Since all metal will go though thermal expansion, and when heat the steel, temperature gap in different zones of the same mold lead to different expansion, so interior strength will exist due to uneven heat. When temperature below steel phase transformation point, uneven heat can lead to thermal stress, when temperature higher, it will lead to steel structure transformation in different time period, namely structural stress, so the more speed steel heated, the gap between mold surface and core side will be larger, and the more stree will be, the more obvious mold transformation will be after heat treatment.

 

2) Precautionary measures

 

Complex molds will be heated slowly under steel phase transformation point, in general, mold deformation under vaccum heat treatment is less than salt bath heat quenching. When pre-heated, low alloy steel molds can be heated one time preheating only under temperature 550-620 ℃, high alloy steel molds can be heated two times preheating (1st temperature 550-620 ℃, 2nd temperature 800-850℃)