Die heat treatment hardness is a very important mechanical performance index, and hardness is not a serious defect. Insufficient hardness or uneven hardness after heat treatment of the mold will reduce the wear resistance and fatigue strength of the mold, resulting in early failure of the mold and severely reducing the service life of the mold. Then let's take a look at the reasons and countermeasures:
Cause
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 when the surface of the mold is not cleaned or quenched.
5) The quenching temperature of the mold is too high, and the amount of retained austenite is too much 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) The cooling rate of the mold after quenching 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 water, 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.
2. Countermeasures
1) Proper selection of mold steel grades, large molds should use high alloy mold steel with high hardenability.
2) Strengthen the inspection of raw materials to ensure that raw materials meet the standards. Reasonable forging of the raw material steel and spheroidizing annealing to ensure good structure. Carbon tool steels are not easily annealed multiple times to prevent graphitization.
3) Strictly carry out 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 well.
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.
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