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Effect of nickel content on rolling-sliding contact fatigue of carburized steels
Novack, Nicholas
Novack, Nicholas
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2020
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2021-07-03
Abstract
The purpose of this study was to evaluate the effects of nickel (Ni) content on the performance of steels subjected to rolling-sliding contact fatigue (RSCF). Bars of four different commercially produced alloys (4120, 4320, 4820, and EN-30B) containing 0.13, 1.72, 3.38, and 4.15 wt pct Ni, respectively, were machined into 25.4 mm diameter test specimens and gas carburized. As-carburized specimens were tested on a modular RCF/RSCF testing machine designed and built at the Colorado School of Mines. Tests were conducted at a Hertzian contact stress of 3.2 GPa with a -20 pct slide ratio; the contacting surfaces were lubricated by a jet oil which was heated to 100 °C. Four specimens of each material condition were tested to failure defined as surface degradation by macropitting. Failures typically occurred between 450,000 and 6,000,000 cycles. Additionally, one specimen of each material condition was subjected to 200,000 cycles to evaluate the accumulation of microstructural damage prior to failure. The microstructures in all materials were observed to change after being subjected to RSCF for 200,000 cycles. All specimens showed evidence of a dark-etching region (DER), and three specimens showed evidence of white etching areas (WEAs). Hardness tests showed evidence of softening in the matrix. The exception to this was in regions which had high initial levels of retained austenite which transformed to martensite, resulting in a harder matrix. All specimen failures were the result of surface-initiated cracks which grew to a size sufficient to cause macropitting. Specimens with low-cycle failures appeared to fail exclusively due to the surface-initiated cracks, whereas specimens with high-cycle failures appeared to develop subsurface cracks prior to the surface-initiated macropitting. The subsurface cracks did not cause damage until they interlinked with surface-initiated cracks, allowing extensive material removal. The effects of several variables on RSCF performance were evaluated using multi variable regression analysis. In general, Ni content increased the fatigue life in the steels tested. Composite surface roughness was identified as a test variable that influenced the test results. Calculations indicate that the oil film was not thick enough to separate the contacting surfaces during the initial test cycles. Larger surface cracks were observed in specimens which failed after fewer cycles.
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