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Vacuum heat treatment measures to prevent tool and die distortion and cracking

For distortion, the following methods can be used to reduce the influence of distortion.

1) Select appropriate tool steel according to the shape, structure and working conditions of the mold.

2) Distortion and cracking tendency should be taken into account when designing the mold.

3) Select appropriate heating process according to the size, structure and material of the mold.

4) Adjust the heating speed.

5) During vacuum heat treatment, auxiliary means such as blocking, binding and mechanical fixation can be used to prevent cracking.

6) Reasonable vacuum hardening and vacuum tempering processes have a huge impact on material distortion, so the hardening cooling medium, hardening temperature and tempering method should be reasonably selected.

For cracking, it is usually classified and discussed according to the hardenability of steel, that is, high hardenability steel, low hardenability steel and large non-hardenability parts. For high permeability steel, longitudinal cracking is its main hardening cracking mode. The principle of preventing hardening cracks in such steel parts is to minimize the time difference of phase change inside and outside the workpiece. The specific measures for crack prevention are as follows.

1) Use process methods such as graded hardening, isothermal hardening and graded hardening plus isothermal hardening to reduce hardening stress.

2) Use a suitable hardening cooling medium.

3) Eliminate factors that cause surface stress concentration on parts as much as possible.

4) Avoid heating defects such as decarburization, carburization, overheating and overburning of steel parts during vacuum heat treatment as much as possible.

For low hardenability steels, arc cracks are a common type of crack. Its occurrence requires three conditions to be met at the same time: ① Rapid overall cooling. ② Inability to quench. ③ Geometrically sensitive parts for arc crack formation.

In the specific production process, it is necessary to analyze the real cause of arc cracks, so that reasonable and effective measures can be analyzed to prevent the occurrence of arc cracks. If the structural design of the material is defective, the structural design of the original mold will be optimized. If cracks occur when there is no problem with the structural design, local strong or weak cooling should be performed on the geometrically sensitive parts to prevent cracking, and then the material hardening process should be considered, and the appropriate vacuum hardening scheme and hardening cooling medium should be selected. Usually, measures are taken from two aspects: the first is to improve the comprehensive metallurgical quality of steel for large parts; the second is to reduce the thermal stress during hardening, thereby improving the vacuum hardening method of large workpieces. For the hardening cracking of large non-hardenable parts, three difficulties usually need to be overcome: the first is that the cracking of large parts generally occurs under oil-cooled vacuum hardening conditions, and oil hardening is a slower hardening method; the second is that the fracture of large parts is mostly caused by metallurgical defects, which cannot be understood in advance and are difficult to change; the third is that large parts are expensive and there are few measures to conduct direct process tests.