cause:
1) The cross-section of the mold is large, and the hardenability of the steel is poor. For example, a steel with low hardenability is selected for a large mold.
2) The carbide segregation in the original structure of the die steel is serious or the structure is coarse, and there are segregation and aggregation of graphitic carbon and carbide in the steel.
3) The die forging process is incorrect, and the spheroidizing annealing is not performed well after forging, resulting in poor spheroidized structure of the die steel.
4) The decarburized layer generated during annealing or quenching heating is not removed from the machined surface of the plastic mold.
5) The quenching temperature of the mold is too high, and the amount of retained austenite after quenching is too much; or the quenching temperature is too low, and the heating and holding time is insufficient, so that the transformation of the mold steel is incomplete.
6) After the mold is quenched and heated, the cooling rate is too slow, the grading and isothermal temperature is too high or the time is too long, and the quenching cooling medium is not properly selected.
7) The alkali bath has too little moisture, or the quenching cooling medium contains too much impurities, or the quenching cooling medium is aging.
8) After the mold is quenched and cooled, the temperature of the quenching cooling medium is too high and the cooling is insufficient.

9) Insufficient tempering and high tempering temperature.