Characterization of the microstructure and hardness of case-carburized gear steel

Wu Chen; Xiaofei He; Wenchao Yu; Jie Shi; Maoqiu Wang; Kefu Yao

doi:10.1016/j.micron.2021.103028

Characterization of the microstructure and hardness of case-carburized gear steel

Micron. 2021 May:144:103028. doi: 10.1016/j.micron.2021.103028. Epub 2021 Feb 2.

Authors

Wu Chen¹, Xiaofei He², Wenchao Yu², Jie Shi², Maoqiu Wang³, Kefu Yao⁴

Affiliations

¹ School of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China; NCS Testing Technology Co., Ltd., No.13 GaoLiangQiao XieJie, Beijing 100081, PR China.
² Institute of Special Steel, Central Iron and Steel Research Institute, No.76 XueYuanNanLu, Beijing 100081, PR China.
³ Institute of Special Steel, Central Iron and Steel Research Institute, No.76 XueYuanNanLu, Beijing 100081, PR China. Electronic address: maoqiuwang@hotmail.com.
⁴ School of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China.

PMID: 33647773
DOI: 10.1016/j.micron.2021.103028

Abstract

The microstructure and hardness of case-hardened steel were investigated after carburizing and austenitizing at 820-900 °C, and oil quenching and tempering at 180 °C. The carburized case had a multiphase microstructure consisting of martensite, carbides, and retained austenite, and the maximum content of the retained austenite was 30%; the particle size range was 2-3 μm. The nano-hardness decreased from about 12 GPa near the surface to about 7 GPa in the core, and the microhardness decreased from 800 HV_0.2 to 450 HV_0.2. The in-depth distribution of the microhardness and nano-hardness showed a similar trend, and the ratio of nano-hardness to microhardness was about 15. The results were attributed to the fine particle size of the retained austenite and its even distribution in the martensite matrix and it could not lower the nano-hardness. The nano-hardness was relatively low in areas of the retained austenite (about 5.5 GPa), and pop-in effects were observed, indicating the phase transformation of the retained austenite during nanoindentation loading.

Keywords: Carburized case; Gear steel; Hardness; Nanoindentation; Retained austenite.