The reason is that a lot of alloy elements are added into the casting during the process of proportioning, among which Mo is a strong carbide forming element, which can refine the grain size, improve the uniformity of carbides and greatly improve the tempering stability of the test steel. Even at higher tempering temperature, the martensite lath morphology of the test steel can still be stable.
The hardness test shows that the hardness of the test steel increases when it is tempered at 550 ℃, which may be the secondary hardening effect caused by the precipitation of carbide forming elements such as Cr and Mo when tempered above 400 ℃. The decrease of shock absorption energy is due to secondary hardening.
In order to meet the application requirements, the strength and toughness of the test steel for lock ring after tempering at different temperatures were studied. The results show that with the increase of tempering temperature, the strength and hardness of the steel gradually decrease, and the elongation and impact energy increase gradually. After 890 ℃× 2 h water quenching and 620 ℃× 2 h water cooling tempering, the steel has the highest strength and plasticity and the best comprehensive mechanical properties. Its yield strength is 917mpa, tensile strength is 978mpa, elongation after fracture is 16%, Brinell hardness is 318hbw, impact energy is 29 J, which can meet the requirements of the standard. Compared with the mechanical properties of 45 steel and 40Cr alloy steel, it can be seen that the yield strength, tensile strength and hardness of the test steel are higher than those of the 45 steel after water cooling at 840 ℃ and air cooling at 600 ℃ for 0.4 H. From the point of view of elongation after fracture, the elongation after fracture of 40Cr steel after 850 ℃× 2H cooling + 520 ℃× 2H water cooling treatment is obviously better than that of 40Cr. To sum up, the test steel has better properties after 620 ℃ tempering treatment, which can meet the performance requirements of large size locking ring.