Comparison of Very High Cycle Fatigue Properties of 18CrNiMo7-6 Steel after Carburizing and Pseudo-carburizing
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Comparison of Very High Cycle Fatigue Properties of 18CrNiMo7-6 Steel after Carburizing and Pseudo-carburizing Peifeng Cheng
, Yunkun Li, Wenchao Yu, Shaopeng Yang, Fangzhong Hu, Jie Shi, Maoqiu Wang, and Li Li
Submitted: 26 August 2020 / Revised: 8 October 2020 / Accepted: 13 October 2020 The very high cycle fatigue (VHCF) properties of case hardening steel 18CrNiMo7-6 after carburizing and pseudo-carburizing have been investigated by means of ultrasonic fatigue tests. Results showed that the pseudo-carburized specimens with the hardness of 442 HV did not show VHCF fracture, and fatigue fracture did not occur even after 109 cycles when the applied stress amplitude was lower than the high cycle fatigue limit of 650 MPa obtained at 107 cycles. When carburized thoroughly with the hardness of 700 HV, the experimental steel showed VHCF phenomenon, and it might fracture even at nearly 109 cycles when the stress amplitude was about 600 MPa. Observations of the fracture surfaces of the fatigue specimens showed that after carburizing, the fatigue crack initiation sites changed from martensite matrix to nonmetallic inclusions, forming granular bright facet (GBF) and fish-eye before final abrupt fracture. The stress intensity factor at GBF area, which was calculated to be 3.5 MPa m1/2, could be used as a critical value for VHCF fracture for the carburized specimens of the experimental steel. The VHCF fracture of the carburized specimens of the experimental steel could be related to the high hardness and large amount of retained austenite as a result of carburizing. Keywords
carburizing, gear steel, S–N curve, stress intensity factor, very high cycle fatigue
1. Introduction Very high cycle fatigue (VHCF) properties of high-strength steels such as bearing steel JIS SUJ2, spring steel JIS SUP7 and bolt steel JIS SCM440 have been extensively investigated in recent years (Ref 1-4). It is known that high-strength steels may fracture at a stress below the classical fatigue limit, which is determined in the high cycle fatigue regime of 107 cycles, when they are stressed in the VHCF regime (Ref 5, 6). Thus, the fatigue limits under the life of 109 cycles, which can be readily measured by ultrasonic fatigue tests (Ref 7), are always used to characterize the VHCF properties of high-strength steels. From the engineering point of view, data of VHCF properties of high-
Peifeng Cheng, School of Materials Science and Engineering, Kunming University of Science and Technology, Kunming 650093 Yunnan, China; and Institute of Special Steel, Central Iron and Steel Research Institute, Beijing 100081, China; Yunkun Li, CN GPower Gearbox Co. Ltd., Chongqing 402263, China; Wenchao Yu, Jie Shi, and Maoqiu Wang, Institute of Special Steel, Central Iron and Steel Research Institute, Beijing 100081, China; Shaopeng Yang, Institute of Special Steel, Central Iron and Steel Research Institute, Beijing 100081, China; and Technology Center, Maanshan Iron and Steel Co. Ltd., Maanshan 243000 Anhui, China; and Fangzhong
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