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        Microstructure Evolution, Fracture and Hardening Mechanisms of Quenched and Tempered Steel for Large Sized Bearing Rings at Elevated Quenching Temperatures

        Bo Jiang,Zhen Mei,Leyu Zhou,Chaolei Zhang,Yazheng Liu 대한금속·재료학회 2016 METALS AND MATERIALS International Vol.22 No.4

        Based on 42CrMo steel, a steel with a higher C and Ni content is developed for use in large sized bearing rings. The impact energy and hardness of the quenched and tempered steel increase with the quenching temperature, but then decrease when the temperature is above 925 °C. When the temperature is below 925 °C, some larger M23C6-type carbides (with average diameters of 255.6 μm) exist in the quenched and tempered microstructure. The number of carbides is reduced as the quenching temperature increases. At the same time, the fracture modes change from microvoid coalescence and quasi-cleavage to microvoid coalescence. The number of round quasi-cleavage fractures, which are formed around the carbides, decrease as the number of carbides decrease. The existence of larger M23C6-type carbides leads to round quasi-cleavage fractures and decrease the impact energy. The precipitation strengthening of M23C6-type carbides increases the hardness at a quenching temperature of 925 °C.

      • KCI등재

        Effect of Transformation Temperature on the Ferrite–Bainite Microstructures, Mechanical Properties and the Deformation Behavior in a Hot-Rolled Dual Phase Steel

        Bo Jiang,Xuewen Hu,Leyu Zhou,Haibo Wang,Yazheng Liu,Fugang Gou 대한금속·재료학회 2021 METALS AND MATERIALS International Vol.27 No.2

        The ferrite–bainite microstructures and mechanical properties in a hot-rolled dual phase steel with different transformationtemperatures were investigated in this paper. The deformation behavior of bainite and ferrite was analyzed by using in situscanning electron microscope. The results showed that the ferrite and bainite can be refined and the volume fraction of bainitecan be slightly increased by 5% by decreasing the ferrite transformation temperature from 690 to 635 °C and increasing thebainite transformation temperature from 400 to 450 °C. The strengthening mechanism was dislocation difference in polygonaland acicular ferrite and then the grain size difference of ferrite. The hole expansion ratio increased from 52.9 to 83.7% withthe ferrite transformation temperature decreasing. The cracks or voids were smaller in size and also the number in steel withthe lower ferrite transformation temperature after expanding. The microstructure separation along the boundary betweenbainite and ferrite and the slip band in the ferrite appeared at the non-uniform deformation stage. However, fewer appearedin the microstructure of steel with lower ferrite transformation temperature. Cooperative deformation of ferrite and bainiteand finer microstructure in steel with lower ferrite transformation temperature were the mechanisms why the cracks couldnot easily nucleated and propagated, respectively. Thus this was also why there was a better hole expansion ratio.

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