The hardness of mold heat treatment is a very important mechanical performance index. Unqualified hardness is a very serious defect. Insufficient or uneven hardness of the mold after heat treatment will reduce the wear resistance and fatigue strength of the mold, leading to early failure of the mold and seriously reducing the service life of the mold.
Causes:
1. The mold cross-section is large and the hardenability of the steel is poor. For example, a large mold uses steel with low hardenability.
2. The carbide segregation in the original structure of the mold steel is serious or the structure is coarse, and graphite carbon and carbide segregation and aggregation exist in the steel.
3. The mold forging process is incorrect, and the spheroidizing annealing is not well performed after forging, resulting in poor mold steel.
4. The decarburization layer produced during annealing or quenching heating is not removed from the mold surface.
5. The mold quenching temperature is too high, and the amount of residual austenite after quenching is too much, or the quenching temperature is too low, and the heating and insulation time is insufficient so that the phase change of the mold steel is incomplete.
6. The cooling speed of the mold after quenching heating is too slow, the grading and isothermal temperature is too high or the time is too long, and the quenching cooling medium is improperly selected.
7. There is too little water in the alkali bath, too many impurities in the quenching cooling medium, or the quenching cooling medium is aged.
8. The temperature of the quenching cooling medium is too high when the mold is quenched and cooled, and the cooling is insufficient.
9. Insufficient tempering and excessive tempering temperature, etc.

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