The hardness of mold heat treatment is a very important mechanical property index.Insufficient or uneven hardness after heat treatment will reduce the wear resistance and fatigue strength of the mold, which will lead to early failure of the mold and seriously reduce the service life of the mold.
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 in the original structure of die steel is severe or large, and graphite carbon and carbide segregation and aggregation exist in the steel.
3) the forging process of the mold is not correct. After the forging, it is not well spheroidized and annealed, resulting in poor spheroidized structure of the mold steel.
4) the decarbonized layer on the surface of the mold is not removed during annealing or quenching heating.
5) the mold quenching temperature is too high, and too much residual austenite after quenching;Or the quenching temperature is too low, heating and holding time is insufficient, so that the die steel phase change is not complete.
6) after quenching and heating, the cooling speed of the mold 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) too little water in alkaline bath, or too much impurity in quenching cooling medium, or quenching cooling medium aging.
8) after quenching and cooling, the temperature of quenching cooling medium is too high and the cooling is insufficient.
9) insufficient tempering and too high tempering temperature.
Countermeasures for 2.
1) correct selection of mold steel, large mold should choose high hardenability of high alloy mold steel.
2) strengthen the inspection of raw materials to ensure that raw materials meet the standards.Reasonable forging and spheroidizing annealing of the defective raw materials to ensure a good structure.Carbon tool steels are not easy to be annealed many times to prevent graphitization.
3) strictly implement the forging process and spheroidizing annealing process to ensure a good preliminary heat treatment structure.
4) before the heat treatment, rust spots and scale on the surface of the mold should be thoroughly removed, and the protection during heating should be paid attention to. Vacuum heating or protective atmosphere heating should be adopted as far as possible, and good deoxidation treatment should be carried out during the heating of salt bath.
5) the process parameters of mold quenching and heating should be correctly formulated to ensure sufficient phase transition and rapid cooling at a cooling rate greater than the critical cooling rate to obtain qualified microstructure.
6) choose the right quenching cooling medium and cooling method, and strictly control the grading and isothermal temperature and time.
7) the moisture content of alkaline bath should be strictly controlled. The quenching cooling medium used for a long time should be filtered and replaced regularly, and kept clean. The quenching cooling characteristic curve should be tested regularly.
8) for large molds, appropriately extend the immersion time of quenching cooling medium to prevent the temperature of quenching cooling medium from getting too high.
9) the mold shall be timely and fully tempered after quenching, and the tempering temperature shall be prevented from being too high.
10) cryogenic processing can be used for high hardness requirements of mould (e.g., - 110 ~ 196 ℃).
11) surface strengthening treatment.