Analysis of Reasons and Countermeasures for Insufficient and Uneven Hardness of Molds
The hardness of mold heat treatment is a very important mechanical performance indicator, and unqualified hardness is a very serious defect. Insufficient or uneven hardness of the mold after heat treatment will reduce its wear resistance and fatigue strength, leading to early failure of the mold and seriously reducing its service life.
1. Causes
1) The cross-section of the mold is large, and the hardenability of the steel is poor. For example, large molds use steel grades with low hardenability.
2) The original structure of mold steel has severe carbide segregation or coarse structure, and there is segregation and aggregation of graphite carbon and carbides in the steel.
3) The forging process of the mold is incorrect, and the spheroidization annealing is not carried out well after forging, resulting in poor spheroidization structure of the mold.
4) The decarburization layer generated during annealing or quenching heating on the surface of the mold has not been completely removed.
5) The quenching temperature of the mold is too high, and there is too much residual austenite after quenching; Or the quenching temperature is too low, and the heating and insulation time is insufficient, resulting in incomplete phase transformation of the mold steel.
6) The cooling rate after quenching and heating of the mold is too slow, the grading and isothermal temperatures are too high or the time is too long, and the selection of quenching cooling medium is inappropriate.
7) Insufficient moisture in the alkaline bath, excessive impurities in the quenching cooling medium, or aging of the quenching cooling medium.
8) After quenching and cooling of the mold, the temperature of the quenching cooling medium is too high and the cooling is insufficient.
9) Insufficient tempering and high tempering temperature.
Analysis of Reasons and Countermeasures for Insufficient and Uneven Hardness of Molds
2. Countermeasures
1) The correct choice of mold steel type, large molds should use high hardenability high alloy mold steel.
2) Strengthen the inspection of raw materials to ensure that they meet the standards. Reasonable forging and spheroidization annealing treatment should be carried out on defective raw material steel to ensure good microstructure. Carbon tool steel is not easily annealed multiple times to prevent graphitization.
3) Strictly implement the forging process and spheroidization annealing process to ensure a good pre heat treatment structure.
4) Before heat treatment, the rust spots and oxide scale on the surface of the mold should be thoroughly removed, and attention should be paid to protection during heating. Vacuum heating quenching or protective atmosphere heating quenching should be used as much as possible. Good deoxidation treatment should be carried out during salt bath heating.
5) Correctly formulate the quenching and heating process parameters for molds, ensure sufficient phase transformation, and rapidly cool at a cooling rate greater than the critical cooling rate to obtain qualified metallographic structures.
6) Correctly select quenching cooling medium and cooling method, strictly control grading and isothermal temperature and time.
7) It is necessary to strictly control the moisture content of the alkaline bath, filter and replace the quenching cooling medium that has been used for a long time, keep it clean, and regularly test its quenching cooling characteristic curve.
8) For large-sized molds, the immersion time in the quenching cooling medium should be appropriately extended to prevent the temperature of the quenching cooling medium from being too high.
9) After quenching, the mold should be tempered promptly and fully, and the tempering temperature should be prevented from being too high.
10) Molds with high hardness requirements can be subjected to cryogenic treatment (such as -110~-196 ℃).
11) Perform surface strengthening treatment.