The hardness of mold heat treatment is a very important mechanical property index,it is a very serious defect when the hardness is not qualified . Insufficient or uneven hardness of the mold after heat treatment will reduce the wear resistance and fatigue strength of the mold and other properties,even resulting in early failure of the mold, seriously shorten the mold life.

The Reasons Causing:
1) The mold section is large and the hardenability of steel is poor. For example, the steel with low hardenability is selected for the large mold.
2) Carbide segregation is serious or large in the original structure of mold steel, and graphite carbon and carbide segregation and aggregation exist in the steel.
3) The mould forging process is not correct, and the mould steel is not spheroidized and annealed well after forging, resulting in poor spheroidized structure of the mould steel.
4) The decarburized layer on the surface of the mold is not removed when annealing or quenching is heated.
5) The mold hardening temperature is too high, and too much austenite remains after quenching; Or the quenching temperature is too low, heat preservation time is insufficient, so that the phase change of die steel is not complete.
6) After quenching and heating, the cooling speed of the mold is too slow, the grading and isothermal temperature are too high or the time is too long, and the quenching and cooling medium is improperly selected.
7) Too little water in the alkali bath, or too much impurities in the quenching cooling medium, or aging of the quenching cooling medium.
8) After quenching and cooling, the temperature of the quenching cooling medium is too high and the cooling is insufficient.
9) Insufficient tempering and excessive tempering temperature.

This is caused by setting tempering temperature too high, fault error of temperature control system or entering furnace when furnace temperature is too high. The process temperature shall be modified, and the temperature control system shall be overhauled and checked, which shall not be higher than the furnace temperature installed.