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Deformation of molds due to heat treatment and its prevention are among the main defects in the mold processing process. For some precision and complex molds, they often become unusable as a result of heat treatment-induced deformation; therefore, controlling the deformation of such molds has always been a key issue in heat treatment manufacturing. As is well known, during heat treatment of molds, especially during the quenching process, temperature differences arise due to the uneven heating and cooling rates in different parts of the mold’s cross-section. Coupled with the asynchronous occurrence of structural transformations, this leads to uneven volumetric expansion and contraction in various parts of the mold’s cross-section as well as uneven structural transformations, thereby generating \"structural stress\" and thermal stress caused by the temperature differences between the inside and outside of the mold. When its internal stress exceeds the yield limit of the mold, it causes the mold to deform. Therefore, reducing and controlling the deformation of precision and complex molds is an important research topic for many heat treatment professionals. This paper attempts to explore measures for reducing and controlling the deformation of precision and complex molds, through research on the deformation conditions and causes thereof, in order to improve the quality and service life of mold products. I. Influence of mold materials 1. Selection of mold material: Considering the simplicity of material selection and heat treatment, a machinery factory chose T10A steel to manufacture complex molds with significantly varying cross-sectional dimensions, where minimal deformation after quenching was required; the desired hardness level was 56HRC–60HRC. After heat treatment, the hardness of the mold met the technical requirements, but the mold suffered significant deformation and became unusable, resulting in its scrapping. Later, the factory began using micro-deformed steel, Cr12 steel, for manufacturing; after heat treatment, the molds met the required hardness and deformation levels. Therefore, to manufacture precision and complex molds that require minimal deformation, micro-deformed steels such as air-quenched steel should be preferred as much as possible. 2. Influence of die material: A factory sent in a batch of relatively complex dies made of Cr12MoV steel; these dies all had ø60 mm circular holes. After heat treatment, the circular holes in some of the dies became elliptical, resulting in the failure of those dies. Generally, Cr12MoV steel is a slightly deformable steel and should not experience significant deformation. We conducted a metallographic analysis on the severely deformed molds and found that the mold steel contained a large amount of eutectic carbides, which were distributed in band-like and massive forms. (1) Causes of mold ellipticity (deformation): This is due to the presence of unevenly distributed carbides in the mold steel, which are oriented in certain directions. The expansion coefficient of these carbides is about 30% lower than that of the steel’s matrix structure; during heating, they prevent the inner cavity of the mold from expanding, and during cooling, they prevent it from contracting. As a result, uneven deformation occurs in the inner cavity of the mold, causing the circular hole to become elliptical. (2) Preventive measures: ① When manufacturing precision and complex molds, it is necessary to choose mold steels with minimal carbide segregation; do not opt for cheaper steel materials produced by smaller steel mills. ②Mold steels with severe carbide segregation require proper forging to break up the carbide clusters, reduce the degree of uneven carbide distribution, and eliminate anisotropy in their properties. ③The forged die steel must undergo quenching and tempering treatment to achieve a sorbite structure with a uniform, fine, and dispersed distribution of carbides, thereby reducing deformation in precision and complex dies after heat treatment. ④For molds that are large in size or cannot be forged, solution treatment combined with fine-grinding can be employed to refine the carbides, ensure a uniform distribution of these carbides, and soften the edges and corners, thereby reducing thermal treatment-induced deformation in the mold. II. Impact of mold structure design: Even when the materials chosen for the mold and the quality of the steel are excellent, improper mold structure design—such as thin edges, sharp corners, grooves, abrupt changes in thickness, and large differences between thickness levels—can lead to significant deformation of the mold after heat treatment. 1. Reasons for deformation: The mold has uneven thickness in different areas or features sharp corners, which leads to differences in thermal stress and structural stress among various parts of the mold during quenching. These differences result in varying rates of volume expansion in different parts, causing the mold to deform after quenching. 