Formation mechanism and elimination of black structure in 2.25Cr-1Mo-0.25V steel

doi:10.13251/j.issn.0254-6051.2023.09.046

Abstract

Abstract: Through analyzing the micro-area chemical composition, optical and SEM microstructure, the “black structure” of large 2.25 Cr-1Mo-0.25V steel forgings was studied. The results show that it is “M₂₃C₆ carbide accumulation area”. By verifying its evolution law, it is found that the “black structure” is the root cause of relative low and scattered low temperature impact property of the forgings, and its formation mechanism is related to the formation of large block M-A islands at grain boundary during quenching and then decomposing and transforming into carbide accumulation during tempering at high temperature tempering, while the formation of large block M-A islands is caused by the accumulated M₂₃C₆ carbides existed in the as-forged steel. Through the process test and based on its formation mechanism, it is further found that the “black structure” can be eliminated by annealing pretreatment above 940 ℃, and the mechanical properties of the forgings after reheat treatment are significantly improved.

Key words: 2.25Cr-1Mo-0.25V steel, black structure, carbide, M-A islands

CLC Number:

TG157

Du Junyi, Zhu Yongyou, Jiang Zhonghua, Duan Hongling. Formation mechanism and elimination of black structure in 2.25Cr-1Mo-0.25V steel[J]. Heat Treatment of Metals, 2023, 48(9): 271-278.

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