Influences of Vanadium and Silicon on Case Hardness and Residual Stress of Nitrided Medium Carbon Steels
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NTRODUCTION
SURFACE hardening processes like nitriding are commonly used to improve the performance of fatigue limited machine components such as transmission gears.[1] Nitriding involves the diffusion of nascent nitrogen into the surface of a steel component typically at temperatures where ferrite is stable. The source of nitrogen can be nitrogen-containing plasma, molten salt, or heated gas. Nitriding is typically performed at relatively low temperatures and does not involve austenite decomposition during cooling, so there are less thermal and transformation strain contributions to distortion compared to carburizing or induction hardening.[2,3] The diffusion of nitrogen into the surface of a JONAH KLEMM-TOOLE, AMY J. CLARKE, JOHN G. SPEER and KIP O. FINDLEY are with the Colorado School of Mines, G.S. Ansell Department of Metallurgical and Materials Engineering, 1500 Illinois St., Golden, CO 80401. Contact e-mail: [email protected] MICHAEL BURNETT is with TimkenSteel, 1835 Dueber Ave. SW, Canton, OH 44706-2728. Manuscript submitted June 23, 2020; Accepted October 11, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
steel results in solid solution strengthening as well as precipitation strengthening if nitride forming elements are present. During nitriding, several binary Fe-M alloys (where M = Cr, V, Ti, Al, Mo, Mn, or Si) have been reported to form nitrides.[4–7] The formation of MX phase (where M= Cr, Al, Mo, Ti, and V and X = N, C) having the B1 rock-salt crystal structure that contains multiple nitride forming elements has also been reported in Fe-Cr-Al,[4,8–10] Fe-Cr-Ti,[10,11] Fe-Cr-V,[10] and Fe-Cr-Mo-V alloys.[12] Alloying with V is of particular interest because the presence of V results in finer MX precipitates after nitriding compared to other nitride formers.[4,6,10] Figure 1(a) shows a compilation of literature data that summarize the influences of V content on increases in maximum case hardness after nitriding. Urao et al. studied the influence of adding 2.91 wt pct V to a Fe-0.14C-0.2Mo (wt pct) alloy on the maximum case hardness after nitriding at 550 °C for 4 hours. While microscopy results were not reported in this work, the increase in maximum case hardness with higher V content was hypothesized to be from an increased amount of nitride precipitates.[13] Miyamoto et al.
Fig. 1—Change in maximum case hardness after nitriding as a function of (a) V and (b) Si content. The change in maximum case hardness is referenced to the condition with the lowest alloy content for each data set. The slope of the dashed line from De Cooman et al. in (b) indicates the expected increases in hardness from Si solid solution strengthening. Data taken from Urao et al.,[13] Miyamoto et al.,[5] Miyamoto et al.,[10] Schwarz et al.,[14] Klemm-Toole et al.,[15] Riofano et al.,[19] Takase et al.,[20] Tomio et al.,[7] De Cooman et al.[21]
reported on the maximum case hardness of a binary Fe-1V (wt pct) alloy plasma nitrided at 570 °C for 20 hours. The increase in maximum case hardness compared to nitrided pure Fe was associa
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