Effect of Microstructural Banding on the Fatigue Behavior of Induction-Hardened 4140 Steel
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ction hardening of steel components is a common industrial practice due in part to its effective and highly adaptable method of creating surface-hardened or through-hardened steel parts. With proper parameters, induction hardening can create an increase in strength and compressive residual stresses at the surface of a part, thus improving fatigue properties.[1] However, there exist concerns about the influence of microstructural banding on the uniformity of the induction process and the subsequent effect on the fatigue behavior of the steel. During induction hardening, prior microstructural banding can have a pronounced effect on the final microstructure. Anderson et al.[2] demonstrated that the orientation of bands affects local case depth and microstructure in induction-hardened 4145M steel. The final microstructure formed during cooling of an induction-hardened part is influenced by the chemical composition of the local region. This fact combined with the chemical segregation present in banded materials can lead to local variations in the case depth, which may
M.L. HAYNE, Graduate Research Assistant, K.O. FINDLEY, Assistant Professor, and C.J. VAN TYNE, FIERF Professor, are with the Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO 80401. Contact e-mail: kfi[email protected] P.I. ANDERSON, Principal Product Engineer – Steel, is with The Timken Company, Canton, OH 44706. Manuscript submitted November 13, 2012. Article published online June 4, 2013 3428—VOLUME 44A, AUGUST 2013
affect mechanical properties such as fatigue, which are often dependent on local microstructures. A result of chemical segregation of substitutional alloying elements, microstructural banding is a common industrial occurrence in cast and hot-rolled steels. Segregation of the substitutional alloying elements is a product of dendritic growth during solidification.[3–7] Long products are broken down and hot rolled after casting, during which time the substitutional alloy element segregation is deformed into bands or layers parallel to the rolling direction. These chemical compositional bands consist of alternating layers of solute-rich and solute-poor regions. The presence of these bands, combined with further deformation or heat treatments, determines if and where microstructural banding will form. Ferrite and pearlite bands are commonly observed, but bands of martensite or bainite are also possible depending on the composition and cooling from the austenitic region. Other studies have examined the effects of microstructural banding on various mechanical properties and processing methods,[2,8–12] but many did not include fatigue and/or induction hardening. Some examples of studies on the effect of microstructure banding on fatigue properties include fatigue crack growth in banded duplex stainless steel,[13] fatigue crack growth in friction stir-welded 2024-T3 aluminum,[14] and short crack growth in banded ferrite-pearlite steels.[15] In low carbon steel, attempts to identify the impact of microstructural banding on m
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