Analysis of abnormal microstructure and properties of 18CrNiMo7-6 steel gear

doi:10.13251/j.issn.0254-6051.2022.04.046

Abstract

Abstract: 18CrNiMo7-6 steel gear was difficult to machine in the root of the gear during the milling process because of its abnormal microstructure and properties. The reasons for abnormal micro structure and properties of 18CrNiMo7-6 steel gear were analyzed by metallographic microscope (OM), field emission scanning electron microscope (FESEM) and EPMA electron probe. The results show that the difficult-machining of the gear can be attributed to the formation of bainite in the root of the gear that results in excessive hardness. The EPMA electron probe verify that the appearance of bainite at tooth root is related to chemical composition segregation. The JMatPro calculation results show that the increase of C and alloying elements expand the temperature range of bainite transformation and reduces the critical rate of that. The bainite structure is formed at a low cooling rate. The influence of Cr and Mn elements on bainite transformation is the most significant factor. When the content of C, Cr and Mn is increased by about 50%, the critical cooling rate of bainite transformation decrease from 0.1 ℃/s to 0.02 ℃/s, and the bainite transformation temperature range at 0.1 ℃/s is expanded to 49 ℃.

Key words: 18CrNiMo7-6 gear steel, bainite, microstructure, composition segregation

CLC Number:

TG142.3

Zhang Ning, Gao Xing, Wang Zhilin, Jiang Bo, Liu Yazheng. Analysis of abnormal microstructure and properties of 18CrNiMo7-6 steel gear[J]. Heat Treatment of Metals, 2022, 47(4): 268-273.

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