The hardening and quenching of Grade 90.9 bolts of 42CrMo steel should be austenitized sufficiently to obtain a martensitic microstructure of about 90%. When the tempering temperature is 540°C, the microstructure and properties are the best. This is tempered at low and medium temperatures. When the martensite and ferrite phases have large differences in strength, stress concentration and segregation can easily occur, resulting in non-uniform microstructure and poor performance. When the 42CrMo steel bolts are tempered at 540°C, the difference in strength of the two phases is reduced, which helps to improve the stress distribution and the homogenization of the structure.
After 42CrMo steel bolts were tempered and tempered, the effect of tempering temperature on the metallographic structure all experienced a process from coarse to small. The different tempering process after quenching is mainly the elimination of stress in the quenched structure, the stabilization of the structure, and the precipitation of carbides in the matrix.
When tempered at less than 400°C, the structure is tempered martensite, undissolved ferrite and a small amount of retained austenite, and carbides precipitated during tempering at medium and low temperatures are finer. With the increase of tempering temperature, carbon atoms are continuously precipitated to form carbides. The first precipitated carbides continue to grow to form tempered bentonite. At 540°C, the carbides continue to grow and granularize, and martensite is formed. Transformation into ferrite structure, the formation of tempered sorbite and insoluble ferrite, at this time, the steel's strength, hardness, plasticity and impact toughness have increased significantly.
Production practice shows that the tempering temperature of 42CrMo steel bolt tempering must be above 540 °C, otherwise the low temperature impact energy -45 °C ≥ 27J may be unqualified.
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