Characterization of a carburized surface layer on an austenitic stainless steel

Characterization tests were made of a commercial carburization treatment that has shown promise for hardening the surfaces of the austenitic stainless steel target vessel of the Spallation Neutron Source against cavitation erosion and pitting caused by the action of pulsed pressure waves in the liquid mercury target. The findings support most of the provider’s promotional claims. The high surface hardness and the thickness of the hardened layer are validated, as are the hardness-depth profiles. The austenite lattice of the layer is enlarged and placed in a state of compressive stress and the surface is plastically distorted by the treatment. The corrosion resistance of the surface in selected acid media is greater than that for untreated austenite. The treated surface is not brittle and is quite resistant to cracking during straining. The maximum carbon content of the layer is measured at 3–4.5 wt%, versus 6–7 wt% cited by the provider. Contrary to the provider’s assertion that all of the carbon is contained in supersaturated solid solution in the austenite phase, some of the carbon is present in an iron carbide phase located non-uniformly at the very surface. Inclusion stringers and δ-ferrite phase in the carburized layer are more prone to corrosion and may provide preferential sites for cavitation pitting.