1、Red heat hardness

Hot working moulds working at high temperatures need to keep their structure and performance stable in order to maintain a high enough hardness, known as red hardness. Carbon tool steels and low-alloy tool steels typically maintain this property in the temperature range of 180 to 250℃, while chrome-molybdenum hot working die steels typically maintain this property in the temperature range of 550 to 600℃. The hardness of steel mainly depends on its chemical composition and heat treatment process.

2、Compressive yield strength and compressive bending strength

Graphite mould often has to bear high strength pressure and bending in the process of use, so the mould material is required to have a certain compressive strength and bending strength. In many cases, the conditions of the compression and bending tests are close to the actual working conditions of the die (for example, the measured compressive yield strength of the die steel is in good agreement with the deformation resistance of the punch). Another advantage of bending test is that the absolute value of strain is large, which can sensitively reflect the difference of deformation resistance of different steel types, different heat treatments and different microstructure states.

3、Hardness

Hardness is the main technical index of die steel. Under high stress, the die must be stiff enough to maintain its shape and size. At room temperature, the hardness of cold working die steel is generally kept around HRC60, while the hardness of hot working die steel is generally kept between HRC40 and 55 according to its working condition. For the same steel, the hardness is proportional to the deformation resistance within a certain hardness range. The plastic deformation resistance of steels with the same hardness but different composition and structure may vary greatly.