At present, in the mold manufacturing, EDM, forming grinding, wire cutting and other new processes have been applied, which have solved the problems of complex mold processing and heat treatment deformation. However, these new processes have not been universally applicable due to various conditions. Therefore, how to reduce the deformation of the heat treatment of the die is still a very important issue.
The general mold requires high precision. After heat treatment, it is inconvenient or even impossible to carry out processing and calibration. Therefore, even after the heat treatment, even if the properties of the structure have reached the requirements, if the deformation is poor, it will still be scrapped due to irremediability. Not only affects the production, but also causes economic losses.
The general law of heat treatment deformation is not discussed here. The following is a brief analysis of some factors that affect the deformation of the mold.
The effect of the material on heat treatment deformation includes both the chemical composition of the steel and the original structure.
From the perspective of the material itself, the heat treatment distortion is mainly influenced by the influence of the composition on the hardenability, the Ms point, and the like.
When the carbon tool steel is quenched with water and oil at normal quenching temperatures, it generates large thermal stress above the Ms point; when it cools down to below the Ms point, austenite transforms to martensite, causing tissue stress, but Due to poor hardenability of carbon tool steel, the value of the tissue stress is not large. In addition to the low Ms point, the plasticity of the steel is already very poor and the plastic deformation is not easy to occur during the transformation of martensite. Therefore, the deformation characteristics caused by the thermal stress are retained, and the mold cavity tends to shrink. However, if the quenching temperature is increased (>850°C), it may also be dominated by the stress of the tissue and the cavity tends to swell.
When using 9Mn2V, 9SiCr, CrWMn, GCr15 steels and other low-alloy tool steels to make molds, the quenching deformation law is similar to that of carbon tool steels, but the deformation amount is smaller than that of carbon tool steels.
For some high-alloy steels, such as Cr12MoV steel, because of its high content of carbon and alloy elements, Ms point is lower, so there are more residual austenite after quenching, which has volume expansion due to martensite Counteracting effect, therefore, the deformation after quenching is quite small, generally with air cooling, air cooling, salt bath quenching, the mold cavity tends to micro expansion; if the quenching temperature is too high, the amount of retained austenite increases, type The cavity may also shrink.
If the mold is made of carbon structural steel (such as 45 steel) or some alloy structural steel (such as 40Cr), because of its higher Ms point, when the surface starts martensite transformation, the core temperature is still higher, and the yield strength is higher. Lower, with a certain plasticity, the surface of the heart of the instantaneous tensile tissue stress, easy to exceed the yield strength of the heart and the cavity tends to swell.
The original structure of the steel also has a certain influence on the quenching deformation. The "raw original structure of steel" referred to here includes the grade of inclusions in steel, the level of banded structure, the degree of segregation of components, the directionality of distribution of free carbides, etc., and the different structures obtained by different pre-heat treatment ( Such as pearlite, tempered sorbite, tempered troostite, etc.). For mold steels, the main considerations are carbide segregation, carbide shape and distribution morphology.
The segregation of carbides in high-carbon high-alloy steels (such as Cr12 steels) has a particularly pronounced effect on quenching deformation. Due to the non-uniformity of the composition after the steel is heated to the austenite state due to segregation of carbides, the Ms points in different regions will be high or low. Under the same cooling conditions, the first transformation of austenite to martensite occurs, and the specific volume of converted martensite leads to a small specific volume. There may even be some low-carbon, low-alloy regions. Martensite is not obtained at all (bainitic, troostite, etc.), all of which result in uneven deformation of the part after quenching.
Different distributions of carbides (in the form of granules or fibers) have different effects on the expansion and contraction of the matrix, and thus also affect the deformation after heat treatment. Generally, the carbides swell in the direction of the carbide fibers and are more pronounced. While the direction perpendicular to the fiber is reduced, but not significant, some factories have made special provisions for this, the surface of the cavity should be perpendicular to the direction of the carbide fibers to reduce the deformation of the cavity, when the carbides are granular Evenly distributed, the cavity shows a uniform expansion and contraction.
In addition, the state of the tissue before the final heat treatment also has a certain impact on the deformation, for example, the original organization of spherical pearlite than the flaky pearlite deformation tendency after quenching is smaller. Therefore, a mold with a strict deformation requirement is often subjected to a tempering treatment after rough processing, and then subjected to a finishing process and a final heat treatment.
The influence of the geometry of the mold on the deformation of the heat treatment actually acts through thermal stress and tissue stress. Since the shape of the mold is various, it is still difficult to summarize the exact deformation rule.
For symmetric molds, the deformation tendency of the cavity can be taken into consideration according to the cavity size, outside dimensions and height. When the mold has a thin wall and a small height, it is easier to be hardened. At this time, the stress of the tissue may play a dominant role. Therefore, the cavity tends to swell. On the contrary, if the wall thickness and height are large, it is difficult to be hardened. At this time, thermal stress may play a leading role. Therefore, the cavity tends to shrink. Here is a general trend, in the production practice, we must consider the specific shape of the parts, the type of steel used and the heat treatment process to consider, through the practice to sum up experience. Due to actual production, the external dimensions of the mold are often not the main working dimensions, and after deformation, they can be corrected by grinding or the like. Therefore, the above analysis mainly focuses on the deformation tendency of the cavity.
