The mold subjected to heat treatment is deformed during quenching and tempering. Therefore, it should be cooled as quickly as possible in order to meet the heat treatment quality requirements. This is in contradiction with the need to resist the deformation as slowly as possible. It is necessary to meet the above requirements at the same time. Very difficult.
In addition to good cutting performance, the mold material also needs to reduce the dimensional deformation during the heat treatment process. Recently, cold forging die steels PD613 and HPM31 have been improved and developed into SLD-Magic and DCMX materials. Compared with JIS SKD11, the performance of these improved steels has been greatly improved. In the metal mold for plastic molding, since the flame-retardant reinforced resin is mainly molded, in addition to the wear resistance and corrosion resistance, it is necessary to fully consider the aging change caused in the molding.
Since the wear resistance is proportional to the hardness, the metal mold material is a steel having a hardness of 60HRC. In addition, it should be noted that the material will become brittle, so the ductility (also known as forgeability) should be considered for molds that may be damaged. Therefore, the material hardness is generally controlled to 56-58 HRC. Further, when it is necessary to consider corrosion resistance, in principle, M (martensitic) stainless steel is used. In the heat treatment that emphasizes corrosion resistance, carbides that reduce the corrosion resistance cannot be precipitated. It is necessary to avoid tempering at temperatures above 450 °C. However, in order to carry out the N-treatment and the coating treatment after the heat treatment, high-temperature tempering is an appropriate treatment.
In order to prevent aging changes, it is necessary to carry out low-temperature treatment, that is, tempering should be carried out at a temperature of 400 ° C or lower at which carbides are not precipitated.