The uneven hardness of die steel after heat treatment will reduce the wear resistance of die and affect the service life of die. The reasons for these phenomena are as follows
1. The steel with low hardenability is selected for the large mould.
2. Carbide segregation is serious in the original structure of die steel.
3. After forging, the forging die is not good and the ball structure is not good.
4. There is annealing decarburization layer on the surface of the die or the decarburization layer is produced during quenching and heating.
5. If the quenching temperature is too high, the retained austenite content is too much; or the quenching temperature is too low, the transformation of die steel is not complete.
6. After quenching and heating, the cooling speed is slow, the grading and isothermal temperature are too high or too long, and the cooling medium is not selected properly.
7. There are too many impurities in quenching medium or aging.
8. After quenching and cooling, the temperature of quenching medium is too high and the cooling is insufficient.
9. Tempering is not enough and tempering temperature is too high.
The preventive measures for die steel are as follows
1. Correct selection of die steel, large-scale mold should choose high hardenability of high alloy die steel.
2. Reasonable forging process and spheroidizing annealing process are selected to ensure good preparation heat treatment structure.
3. Before heat treatment, rust spots and oxide scales on the die surface should be completely eliminated, and attention should be paid to the protection during heating. If possible, vacuum heating or protective atmosphere heating should be used as far as possible, and good decarburization treatment should be carried out when heating in salt bath.
4. Correct formulation of die quenching and tempering process specifications, tempering should be sufficient.
5. Appropriate cooling medium and cooling method should be selected.
6. The water content of alkali bath should be strictly controlled for a long time.
7. Cryogenic treatment can be used for the mould with high hardness requirement.
8. In order to improve the service life of the die after quenching, the steel can be hardened.