45# steel is commonly used in the production of large-size 8.8-grade high-strength bolts. The heat treatment quenching and tempering process uses 830 to 840°C quenching and medium and high-temperature tempering. As the diameter is larger, the aqueous solution is generally used for cooling, but the water quenching tends to cause cracks and large deformations in the steel, which is particularly serious at the head and hexagonal edges. Therefore, improving the heat treatment process of 45# steel has attracted great interest from its peers.
For many years, everyone used to solve the 45# steel quench cracking problem with subcritical quenching. When subcritical quenching heating, due to the presence of undissolved ferrite in the structure, it will be embedded in the martensite after quenching to become residual ferrite, if according to the latest GB/T3098.1-2010 standard, martensite after quenching should reach about 90%, it is judged as a substandard product.
According to the latest information, the critical point temperatures for Ac1 and Ac3 of 45# steel are 724°C and 802°C, respectively. However, 800°C is selected to be close to the critical temperature. From the metallographic examination, it can be seen that the residual ferrite is similar to that of none. Critical state.
From the metallographic examination, it can be seen that with increasing the quenching temperature from 770°C to 840°C, the amount of residual ferrite is not obvious, and conventional quenching is performed at 830-840°C. There is no residual ferrite in the structure. At the lower quenching temperature of 800°C, the coexistence of lath-like and sheet-like martensite tends to refine and more uniform, and the quenching stress is relatively small, effectively reducing the risk of quench cracking.
From the test of mechanical properties, it is known that subcritical quenching at 770 to 790°C causes the residual ferrite to be embedded in the martensitic matrix, causing segregation of martensite. When tempering at medium and high temperatures, it will affect the tempered brucellus, The continuity of tempered sorbite, to a certain extent, has a debilitating effect on the strength. At the same temperature, the strength obtained after 800°C or conventional quenching and tempering is similar. The effect of tempering on the strength, in addition to the tempering temperature, mainly depends on the degree of solid solution strengthening of the martensite and the relative relationship with the carbides.
For the quenching at 800°C, the martensite in the 800°C and conventional quenched structure is not affected by the solidification of martensite due to the presence or absence of ferrite in the microstructure. The solubilities should all be similar, so after subcritical quenching + tempering near Ac3 (800°C), the strength is not much different from the strength after normal quenching and tempering. To this end, 45# steel high-strength bolt hardening hardening process is recommended for 800 °C quenching.
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