Effects of alloying on nitriding behavior and fatigue performance of medium carbon bar steels

Abstract

Nitriding is a surface hardening technique commonly applied to transmission gears for improved fatigue resistance. Steel alloys used for nitrided transmission components often do not have optimized compositions and thermal processing routes to maximize fatigue performance after nitriding. Furthermore, significant cost and processing time savings could be realized if nitriding could be performed without first heat treating to form tempered martensite. Additions of vanadium (V) and silicon (Si) to a series of experimental medium carbon steels were evaluated in order to improve fatigue performance after nitriding. ThermoCalc© simulations were used to design the compositions of four medium carbon steels based on a full factorial matrix of two levels of Si (0.7 and 1.5 wt pct) and two levels of V (0.1 and 0.23 wt pct). Each alloy was heat treated to form tempered martensite, which is common industrial practice. The alloys were also heat treated to form bainite, which is consistent with the microstructure commonly observed in the as-forged condition in medium carbon steels used for nitriding. The bainite conditions were also tempered prior to nitriding. Gas nitriding was performed on all four alloys with bainite and martensite microstructures. Higher V contents result in higher core and case hardness values primarily by increasing the volume fraction and resulting precipitation strengthening of MX precipitates. Higher Si contents result in higher core hardness values primarily by increased solid solution strengthening; refinement of cementite size and grain size is also observed in bainite microstructures. Increased Si contents result in higher case hardness values due to increased precipitation strengthening from higher volume fractions of (Si,Mn) nitrides in addition to the strengthening mechanisms observed in the core. Higher V and Si contents both lead to greater magnitudes of compressive residual stress after nitriding due to increases in the volume fractions of MX and (Si,Mn) nitrides, respectively. Cantilever bending fatigue testing was performed on nitrided specimens and specimens that simulate the core microstructure after nitriding. The distribution of applied and residual stresses near the surface of nitrided fatigue specimens was analyzed to determine vulnerable regions of crack initiation. The analysis correlated with experimentally observed failure locations. Increases in V and Si content lead to higher core hardness and higher core fatigue strength, which increase the applied stress needed for subsurface fatigue crack initiation. Increases in V and Si content also increase the magnitude of compressive residual stress, which increases the applied stress needed for surface initiated failure. The combination of increases in core fatigue strength and increases in the magnitude of compressive residual stress from higher V and Si contents result in superior fatigue performance after nitriding compared to other nitrided medium carbon steels reported in literature. The improvements in fatigue performance by alloying with V and Si may allow for nitriding to be performed in the as-forged condition resulting in decreased costs and processing time.