Analysis of Factors Influencing Mold Deformation and Corresponding Mitigation Measures

We will not discuss the general principles governing thermal treatment deformation here. Instead, we will briefly analyze some of the factors that influence mold deformation.

　　1. The Influence of Mold Materials on Thermal Treatment Deformation
　　The influence of materials on thermal treatment deformation includes two aspects: the chemical composition of steel and its original microstructure. From the material’s perspective, thermal treatment deformation is primarily affected by the composition through its impact on hardenability, the Ms point, and other related factors.
　　When carbon tool steel is subjected to dual-medium quenching—water-oil—at a normal quenching temperature, significant thermal stresses are generated above the Ms point. As the temperature cools below the Ms point, austenite transforms into martensite, giving rise to microstructural stresses. However, since carbon tool steel has poor hardenability, the magnitude of these microstructural stresses is relatively small. Moreover, given that its Ms point is not particularly high, by the time martensitic transformation occurs, the steel’s ductility has already become very poor, making plastic deformation unlikely. Consequently, the deformation characteristics induced by thermal stresses remain largely intact, causing the mold cavity to tend toward shrinkage. Nevertheless, if the quenching temperature is raised (to above 850℃), microstructural stresses may come to dominate, potentially causing the mold cavity to expand instead.
　　When molds are made from low-alloy tool steels such as 9Mn2V, 9SiCr, CrWMn, and GCr15, their quenching deformation patterns are similar to those of carbon tool steels, but the amount of deformation is smaller than that in carbon tool steels.
　　For certain high-alloy steels, such as Cr12MoV steel, due to their relatively high carbon and alloy element content, the Ms point is lower. Consequently, after quenching, these steels retain a significant amount of residual austenite, which counteracts the volume expansion caused by martensite formation. As a result, the deformation after quenching is quite minimal. Typically, when using air cooling, fan cooling, or nitrate salt baths for quenching, the mold cavity tends to expand only slightly. However, if the quenching temperature is too high, the amount of residual austenite will increase, potentially causing the mold cavity to shrink instead.
　　If molds are made from carbon structural steels (such as 45 steel) or certain alloy structural steels (such as 40Cr), due to their relatively high Ms point, when the surface begins to undergo martensitic transformation, the core temperature is still relatively high, resulting in lower yield strength and some degree of plasticity. Consequently, the instantaneous tensile residual stresses acting on the core from the surface easily exceed the core’s yield strength, causing the cavity to tend toward expansion.
　　The original microstructure of steel also has a certain influence on quenching distortion. The “original microstructure of steel” referred to here includes the grade of inclusions in the steel, the degree of banding, the extent of compositional segregation, the directional distribution of free carbides, as well as the different microstructures obtained through various pre-heat treatments (such as pearlite, tempered sorbite, and tempered troostite). For die steels, the primary factors to consider are carbide segregation, as well as the shape and distribution pattern of carbides.
　　In high-carbon, high-alloy steels (such as Cr12-type steel), the segregation of carbides has a particularly pronounced effect on quenching distortion. Because carbide segregation leads to compositional non-uniformity after the steel is heated to the austenitic state, the Ms points—where the austenite starts to transform into martensite—vary from one region to another. Under identical cooling conditions, the transformation from austenite to martensite thus occurs at different times in different regions. Moreover, the resulting martensites exhibit varying specific volumes depending on their carbon content; in some low-carbon, low-alloy regions, martensite may not form at all, instead giving rise to bainite or troostite. All these factors contribute to uneven deformation of the parts after quenching.
　　Different morphologies of carbide distribution—whether in the form of particles or fibers—affect the expansion and contraction of the matrix differently, thereby influencing the deformation after heat treatment. Generally, along the direction of the carbide fibers, the cavity tends to expand more noticeably; whereas perpendicular to the fiber direction, the cavity shrinks, though not significantly. Some manufacturers have specifically stipulated that the surface on which the cavity is located should be oriented perpendicular to the direction of the carbide fibers, in order to minimize cavity deformation. When the carbides are uniformly distributed in a particulate form, the cavity exhibits uniform expansion and contraction.
