Study On Microstructure And Hardness Of42CrMo Steel Hot-rolled Rod

The42CrMo hot rolled round bar which produced by Small rolling company Brinell hardness is too high, so that cold shear and rod, drawing, processing is easy to form crack, fracture and other defects, caused very big effect to the sale and use of products. We found that core region of42CrMo hot rolled round bar has a large number of acicular martensite. The Acicular martensite has high mechanical property and hardness, but the plasticity is poor. A lot of this microstructure is bound to cause great influence on material properties This essay reference a large number of documents, by pickling, metallurgical,microscope, hardness tester, SEM, EDS, direct reading spectrometer and OPA, the small rolling42CrMo hot rolled round bar high hardness and martensite are analyzed.Analysis and showed that the acicular martensite microstructure of hard and brittle has led to high hardness, and the segregation of alloy elements is a major cause of forme acicular martensite.According to the results of the analysis, this paper, diffusion annealing test, slow cooling test at high temperature and low temperature,investigates had been done in order to study how to reduction or elimination of42CrMo steel core martensite microstructure and hardness, improving its mechanical properties. Thereby reducing the cracking phenomenon of42CrMo steel during subsequent cold shear, cold drawing, rod and induction heating, improve product quality, reduce the amount of customer objection.By the high temperature diffusion annealing test.The results show that it is difficult to make it even unless have enough time (30～40hours),because the diffusion of42CrMo steel alloy elements is very slow. Therefore, the high temperature diffusion annealing is difficult for industrial production.By slow cooling tests at high temperature show that, due to steel cooling process, high temperature cooling conditions are consistent with low temperature. Due to the large temperature difference at high temperature, so the cooling speed is fast. Therefore, the cooling speed is reduced at high temperature that can reduce the cooling speed in the cooling process.The test results show that the cooling rate dropped to0.8℃/s,42CrMo steel will be completely transformed into the pearlite and ferrite.Slow cooling tests at low temperature show that it can effectively improve the hardness, microstructure and mechanical properties of42CrMo steel,if the round steel cooling to a certain temperature then put into the slow cooling box. The test results show that:when the slow cooling temperature increased from200℃to400℃, martensite proportion, Brinell hardness, tensile strength decreased, the contraction section increased and performance improved significantly. But increased from400℃to600℃, the performance improvement was found to be insignificant. Therefore, the temperature of400℃～500℃is the right temperature for slow cooling starting.