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Effect of nickel and retained austenite on rolling sliding contact fatigue resistance in carburized steels, The
Orr, Ashlyn
Orr, Ashlyn
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2024
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The objective of this study is to investigate the influence of nickel (Ni) content and retained austenite (RA) on rolling-sliding contact fatigue (RSCF) life in carburized steel. In order to evaluate Ni and retained austenite effects, this study utilizes carburized steel specimens of 4120 (0.13 wt pct Ni, 15.2 vol pct RA), 4820 (3.38 wt pct Ni, 20.2 vol pct RA), and EN 30B (4.15 wt pct Ni, 34.2 vol pct RA) which were subjected to RSCF testing. The 4120, 4820, and EN 30B were gas carburized with a resulting case depth of approximately 1.1, 1.1, and 1.2 mm, respectively, based on a hardness of 550 HV. Specimens were surface ground to an average surface roughness of 0.2 µm to decrease the effect of as carburized surface roughness on the fatigue life. The ground specimens underwent RSCF testing with a surface stress of 2.5 GPa and a slide to roll ratio of -20 pct, until a pit formed, as detected by an accelerometer. The pits that form on the surface of the specimens were analyzed with secondary electron microscopy, macrophotographs, and light optical microscopy. The pits that form from the RSCF testing conditions are surface-initiated. The fatigue life of the 4820 and EN 30B conditions is higher than the fatigue life of the 4120 specimens, suggesting that the higher Ni level is beneficial to the fatigue life. A Weibull analysis showed that the EN 30B and 4820 conditions consistently have a longer RSCF life than the 4120. The RSCF lives of the ground specimens were compared to previous RSCF testing on as carburized specimens. Generally, the surface grinding results in a lower degree of scatter in the RSCF lives.
After RSCF testing of ground specimens, the 4120 and EN 30B conditions were cryotreated to decrease the RA content and evaluate how the decrease in RA impacted the RSCF lives of the conditions. The cryotreated 4120 and EN 30B conditions had 10 and 13 vol pct RA, respectively. The specimens were tested with a surface stress of 3.2 GPa and slide to roll ratio of -20 pct. Despite similar levels of RA, the EN 30B condition outperformed the 4120 condition. The improved performance of the EN 30B is evidence that difference in fatigue lives is not associated with RA differences, perhaps suggesting the intrinsic effects of Ni are more beneficial to RSCF life.
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