　　Moreover, the microstructural state prior to final heat treatment also has a certain influence on deformation. For example, steels with a spheroidal pearlite microstructure exhibit less tendency to deform after quenching than those with a lamellar pearlite microstructure. Therefore, for dies with stringent deformation requirements, it is common practice to first perform a tempering treatment after rough machining, followed by finish machining and then the final heat treatment.

　　2. The Influence of Mold Geometry on Deformation
　　The influence of mold geometry on thermal treatment deformation actually still operates through thermal stresses and microstructural stresses. Given the wide variety of mold shapes, it remains quite challenging at present to derive precise deformation patterns from them.
　　For symmetrical molds, the tendency for cavity deformation can be assessed based on the cavity dimensions, external dimensions, and height. When the mold walls are thin and the height is small, the mold is more likely to achieve complete quenching; in such cases, microstructural stresses tend to play a dominant role, causing the cavity to expand. Conversely, when the walls are thick and the height is large, the mold is less likely to undergo complete quenching; here, thermal stresses may become the dominant factor, leading to a tendency for the cavity to shrink. What has been described above represents a general trend. In actual production practice, however, it is also necessary to take into account the specific shape of the part, the type of steel used, and the heat treatment process employed, continually refining and accumulating experience through practical application. Since, in real-world production, the external dimensions of the mold are often not the primary working dimensions—and since any deformation can be corrected later through machining operations such as grinding—the analysis presented above focuses primarily on the deformation tendencies of the cavity itself.
　　The deformation of asymmetric molds is also the result of the combined effects of thermal stress and microstructural stress. For example, in molds with thin walls and thin edges, since the mold walls are thin, the temperature difference between the inner and outer surfaces during quenching is small, resulting in relatively low thermal stress. However, such molds tend to be fully hardened, leading to greater microstructural stress and causing the mold cavity to expand.
　　To minimize mold deformation, the heat treatment department should work closely with the mold design department to refine the mold design—for example, by avoiding, as much as possible, mold structures with dramatically varying cross-sectional dimensions; striving for symmetry in mold shapes; and using modular designs for complex molds.
　　When it is impossible to alter the mold’s shape, other measures can still be taken to reduce deformation. The overarching principle behind these measures is to improve cooling conditions so that all parts of the mold cool uniformly. In addition, various forced cooling techniques can be employed to limit quenching-induced deformation of the part. For example, adding process holes is one such measure aimed at achieving uniform cooling—by creating openings in certain areas of the mold, all sections can cool evenly, thereby minimizing deformation. Another approach is to wrap the outer periphery of molds that tend to expand significantly after quenching with asbestos, thereby increasing the cooling differential between the inner cavity and the outer layer and inducing shrinkage of the cavity. Leaving ribs or adding reinforcing elements to the mold is yet another forced measure for reducing deformation; this technique is particularly effective for concave dies where the cavity tends to expand, as well as for molds whose grooves are prone to either expansion or contraction.

　　3. The Influence of Heat Treatment Processes on Mold Deformation
　　1. The effect of heating rate
　　Generally speaking, during quenching heating, the faster the heating rate, the greater the thermal stress generated within the die, making it more prone to deformation and cracking. This is particularly true for alloy steels and high-alloy steels, which have poor thermal conductivity; therefore, preheating is especially important for these materials. For some highly alloyed dies with complex shapes, multiple-stage preheating may even be necessary. However, in certain specific cases, rapid heating can sometimes actually help reduce deformation. In such instances, only the surface of the die is heated, while the core remains “cold.” As a result, both microstructural stress and thermal stress are reduced accordingly. Moreover, since the core exhibits greater resistance to deformation, overall quenching distortion is minimized. According to experience from several factories, this approach has shown some effectiveness in addressing deformation related to hole spacing.
