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These three factors are the key to mold deformation (2)

2019-01-10 13:33

These three factors are the key to mold deformation (2) At present, in the mold manufacturing, new processes such as EDM, profile grinding and wire cutting have been applied, which has solved the

These three factors are the key to mold deformation (2)
    At present, in the mold manufacturing, new processes such as EDM, profile grinding and wire cutting have been applied, which has solved the problem of processing and heat treatment deformation of complex molds. However, these new processes have not been universally applied due to various conditions. Therefore, how to reduce the heat treatment deformation of the mold is still a very important issue.
    Generally, the mold requires high precision. After heat treatment, it is inconvenient or even impossible to process and calibrate. Therefore, even after the heat treatment, even if the structural properties have reached the requirements, if the deformation is too poor, it will be scrapped because it cannot be saved. Not only affects production, but also causes economic losses.
The general rule of heat treatment deformation is not discussed here. The following is a brief analysis of some factors affecting mold deformation.
Effect of heat treatment process on mold deformation
1, the impact of heating speed
    In general, the faster the heating rate during quenching heating, the greater the thermal stress generated in the mold, which tends to cause deformation and cracking of the mold. Especially for alloy steel and high alloy steel, it is necessary to pay attention to preheating due to poor thermal conductivity. For some high-alloy molds with complex shapes, multiple stages of preheating are required. However, in some cases, rapid heating can sometimes reduce the deformation, and only the surface of the mold is heated, and the center remains "cold", so the tissue stress and thermal stress are reduced accordingly, and the deformation resistance of the core is relatively large. Therefore, the quenching deformation is reduced, and according to some factory experience, it has a certain effect in solving the hole pitch deformation.
2, the effect of heating temperature
    The quenching heating temperature affects the hardenability of the material and acts on the composition and grain size of the austenite.
(1) From the viewpoint of hardenability, the high heating temperature will increase the thermal stress, but at the same time increase the hardenability, so the structural stress also increases and gradually becomes dominant.
    E.g. Carbon tool steel T8, T10, T12, etc., when quenching at normal quenching temperature, the inner diameter shows a tendency to shrink. However, if the quenching temperature is increased to ≥850 °C, the microstructure stress gradually dominates due to the increased hardenability. Therefore, the inner diameter may appear to be a tendency to swell.
(2) From the austenite composition, the increase in the quenching temperature increases the carbon content of the austenite, and the squareness of the martensite increases (the specific volume increases) after quenching, thereby increasing the volume after quenching.
(3) From the point of view of the influence on the Ms point, if the quenching temperature is high, the austenite grains are coarse, which tends to increase the deformation cracking tendency of the parts.
    In summary, for all steel grades, especially 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 correctly select the quenching heating temperature.
    In general, choosing an excessively high quenching heating temperature is not beneficial for deformation. A lower heating temperature is always used without affecting the performance. However, for some steel grades with more retained austenite after quenching (such as Cr12MoV, etc.), the deformation of the mold can also be adjusted by adjusting the heating temperature and changing the amount of retained austenite.
3, the impact of quenching cooling rate
    In general, increasing the cooling rate above the Ms point causes a significant increase in thermal stress, and as a result, the deformation caused by thermal stress tends to increase; increasing the cooling rate below the Ms point mainly causes the deformation tendency caused by the tissue stress. Increase.
    For different steel grades, due to the different heights of the Ms points, there are different deformation tendencies when using the same quenching medium. If different quenching media are used for the same steel grade, they have different deformation tendencies due to their different cooling capacities.
For example, carbon tool steel has a relatively low Ms point, so when using water cooling, the influence of thermal stress tends to prevail; while when it is cold, it may be that tissue stress prevails.
    In actual production, when the mold is often graded or graded-isothermally quenched, it is usually not completely hardened, so the effect of thermal stress is often used, so that the cavity tends to shrink, but since the thermal stress is not very large at this time, Therefore the total amount of deformation is relatively small. If water-oil two-liquid quenching or oil quenching is used, the thermal stress caused by the larger, the cavity shrinkage will increase.
4, the impact of tempering temperature
    The effect of tempering temperature on deformation is mainly due to the structural transformation during tempering. If the phenomenon of "secondary quenching" occurs during the tempering process, the retained austenite transforms into martensite, and the specific volume of the formed martensite is larger than that of the retained austenite, which will cause the swelling of the mold cavity; For some high-alloy tool steels, such as Cr12MoV, when high-temperature quenching is used mainly for red hardness, when tempering multiple times, the volume will swell once every time.
    If tempered in other temperature regions, the specific volume is reduced due to the transformation of the quenched martensite to the tempered martensite (or tempered sorbite, tempered troostite, etc.), and thus the cavity tends to shrink.
    In addition, when tempering, the relaxation of residual stress in the mold also affects the deformation. After the mold is quenched, if the surface is in a tensile stress state, the size will increase after tempering; conversely, if the surface is in a compressive stress state, shrinkage will occur. However, the former is the main one of the two effects of organizational transformation and stress relaxation.
 
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These three factors are the key to mold deformation (2)

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