated structure. It reduces hardness and significantly lowers wear resistance, thereby affecting the service life of bearings used in idler roller components. Quenching cracks: Cracks that form in roller bearing components during the quenching and cooling process due to internal stresses are known as quenching cracks. The causes of such cracks include: excessive quenching heating temperature or too rapid cooling, whereby the thermal stress and the structural stress resulting from changes in the volume and density of the metal exceed the steel’s fracture resistance ； The existing defects on the working surface (such as surface micro-cracks or scratches), or the internal defects in the steel (such as inclusions, severe non-metallic inclusions, white spots, residual shrinkage cavities, etc.), cause stress concentration during quenching ； Severe surface decarburization and carbide segregation ； Insufficient or delayed tempering of parts after quenching ； Excessive cold stamping stress caused by previous processes, forging folds, deep turning marks, sharp edges in oil grooves, etc. In summary, the causes of quenching cracks may be one or more of the factors mentioned above, with the presence of internal stress being the main reason for their formation. The quenching cracks are deep and elongated; the fracture surface is straight, with no oxidation color on the broken area. On the bearing rings, it is usually a longitudinal straight crack or an annular crack ； The shapes on bearing steel balls are S-shaped, T-shaped, or ring-shaped. The microstructural feature of quenching cracks is the absence of decarburization on both sides of the crack, which clearly distinguishes them from forging cracks and material cracks. Thermal treatment deformation: During heat treatment, NACHI bearing parts are subject to thermal stresses and structural stresses. These internal stresses can either add up to each other or partially cancel each other out; the situation is complex and variable, as these stresses can change depending on factors such as the heating temperature, heating rate, cooling method, cooling rate, as well as the shape and size of the parts. Therefore, thermal treatment deformation is inevitable. Understanding and mastering its variation patterns allows the deformation of bearing components (such as the elliptical shape of the rings or increases in size) to be kept within controllable limits, which facilitates production processes. Of course, mechanical impacts during the heat treatment process can also cause deformation of the parts, but this deformation can be reduced or avoided by improving the processing methods. Surface decarburization occurs when bearing components for idler rollers are heated in an oxidizing medium during heat treatment; oxidation takes place on the surface, resulting in a decrease in the carbon content in that surface layer and thus surface decarburization. If the depth of the surface decarburized layer exceeds the remaining amount after the final machining, the part will become unusable. The determination of the depth of the surface decarburized layer in metallographic testing can be carried out using metallographic methods and microhardness testing. The measurement method based on the microhardness distribution curve of the surface layer can be used as an adjudicative criterion. Soft spots refer to the condition in which the surface of roller bearing components has insufficient local hardness, caused by factors such as inadequate heating, poor cooling, or improper quenching procedures. It can cause a severe decline in surface wear resistance and fatigue strength, similar to surface decarburization.