　　2. The effect of heating temperature
　　The high or low temperature of quenching heating affects the hardenability of the material and also influences the composition and grain size of austenite.
　　(1) From the perspective of hardenability, a higher heating temperature will increase thermal stresses but simultaneously enhance hardenability. Consequently, microstructural stresses also rise and gradually become dominant. For example, carbon tool steels such as T8, T10, and T12 typically exhibit a tendency toward shrinkage in their inner diameters when quenched at conventional temperatures. However, if the quenching temperature is raised to ≥850℃, the enhanced hardenability causes microstructural stresses to gradually take over as the dominant factor, and as a result, the inner diameters may instead show a tendency toward expansion.
　　(2) From the perspective of austenitic composition, increasing the quenching temperature leads to a higher carbon content in the austenite. As a result, the tetragonal distortion of the martensite formed after quenching increases (leading to an increase in specific volume), thereby causing the overall volume to expand after quenching.
　　(3) Looking closely at the effect on the Ms point, a higher quenching temperature results in coarser austenite grains, which in turn increases the tendency for parts to deform and crack.
　　In summary, for all steel grades—especially certain medium- and high-alloy steels with high carbon content—the magnitude of the quenching temperature significantly affects the quenching distortion of dies. Therefore, it is crucial to select the quenching heating temperature correctly.
　　Generally speaking, selecting an excessively high quenching heating temperature is detrimental to minimizing deformation. As long as the service performance is not compromised, it is always advisable to use a lower heating temperature. However, for certain steel grades—such as Cr12MoV—that retain a relatively large amount of residual austenite after quenching, the amount of residual austenite can be adjusted by modifying the heating temperature, thereby helping to control mold deformation.
　　3. The Influence of Quenching Cooling Rate
　　In general, increasing the cooling rate above the Ms point significantly enhances thermal stresses, thereby causing the deformation induced by thermal stresses to tend to increase. Conversely, increasing the cooling rate below the Ms point primarily leads to an increase in deformation caused by microstructural stresses.
　　For different steel grades, since the Ms point varies in height, they exhibit different deformation tendencies when using the same quenching medium. Even for the same steel grade, if different quenching media are employed, their varying cooling capacities will lead to different deformation tendencies as well. For example, carbon tool steels have a relatively low Ms point; thus, when quenched in water, thermal stresses tend to dominate; whereas when quenched in oil, organizational stresses may take the upper hand.
　　In actual production, when molds undergo staged or staged-isothermal quenching, they usually do not achieve full-through hardening. As a result, thermal stresses tend to dominate, causing the cavity to shrink. However, since the thermal stresses at this stage are not very high, the overall deformation remains relatively small. If water-oil dual-liquid quenching or oil quenching is employed instead, the resulting thermal stresses will be greater, leading to an increased amount of cavity shrinkage.
　　4. The effect of tempering temperature
　　The effect of tempering temperature on deformation is primarily attributable to the microstructural transformations that occur during the tempering process. If a “secondary quenching” phenomenon occurs during tempering, residual austenite will transform into martensite. Since the specific volume of the newly formed martensite is greater than that of the residual austenite, this will cause the mold cavity to expand. For certain high-alloy tool steels such as Cr12MoV, when high-temperature quenching is employed primarily to achieve red hardness and multiple tempering cycles are carried out, the mold cavity will expand with each tempering cycle. However, if tempering is performed in other temperature ranges, since the quenched martensite transforms into tempered martensite (or tempered sorbite, tempered troostite, etc.), the specific volume decreases, causing the mold cavity to tend toward contraction.
　　In addition, during tempering, the relaxation of residual stresses in the mold also affects deformation. After quenching, if the mold surface is under tensile stress, its dimensions will increase upon tempering; conversely, if the surface is under compressive stress, the mold will shrink. However, among these two effects—microstructural transformation and stress relaxation—the former is the primary contributing factor.