The deformation of asymmetrical molds is also the result of a combination of thermal stress and tissue stress. For example, for the thin-walled thin edge die, because the die wall is thin, the internal and external temperature difference during quenching is small, so the thermal stress is small; but it is easy to harden through, and the tissue stress is large, so the deformation tends to expand in the cavity.
In order to reduce the deformation of the mold, the heat treatment department should work together with the mold design department to improve the mold design, such as avoiding as much as possible the difference in cross-sectional size of the mold structure, the shape of the mold to seek symmetry, complex mold assembly structure.
When the shape of the mold cannot be changed, some other measure can also be taken in order to reduce the deformation. The overall consideration of these measures is to improve the cooling conditions so that the parts can be uniformly cooled; in addition, various coercive measures can be assisted to limit the quench deformation of the parts. For example, adding a process hole is a measure to uniformly cool the parts, that is, to open holes in certain parts of the mold so that all parts of the mold can be uniformly cooled to reduce distortion. It is also possible to enclose the periphery of the mold, which is likely to expand after quenching, with asbestos in order to increase the cooling difference between the inner hole and the outer layer and shrink the cavity. Retaining or reinforcing the ribs on the mold is another measure to reduce the deformation. It is especially suitable for the cavity swelled die, and the die with which the notch expands easily or shrinks.
1, the impact of heating speed
Generally speaking, when the heating is quenched, the faster the heating rate, the greater the thermal stress generated in the mold, which will easily cause deformation and cracking of the mold. Especially for alloy steel and high-alloy steel, due to its poor thermal conductivity, special attention must be paid to preheating For some high-alloy molds with complex shapes, multiple stages of preheating are required. However, in some cases, the use of rapid heating may sometimes reduce the deformation. At this time, only the surface of the mold is heated and the center remains “cold.” Therefore, the tissue stress and thermal stress are reduced accordingly, and the core deformation resistance is relatively high. , thereby reducing the quench deformation, according to some factory experience, to solve the hole pitch deformation has a certain effect.
2, the influence of heating temperature
The quenching heating temperature affects the hardenability of the material, and at the same time it contributes to the austenite composition and grain size.
(1) From the aspect of hardenability, a high heating temperature will increase the thermal stress, but at the same time, the hardenability will increase. Therefore, the structural stress also increases, and it gradually dominates.
E.g. Carbon tool steel T8, T10, T12, etc., in the general quenching temperature quenching, the inner diameter tends to shrink, but if the quenching temperature is increased to ≥ 850 °C, due to the increase of hardenability, the organizational stress gradually dominated. Therefore, the inner diameter may show the tendency to swell.
(2) From the perspective of the austenite composition, the increase in the quenching temperature increases the carbon content of the austenite, and the squareness of the martensite after quenching increases (the specific volume increases), thereby increasing the volume after quenching.
(3) From the impact of the Ms point, the quenching temperature is high, then the austenite grains are coarse, which will increase the deformation cracking tendency of the parts.
To sum up, for all steel types, especially for some high carbon medium and high alloy steels, the quenching temperature will obviously affect the quenching deformation of the mold. Therefore, it is very important to select the quenching heating temperature correctly.
In general, choosing an excessively high quenching temperature is not good for deformation. The lower heating temperature is always used without affecting the performance. However, for some steels with a lot of retained austenite after quenching (such as Cr12MoV, etc.), the amount of retained austenite can also be changed by adjusting the heating temperature to adjust the deformation of the mold.
3, the effect of quenching cooling rate
In general, increasing the cooling rate above the Ms point will increase the thermal stress significantly. As a result, the deformation caused by the thermal stress tends to increase. The increase in the cooling rate below the Ms point will mainly cause the deformation induced by the tissue stress. Increase.
For different steel grades, there are different deformation tendencies when the same quenching medium is used due to the difference in the Ms point. If different quench media are used for the same steel type, they have different deformation tendencies due to their different cooling capacities.
For example, the carbon tool steel is relatively low at Ms, so when water cooling is used, the influence of thermal stress often prevails; when cold is used, the stress of tissue may prevail.
In actual production, when the mold is often graded or graded - isothermal quenching, it is usually not completely hardened, so it is often the main role of thermal stress, so that the cavity tends to shrink, but because the thermal stress is not great at this time, Therefore, the total deformation is relatively small. If the water-oil double-liquid quenching or oil quenching is used, the thermal stress caused is greater and the cavity shrinkage will increase.
4, the effect of tempering temperature
The effect of tempering temperature on deformation is mainly due to the transformation of the microstructure during the tempering process. If a "secondary quenching" occurs during the tempering process, the retained austenite is transformed into martensite. Since the specific volume of the resulting martensite is larger than that of retained austenite, the expansion of the mold cavity will be caused. For some high-alloy tool steels, such as Cr12MoV, when high-temperature quenching is mainly required for red hardness, and multiple tempering, the volume expands once each time.
If the tempering is performed in other temperature regions, the specific volume decreases due to the transformation of quenched martensite to tempered martensite (or tempered sorbite, tempered troostite, etc.), and the cavity tends to shrink.
In addition, the relaxation of the residual stress in the mold during tempering also affects the deformation. After the mold is quenched, if the surface is under tensile stress, the dimensions will increase after tempering; conversely, if the surface is under compressive stress, shrinkage occurs. However, of the two effects of organizational change and stress relaxation, the former is the main one.