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Heat treatment distortion and cracking of tools and dies

Principle of distortion and cracking in vacuum heat treatment of tools and molds

During vacuum heat treatment, tool steel may be distorted and cracked, which will seriously affect the performance of steel. There are two main reasons: one is that during the heating and cooling process of steel, the temperature difference between the surface and the center will cause different expansion and contraction in different areas of the material; the other is that the volume changes of various tissues in the material are different when phase change occurs. These two reasons will cause stress inside the material, thereby causing distortion or cracking. In addition, the faster the heating and cooling frequency of steel, the larger the size of the cross section, the smaller the thermal conductivity, and the greater the internal stress inside the material. When the internal stress exceeds the yield strength of the steel, it will cause material distortion, and when it exceeds the fracture strength of the material, the material will crack. Generally speaking, the stress caused by different and uneven thermal expansion and contraction is called thermal stress; the stress caused by different and uneven phase change is called tissue stress.

The chemical composition of steel, vacuum heat treatment process parameters, and the geometric shape and size of the workpiece are the three major factors affecting the distortion of tool steel. For the chemical composition of steel, C is the element that has the greatest impact on quenching distortion, because as the C content increases, the martensite content in the steel increases, the martensite changes more, and the quenching distortion caused by the microstructure phase change increases. However, as the C content increases, the influence of martensite phase change can be reduced due to the increase in the content of retained austenite. Most alloying elements will improve the hardenability of steel and reduce the critical quenching speed. Therefore, vacuum hardening operations can be performed in a mild medium to reduce internal stress and reduce the probability of distortion. For vacuum heat treatment processes, increasing the vacuum hardening heating temperature will generally increase the distortion of the material. The heating rate and cooling rate also have a greater impact on quenching distortion. If the heating rate and cooling rate are too fast, the thermal stress inside the material will increase. Therefore, in order to reduce the probability of distortion, it is necessary to preheat in advance and heat evenly, and use a reasonable cooling medium for quenching. The geometric dimensions of the workpiece have a great influence on the distortion of vacuum heat treatment. Usually, it is necessary to avoid sharp corners and concave shapes when designing the workpiece, but due to the great variability of the shape of the workpiece, it is difficult to derive a relatively uniform rule.

According to the distribution state of vacuum heat treatment cracks on steel, they can be divided into three forms: longitudinal, arc and mesh. Longitudinal cracks are caused by tangential tensile stress exceeding the fracture strength of the material; arc cracks are caused by axial tensile stress exceeding the fracture strength of the material; mesh cracks are formed under multi-directional tensile stress.

Longitudinal cracks usually appear in high hardenability steels, because the surface of the steel will produce large tangential tensile stress when it is hardened. In addition to longitudinal cracks during hardening, cracks or defects may exist in the raw materials before vacuum heat treatment.

Arc cracks are usually formed in the following situations: the first is in high carbon steel, when the cross section is not fully hardened, the second is that the geometric shape of the steel part has obvious sharp corners and groove defects; the third is that during surface quenching, there is a large tangential or axial tensile stress in the transition zone between the hardened zone and the non-hardened zone, forming a transition zone crack.