Phase Equilibria and Phase Transformations in Stainless Steels Containing "Colossal" Concentrations of Interstitial Carbon

Case Western Reserve University, Cleveland OH

Investigators

Abstract

TECHNICAL: Phase equilibria and phase transformations will be studied in stainless steels carburized at low temperatures (725-750K) under paraequilibrium conditions. Prior experience with austenitic stainless steels has shown that "colossal" concentrations of interstitial carbon can be achieved using paraequilibrium carburization and lead to very much improved properties, including "superhardness" and enhanced wear, fatigue, and corrosion resistance. Under a NSF SGER program, PI have demonstrated that low temperature paraequilibrium carburization is also possible for at least some ferritic and martensitic stainless steels and that a carburization-induced ferrite-to-austenite phase transformation can occur in some alloys. PIs have put forth a combined computational/experimental program to study low temperature paraequilibrium carburization of this class of stainless steels. The computational work will employ the CALPHAD formalism and explore the composition space where ferritic alloys should develop a colossal interstitial carbon supersaturation and remain ferritic through the entire process. A second group of ferritic alloys will transform to austenite during carburization, and the composition space where this is possible will likewise be explored computationally. A suite of ferritic alloys predicted to display both types of behavior will then be melted, rolled into sheet form, carburized, and characterized. A further important aspect of the prior work on austenitic steels is the strong compositional dependence of carbon diffusivity at the low temperatures and large carbon concentrations inherent in paraequilibrium carburization. This will be explored in detail. PI would be able to explore the concentration dependence of carbon diffusivity in ferritic alloys, again at the low temperatures and large carbon concentrations afforded by paraequilibrium carburization. Such studies of carbon diffusion would not be possible absent the greatly enhanced solubility of carbon in ferritic alloys that can be achieved by low temperature carburization. Stainless steels carburized under paraequilibrium conditions have the promise to greatly extend the application arena where these materials can be used. As mentioned already, the process is inexpensive, and uses conventional carburization gases at 1 atm pressure in a low temperature furnace. It is fully conformal and entails no dimensional changes or loss of component ductility. Hence, finished components can be carburized as a final step in their manufacture. NON-TECHNICAL: This transformative research will provide the scientific underpinning for an exciting disruptive technology. The use of carburized stainless steels with their improved properties will allow substitution of these materials for more expensive materials in many structural applications. The work on ferritic and martensitic stainless steels is quite novel, and reveals that an understanding of a materials system, particularly under conditions far from true thermodynamic equilibrium, can lead to exciting breakthroughs even for so-called "mature" materials and industries. The research suggested here will also have very positive impact on the graduate materials science and engineering program at Case Western Reserve University, particularly for the two graduate students who will be supported. The students' research will entail their acquiring skills in a host of mainstream materials science techniques, including modeling and microstructural characterization, and thermodynamic and kinetic analyses.