2. Preventive measures: When designing molds, it is necessary to minimize significant differences in thickness and structural asymmetry, as much as possible, while meeting the actual production requirements; at the junctions where the thickness of the mold changes, smooth transitions should be used in the design. Based on the deformation pattern of the mold, a machining allowance is reserved to prevent the mold from becoming unusable due to deformation after quenching. For molds with particularly complex shapes, a combined structure can be used to ensure uniform cooling during quenching. III. Molding manufacturing processes and the impact of residual stress: Factories often find that some molds with complex shapes and high precision requirements experience significant deformation after heat treatment. Upon thorough investigation, it is discovered that no pre-heat treatment was carried out on these molds during their mechanical processing or final heat treatment. 1. Causes of deformation: The accumulation of residual stresses from the machining process and the stresses resulting from quenching increases the deformation of the mold after heat treatment. 2. Preventive measures: (1) A stress-relief annealing should be carried out after rough machining and before semi-finishing, that is, at (630–680)°C for (3–4) hours; after that, the material is cooled in the furnace until its temperature drops below 500°C before being taken out for air cooling. A stress-relief treatment at 400°C for (2–3) hours can also be used. (2) Reduce the quenching temperature to decrease the residual stress after quenching. (3) Quenching in oil at 170ºC followed by air cooling (step quenching). (4) The use of isothermal quenching can reduce quenching residual stress. The above measures can reduce the residual stress in the mold after quenching, resulting in less deformation of the mold. IV. Effects of the heating process in heat treatment 1. Effect of heating rate It is generally believed that deformation after heat treatment is caused by cooling, but this is incorrect. For molds, especially complex ones, the correctness of the processing techniques has a significant impact on the deformation of the molds. A comparison of various heating techniques for such molds shows that a faster heating rate tends to result in greater deformation. (1) Reasons for deformation: Any metal expands when heated. In the case of steel, when heated, the temperature varies across different parts within the same mold; this uneven heating inevitably leads to inconsistent expansion of those parts, thereby generating internal stresses due to the uneven heating. At temperatures below the phase transformation point of steel, uneven heating primarily generates thermal stress; whereas uneven heating above the phase transformation temperature also leads to heterogeneity in the structural transformation, thereby producing structural stress as well. Therefore, the faster the heating rate, the greater the temperature difference between the surface and the core of the mold, the higher the stress, and the greater the deformation that occurs after heat treatment of the mold. (2) Preventive measures  When heating complex molds below the phase transition temperature, it should be done gradually. Generally speaking, deformation caused by vacuum heat treatment of molds is much less than that resulting from quenching in a salt bath furnace. ‚Preheating is employed; for low-alloy steel molds, a single preheating step at 550°C–620°C can be used ； For highly alloyed steel molds, secondary preheating (550°C–620°C and 800°C–850°C) should be used. 2. Influence of heating temperature: To ensure that the mold achieves a high hardness, manufacturers believe it is necessary to increase the quenching heating temperature. However, production experience shows that this approach is inappropriate; for complex molds, when heat treatment is carried out at normal heating temperatures, the deformation resulting from heating to the upper allowable temperature is much greater than that resulting from heating to the lower allowable temperature. (1) Reasons for deformation: As is well known, the higher the quenching heating temperature, the larger the grain size of the steel becomes. Since larger grains increase the hardenability, the stress generated during quenching and cooling also increases. Furthermore, since complex molds are mostly made of medium- to high-alloy steels, if the quenching temperature is high, the low Ms point leads to an increase in the amount of retained austenite in the microstructure, thereby increasing deformation after heat treatment of the mold. (2) Preventive measures: Select the heating temperature appropriately while ensuring the technical requirements of the mold; prefer to use the lowest possible quenching heating temperature in order to reduce stresses during cooling, thereby minimizing complex heat treatment distortions. V. Effects of residual austenite: In some high-alloy die steels, such as Cr12MoV die steel, the length, width, and height of the dies decrease after quenching and low-temperature tempering. This is caused by an excessive amount of residual austenite remaining in the dies after quenching. 1. Reasons for deformation: After quenching, alloy steels such as Cr12MoV steel contain a large amount of residual austenite. Different microstructures in the steel have different specific volumes, with austenite having the lowest specific volume. This is the main reason for the volume reduction that occurs in high-alloy steel molds after low-temperature tempering following quenching. The specific volumes of the various steel microstructures decrease in the following order: martensite – tempered sorbite – pearlite – austenite. 