SGER: Paraequilibrium Carburization of Ferritic and Martensitic Stainless Steels

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TECHNICAL: Research directed towards low temperature paraequilibrium carburization of ferritic stainless steels will be done. Paraequilibrium here refers to conditions where carbon interstitials are mobile, whereas substitutional solutes such as Cr and Ni are effectively frozen because of the low temperatures of the carburization heat treatment. Prior studies have demonstrated that carburization of austenitic stainless steels, e.g. 316L, allows surface carbon concentrations of ~ 14 at % to be realized but absent carbide formation, and with a case ~ 25 um thick, having a Vickers hardness of 1200. Such hardened austenitic stainless steels have improved wear resistance and enhanced corrosion resistance, without significant attenuation of the inherent ductility of austenitic steels. Further, fatigue life of these austenitic steels containing such hardened cases are significantly increased, because of the residual compressive stresses generated during carburization. The high risk, high profit payoff research is intended to explore the potential for paraequilibrium carburization of ferritic stainless steels. Preliminary experiments involving a PH 13-8 Mo stainless steel which has a bcc matrix indicate that Vickers hardnesses up to 1400 may be possible. Knowledge of paraequilibrium carburization of ferritic materials is virtually nil. PIs will undertake experiments with EBRITE (Registered), a commercial alloy with a particularly simple chemistry, to establish conditions under which low temperatures carburization absent carbide formation may be possible for ferritic stainless steels. In addition the response of two martensitic stainless steels, M152 and Ph-17-4, will be examined in this exploratory project. NON-TECHNICAL: The broader impact of the research concerns extending the utility of ferritic stainless steels in many applications. In particular, being able to improve their wear resistance, fatigue resistance and corrosion resistance, without attenuation of other properties, by a low cost conformal process that can be applied to finished components, as carburization involves virtually no dimensional changes, can be considered a disruptive technology. With the improved properties resulting from paraequilibrium carburization, the costs: benefit ratio changes in such a way that ferritic stainless steel will be preferred materials in many applications that now require more expensive materials, or will extend the lifetime of components currently fabricated from stainless steels. On the pedagogical front, the combination of materials science subdisciplines that have to be brought to bear on this research - thermodynamics, kinetics, microstructural characterization, and mechanical behavior - makes this topic an ideal topic for graduate education in materials science.