Nano- and Microhardness Distribution in the Carburized Case of Nb-Microalloyed Gear Steel
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JMEPEG https://doi.org/10.1007/s11665-020-04992-7
Nano- and Microhardness Distribution in the Carburized Case of Nb-Microalloyed Gear Steel W. Chen, X.F. He, W.C. Yu, J. Shi, M.Q. Wang, and K.F. Yao (Submitted May 18, 2020; in revised form June 26, 2020) The characteristics of in-depth hardness distribution in the high-temperature carburized case of Nbmicroalloyed gear steel before and after cryogenic treatment were studied by nano-indentation method and Vickers hardness method. The results showed that there was no significant difference in Vickers hardness of the carburized case before and after cryogenic treatment, while the nano-hardness of the carburized case was increased after cryogenic treatment. The change of nano-hardness could be related to the decrease in retained austenite content. The retained austenite content in the surface of the carburized case was 26%, which was reduced to 18% after cryogenic treatment at 2 196 °C, with the nano-hardness increased from 11.8 to 12.6 GPa. Nano-indentation can be used to reliably measure the properties of the high-temperature carburized case with fine retained austenite for Nb-microalloyed gear steel. Keywords
carburized case, gear steel, nano-hardness, retained austenite, Vickers hardness
1. Introduction High-temperature carburizing for gears has several advantages in comparison with conventional carburizing at 930 °C, because it can reduce process time, improve efficiency and lessen gas pollution (Ref 1, 2). New gear steels with fine grain sizes are needed for high-temperature carburizing process since austenite grains tend to coarsen at higher temperatures. Nbmicroalloying has been widely regarded as the best way to develop new gear steels for high-temperature carburizing, due to the pinning effect of niobium precipitates on austenite growth (Ref 3-6). On the other hand, Nb-microalloyed gear steels have been found to have excellent performance including fatigue resistance, even after high-temperature carburizing (Ref 7, 8). After carburizing, gears usually have very high hardness and wear resistance in the case and excellent toughness in the core. The high hardness in the carburized case is due to the highcarbon martensite in the microstructure. However, there is also a high fraction of retained austenite in the carburized case, which may influence the case properties remarkably after phase transformation (Ref 9-14). However, the mechanisms of the transformation as well as the properties of the new phases are not very well understood yet. Because of these complex
W. Chen, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, PeopleÕs Republic of China; and Institute of Special Steel, Central Iron and Steel Research Institute, Beijing 100081, PeopleÕs Republic of China; X.F. He, W.C. Yu, J. Shi, and M.Q. Wang, Institute of Special Steel, Central Iron and Steel Research Institute, Beijing 100081, PeopleÕs Republic of China; and K.F. Yao, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, PeopleÕs Republic of C
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