The cracks of the lost foam mold after quenching are a major defect in the heat treatment process of the mold, which will lead to the scrapping of the processed mold and cause great losses in production and economy.
(1) Causes
There is severe network carbide segregation in the mold material. There is machining or cold plastic deformation stress in the mold. Improper heat treatment operation (heating or cooling too fast, improper selection of quenching cooling medium, cooling temperature is too low, cooling time is too long, etc.).
The complex shape of the mold, uneven thickness, sharp corners and threaded holes, etc., cause excessive thermal stress and tissue stress. Quenching heating temperature is too high to produce overheating or overburning. Tempering is not timely after quenching or the tempering holding time is insufficient. When repairing and quenching heating, heating and quenching are performed again without intermediate annealing. Heat treatment, improper grinding process. When EDM after heat treatment, high tensile stress and microcracks exist in the hardened layer.
(2) Preventive measures
Strictly control the internal quality of mold raw materials, improve forging and spheroidizing annealing processes, eliminate mesh, band, and chain carbides, and improve the uniformity of spheroidizing structures. After machining or cold plastic deformation, the mold should be subjected to stress relief annealing (>600°C) and then heating and quenching. For molds with complex shapes, asbestos should be used to block the threaded holes, and dangerous sections and thin walls should be wrapped, and graded quenching or austempering should be used.
Annealing or high temperature tempering is required when repairing or refurbishing molds. Preheating should be taken when quenching and heating, precooling measures should be taken when cooling, and a suitable quenching medium should be selected. The quenching heating temperature and time should be strictly controlled to prevent the mold from overheating and overburning.
After the mold is quenched, it should be tempered in time, and the holding time should be sufficient, and the high-alloy complex mold should be tempered 2-3 times. Choose the correct grinding process and suitable grinding wheel. Improve the mold electric discharge machining process, and start the operation of stress relief tempering.