Possible causes of insufficient or uneven hardness:
1) The mold has a large cross section, and the hardenability of the steel is poor. For example, a large mold has a steel with low hardenability.
2) Carbide segregation or coarse structure in the original microstructure of the die steel, segregation and aggregation of graphite carbon and carbide in the steel.
3) The mold forging process is not correct, and the spheroidizing annealing is not performed well after forging, so that the spheroidized structure of the mold steel is poor.
4) The decarburized layer produced on the surface of the mold without de-annealing or quenching heating.
5) The quenching temperature of the mold is too high, and the amount of retained austenite is excessive after quenching; or the quenching temperature is too low, and the heating and holding time is insufficient, so that the phase transformation of the mold steel is incomplete.
6) After the quenching of the mold, the cooling rate is too slow, the grading and isothermal temperature are too high or the time is too long, and the quenching cooling medium is improperly selected.
7) The alkali bath has too little moisture, or the quenching cooling medium contains too much impurities, or the quenching cooling medium is aged.
8) When 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 excessive tempering temperature.
For the above 9 points analysis reasons, we find the corresponding solution.
1) Proper selection of mold steel grades, large molds should use high alloy mold steel with high hardenability.
2) Strengthen raw material inspection to ensure that raw materials meet standards. Reasonable forging of the raw material steel and spheroidizing annealing to ensure good organization. Carbon tool steels are not easily annealed multiple times to prevent graphitization.
3) Strictly perform the forging process and the spheroidizing annealing process to ensure a good preliminary heat treatment structure.
4) Before heat treatment, thoroughly remove the rust and scale on the surface of the mold, and pay attention to the protection during heating. Try to use vacuum heating or quenching or protective atmosphere to heat and quench. When the salt bath is heated, it should be deoxidized.
5) Properly formulate the mold quenching heating process parameters to ensure sufficient phase change, and rapidly cool at a cooling rate greater than the critical cooling rate to obtain a qualified metallographic structure.
6) Correctly select the quenching cooling medium and cooling method, and strictly control the classification and isothermal temperature and time.
7) To strictly control the moisture content of the alkali bath, the quenching and cooling medium used for a long time should be regularly filtered and periodically replaced, and kept clean, and the quenching cooling characteristic curve is periodically checked.
8) For large-size molds, properly extend the time of immersion in the quenching cooling medium to prevent the temperature of the quenching cooling medium from being too high.
9) After the mold is quenched, it should be tempered in time and fully, and the tempering temperature should be prevented from being too high.
10) For molds with high hardness requirements, cryogenic treatment (eg -110~-196°C) can be used.
11) Perform surface strengthening treatment.