Enhanced Corrosion Resistance of Stainless Steel Carburized at Low Temperature
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CASE hardening of stainless steels by traditional carburization techniques has seldom been employed in applications requiring corrosion resistance, due to the resultant formation of chromium carbides, which depletes chrome and decreases the resistance to corrosion. These carbides also compromise fatigue resistance. Williams et al.[1–4] have developed a low-temperature (450 C to 500 C) process for introducing carbon into stainless steels without formation of carbides. The surface modification technique has been termed ‘‘low temperature colossal supersaturation’’ (LTCSS) and is a paraequilibrium process. Paraequilibrium in this context refers to the fact that diffusion kinetics of substitutional solutes such as Cr and Ni in a 316-type composition are orders of magnitude slower than interstitial solutes, such that they are effectively immobile, whereas carbon can diffuse to depths of 20 to 30 lm in industrially relevant times. This kinetic limitation prevents formation of carbides and allows carbon to be introduced at concentrations orders of magnitude greater than the equilibrium solubility. Details on the LTCSS process can be found elsewhere.[1–8] Because the carburization process involves a gaseous source of carbon at 1 atm pressure, it F.J. MARTIN, P.M. NATISHAN, and R.A. BAYLES, Doctors, Chemistry Division, and R.J. RAYNE, Research Engineer, Materials Science and Technology Division, are with the United States Naval Research Laboratory, Washington, DC 20375. Contact e-mail: [email protected] E.J. LEMIEUX, Research Scientist, is with Chemistry Division, United States Naval Research Laboratory, Key West, FL 33040. T.M. NEWBAUER, Research Engineer, is with SAIC/NRL Operations, Key West, FL 33040. H. KAHN, G.M. MICHAL, F. ERNST, and A.H. HEUER, Professors, are with Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, OH 44106. This article is based on a presentation given at the ‘‘International Conference on Surface Hardening of Stainless Steels,’’ which occurred October 22–23, 2007 during the ASM Heat Treating Society Meeting in Cleveland, OH under the auspices of the ASM Heat Treating Society and TMS. Article published online June 24, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
is conformal and not restricted to flat surfaces or line-ofsight surface access. The LTCSS treatment of austenitic stainless steels has been shown[5,6] to dramatically improve hardness, wear resistance, and fatigue resistance, with little attenuation of the inherently good ductility of these materials. Under these conditions, and provided the surface has been properly activated,[6] surface carbon concentrations of ~15 at. pct can be generated in the near-surface region encasing the entire treated component. Vickers hardness values of ~11 GPa (comparable to a Rockwell C hardness >70) have been obtained, representing a significant increase in hardness over the core material (Vickers hardness of ~4 GPa). The large concentration of interstitial carbon in near-surface regions also induces a significant l
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