Abstract
As research of the high cycle fatigue of carburized gear steel could not meet the status quo of longer and longer service lives, research of very high cycle fatigue (VHCF) performance has become the focus of current research. The VHCF properties of case-hardening steel 18CrNiMo7-6 after being carburized with gradient hardness distribution were investigated by means of ultrasonic fatigue tests. The results showed that the carburized specimens with a case hardness of 705 HV and core hardness of 530 HV showed VHCF phenomenon, and the fatigue lives continuously increased to even 109 cycles as the stress amplitude decreased to about 500 MPa. Observations of the fracture surfaces of the fatigue specimens showed that the fatigue crack initiation sites were located in the transition area with the hardness at about 580 HV. It was found that the transition area had low VHCF properties, since the core did not show VHCF phenomenon, and the case had a higher hardness. A fine microstructure was observed in the granular bright facet (GBF) area, and the stress intensity factor ΔKGBF was measured to be 3.04 MPam−1/2. The 109 cycles fatigue life was predicted based on the inclusion size, and the 1010 cycles fatigue life was 490 MPa based on the prediction model.
Highlights
The high-frequency facilities uesd to study the VCHF performance of homogeneous materials have become a subject of extensive research in recent decades
Researchers found that when the fatigue life was beyond 109 cycles, fatigue failure could still occur and that the conventional high cycle fatigue limit could not reflect from the very high cycle fatigue (VHCF) S-N data [1,2,3]
The granular bright facet (GBF) area can be clearly seen around a nonmetallic inclusion, which acted as the crack initiation site (Figure 8b,d)
Summary
The high-frequency facilities uesd to study the VCHF performance of homogeneous materials have become a subject of extensive research in recent decades. Hong et al [13,14] reported the microstructure distribution and formation mechanism of a GBF region of high-strength steel under ultra-high cycle fatigue and proposed the numerous cyclic pressing (NCP) model of the nanograin layer in the crack initiation region and indicated that repeated cyclic stress was the main reason for the formation of a nanograin layer in the GBF region. Nehila et al [18,19] indicated the ultra-high cycle fatigue performance of case-hardened steel with a hardness gradient in the early stage and found that the failure of the fatigue specimens would still occur beyond 107 cycles, and the stress amplitude of the surface failure was much higher than that of the internal failure. As the core component of carburized gear steel 18CrNiMo7-6 used in heavy-duty locomotive and rail transit, and the demand for the long lives of these important components, it is important to study the ultra-high cycle fatigue life performance of carburized gear steel 18CrNiMo7-6 [23,24]
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