2. Preventive measures: (1) Appropriately reduce the quenching temperature. As mentioned earlier, the higher the quenching heating temperature, the greater the amount of residual austenite; therefore, selecting an appropriate quenching heating temperature is an important measure to reduce mold shrinkage. Generally, while ensuring the technical requirements of the mold, its overall performance should be taken into consideration, and the quenching heating temperature of the mold should be appropriately reduced. (2) Some data show that after quenching, Cr12MoV steel molds contain half as much residual austenite when tempered at 500ºC compared to when tempered at 200ºC; therefore, while ensuring compliance with the technical requirements for the molds, the tempering temperature should be increased appropriately. Production experience shows that Cr12MoV steel molds exhibit the least deformation when tempered at 500ºC, with only a slight decrease in hardness (2HRC–3HRC). (3) Cold treatment after mold quenching is the best process for reducing the amount of residual austenite, and it is also the most effective measure to minimize mold deformation and stabilize dimensional changes during use; therefore, deep cryogenic treatment is generally applied to precision and complex molds. VI. Influence of cooling media and cooling methods The deformation that occurs during mold heat treatment usually appears after quenching and cooling. Although various factors mentioned above play a role, the impact of the cooling process cannot be ignored either. 1. Causes of deformation: When the mold is cooled below the Ms point, the steel undergoes phase transformation. In addition to the thermal stresses resulting from uneven cooling, there are also structural stresses caused by the asynchronous nature of this phase transformation. The faster the cooling rate and the more uneven the cooling, the greater the stresses generated, and consequently the greater the deformation of the mold. 2. Preventive measures: (1) While ensuring the required hardness of the mold, pre-cooling should be utilized as much as possible; for carbon steel and low-alloy mold steels, pre-cooling can be carried out until the edges and corners turn black (720°C~760°C). For steel grades in which supercooled austenite is more stable in the pearlite transformation zone, it can be pre-cooled to around 700°C. (2) Using staged cooling quenching can significantly reduce the thermal stress and structural stress generated during the quenching of molds, and it is an effective method for minimizing deformation in some complex molds. (3) For some precision and complex molds, isothermal quenching can significantly reduce deformation. VII. Improving heat treatment processes to reduce deformation of molds. Deformation of molds after quenching is inevitable no matter what methods are used; however, for precision and complex molds where strict control over the degree of deformation is required, the following methods can be employed to control it. 1. For precision and complex molds for which the basic hardness requirement is not high but a high surface hardness is required, quenching and tempering treatment can be applied after rough machining of the mold, followed by low-temperature nitriding treatment at 500ºC–550ºC. Due to the low nitriding temperature, no phase changes occur in the matrix structure; moreover, cooling to room temperature reduces cooling stresses, resulting in minimal deformation of the mold. 2. Pre-heat treatment: For precision and complex molds, if the required hardness level is not very high, pre-hardened steel that has undergone pre-heat treatment can be used. The mold steel (such as 3Cr2Mo and 3CrMnNiMo steels) is subjected to pre-heat treatment to achieve the desired hardness level at the time of use (with lower hardness levels ranging from 25HRC to 35HRC, and higher hardness levels ranging from 40HRC to 50HRC). After that, the mold is shaped without further heat treatment, thereby ensuring the precision of these complex molds. 3. Use age-hardening die steels: For precision and complex molds, age-hardening steels can be employed. For example, PMS (1Ni3Mn2CuAlMo) steel is a new type of age-hardening die steel; after solution treatment and quenching at 870ºC, its hardness is around 30 HRC, which facilitates mechanical processing. After the mold is shaped, age treatment at around 500ºC yields a higher hardness of 40 HRC to 45 HRC. The deformation of the mold is minimal, and only polishing is required – making it an ideal steel for precision and complex molds. VIII. Conclusion: The reasons for deformation in precision and complex molds are often complex. However, by understanding the patterns of deformation, analyzing its causes, 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 appropriate materials. For precision and complex molds, micro-deformed die steels with high quality materials (such as air-quenched steel) should be selected. Die steels with severe carbide segregation require proper forging followed by quenching and tempering treatment, while larger molds or those that cannot be forged can undergo solution treatment combined with grain refinement. (2) The mold structure design should be reasonable; the thickness differences should not be excessive, 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, precise, 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) While ensuring the hardness of the mold, pre-cooling, staged cooling quenching, or tempering quenching processes should be employed as much as possible. (6) For precision and complex molds, where conditions permit, vacuum heating quenching and cryogenic treatment after quenching should be used as much as possible. (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. 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.