Ferritic, martensitic, and duplex stainless steels, optimally carburized at temperatures below 500 °C, possess a hardened case containing a remarkable class of carbide phases that do not degrade, and in certain cases actually improve the corrosion resistance of these important steels. Analyses of these carbides have determined that they have the same metal compositions as those of the bulk stainless steels in which they formed, and thus are termed paraequilibrium carbides. Inasmuch as the formation of these paraequilibrium carbides do not deplete the surrounding matrix of Cr, "sensitization" and loss of corrosion resistance does not occur. The proposed research will fully characterize the structure, microstructure, and stoichiometry of these paraequilibrium carbides, and their effect on electro-chemical, tribological, and mechanical properties of the interstitially hardened steel surfaces. A range of processing parameters, e.g., time/temperature and carbon activity in the process gas atmosphere, will be applied to these stainless steels to generate a broad spectrum of paraequilibrium carbide phases. Thermodynamic and kinetic modeling will be performed in parallel with the processing and characterization activities to provide in-depth understanding of this exciting technology.
NON-TECHNICAL SUMMARY: Low-temperature carburization of austenitic (high nickel content) stainless steels is now employed on an industrial scale in a number of important applications. Similar technology is not now generally available for ferritic, martensitic, and duplex stainless steels. Successful low-temperature carburization of this important group of magnetic stainless steels---which furthermore are less costly due to their greatly reduced nickel content---will have broad impact. A properly hardened carburized surface will enable these stainless steels to establish new applications and new markets, based upon improved wear and corrosion resistance, in addition to their other useful properties. Finally, the student researchers doing their thesis research on this topic will experience a rich array of activities involving processing, characterization and modeling, all against the backdrop of a constrained optimization project.
Ferrite grains with low uniform contrast under TEM were observed following low-temperature (less than 400 °C) gas-phase nitriding in both 17-7 precipitation hardening and 2205 duplex stainless steels. A VERY Surprising Result These ferrite grains possess nitrogen (and carbon) concentrations as high as 20-25 at. pct.—orders of magnitude higher than those predicted by thermodynamic calculations (less than 0.001 at. pct.), while displaying no measurable expansion of the bcc ferrite lattice. HR STEM data reveals segregation of Fe and Cr in the ferrite at the nm scale, which proceeds in the elastically soft directions until the grain is consumed. Such a large supersaturation with no detectable lattice expansion is ascribed to the strong nitrogen-dislocation interaction, which favors nitrogen dissolution in solid solution. Rather than nitride formation, this imparts a nearly four-fold hardening compared to the non-treated material. An extremely high dislocation density, on the order of 1016 m-2, has been estimated within the nitrogen-enriched region from HR images (Figure 1b). The increased kinetics of the so-called spinodal decomposition into Fe-rich and Cr-rich regions is believed to result from the extraordinarily high dislocation density. BF TEM: Bright-field transmission electron microscopy HR STEM: High-resolution scanning transmission electron microscopy XEDS: X-ray energy dispersive spectroscopy
