In production, 45# steel is used to manufacture large-size 8.8 high-strength bolts. The heat treatment and quenching process is 830~840°C quenching + medium and high temperature tempering. Due to the large diameter, it is generally cooled by an aqueous solution, but water quenching tends to cause cracks and large deformation in the steel, especially at the head and hexagonal edges. Therefore, improving the heat treatment process of 45# steel has aroused great interest from peers.
For many years, everyone used to solve the quenching cracking problem of 45# steel by sub-temperature quenching. When quenching by sub-temperature, due to the presence of undissolved ferrite in the structure, it is embedded in the martensite after quenching and becomes residual ferrite. According to the latest GB/T3098.1-2010 standard, the martensite should be about after quenching. 90%, it is judged as a non-conforming product.
According to the latest data, the critical point temperatures of Ac1 and Ac3 of 45# steel are 724 °C and 802 °C, respectively, and 800 °C is selected just near the critical temperature. From the metallographic examination, it can be seen that residual ferrite seems to be absent. a critical state.
From the metallographic examination, it can be seen that as the quenching temperature is raised from 770 ° C to 840 ° C, the amount of residual ferrite is not obvious, and 830-840 ° C is conventionally quenched, and there is no residual ferrite in the structure. At 800 ° C quenching at a lower temperature, the lath and sheet martensite coexist, tend to be finer and more uniform, and the quenching stress is smaller, effectively reducing the risk of quenching cracking.
From the mechanical properties test, it can be known that the sub-temperature quenching at 770-790 °C embeds residual ferrite in the martensite matrix, causing a splitting effect on martensite. When tempering at medium and high temperatures, it will affect the tempered torsite. The continuity of the tempered sorbite has a certain degree of weakening 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 strength is not only related to the tempering temperature, but also depends on the degree of solid solution strengthening of martensite and the relative relationship with carbide.
For 800 °C quenching, martensite in 800 ° C and conventional quenched microstructures is not only from the morphology of the tissue, but also because the ferrite in the microstructure has little effect on the solid solubility of martensite. The solubility should be similar, so after the sub-temperature quenching + tempering near Ac3 (800 ° C), the strength is not much different from the strength after conventional quenching and tempering. To this end, the 45# steel high-strength bolt quenching and toughening process is recommended to be 800 ° C quenching water. 
Editor in charge: Hu Na
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