Effect of Plastic Pre-straining on Residual Stress and Composition Profiles in Low-Temperature Surface-Hardened Austenit
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SURFACE engineering techniques are widely used to modify material surface properties. Through thermal, mechanical and chemical surface treatments, it is possible to tailor the resistance against wear, corrosion, and fatigue and thus enhance the component performance.[1] Most surface engineering techniques are associated with the introduction of residual stress-depth profiles, which can have a detrimental or a favorable effect on the performance of materials and components under certain conditions.[2] Among surface engineering techniques, a growing interest is observed in low-temperature gaseous processes of stainless steels, because, compared to existing treatments, these techniques allow a significant improvement of wear and fatigue resistance, without impairing (but rather improving) the material´s corrosion performance.[3–5] The significant improvement of the material surface properties after low-temperature nitriding (LTN), nitrocarburizing (LTNC), or carburizing (LTC) is due to the dissolution of a colossal amount of nitrogen and/or carbon in the stainless steel matrix, forming a supersaturated solid solution known as expanded austenite.[6,7] The interstitial concentration profile of the dissolved nitrogen and carbon atoms has been reported to lead to the development of enormous compressive residual FEDERICO BOTTOLI, Ph.D. Student, GRETHE WINTHER, Associate Professor, THOMAS L. CHRISTIANSEN, Senior Researcher, and MARCEL A.J. SOMERS, Section Head, Professor, are with the Department of Mechanical Engineering, Technical University of Denmark, Produktionstorvet b. 425, 2800 Kgs. Lyngby, Denmark. Contact e-mail: [email protected] Manuscript submitted November 26, 2015. Article published online June 15, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
stresses in the near surface region parallel to the material’s surface, from 2 to 3 GPa after carburizing up to 8 GPa after nitriding.[8–10] The composition-induced stresses are usually evaluated by angle-dispersive diffraction methods using the radiation produced by conventional X-ray tubes. Quantitative assessment of residual stresses in expanded austenite is challenging as the several properties influencing the lattice spacing measured in an X-ray diffraction experiment can change considerably within the depth range probed in an experiment.[2] (Steep) gradients in the local lattice spacing, as a consequence of gradients in composition, stress, and stacking fault density can have a significant effect on the result obtained in residual stress determination.[2,9–11] Furthermore, elastic constants[9] and thermal expansion coefficients[12] (and magnetic properties[13]) depend strongly on the interstitial content dissolved in expanded austenite. Extensive research has been carried out with both destructive and non-destructive XRD methods to evaluate the magnitude of the compressive stresses and to take into account the influence of the various parameters.[8–11,14,15] The conventional ‘‘sin2w’’ method, using symmetric Bragg–Brentano geometry, leads to a significant variation of t
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