Case-hardening'' of the Ni-base superalloy IN718 has been achieved by low-temperature gas-phase carburization. After carburization under optimum conditions, the hardened surface layer (the ''case'') has about twice the hardness of the core (HV of %800) and contains %12 at pct carbon in interstitial solid solution. This causes a lattice parameter expansion of %1 pct perpendicular to the surface and, because of the mechanical constraint provided by the noncarburized core below, develops a large biaxial surface compressive residual stress (%1.9 GPa) parallel to the surface. Microstructural studies and X-ray diffractometry reveal no carbide precipitates in the case. In agreement with this observation, low-temperature carburization does not compromise the ductility and actually improves the crevice corrosion resistance of the alloy.

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The mechanical properties and corrosion resistance of duplex (ferrite-austenite) grade 2205 stainless steel have been substantially improved by interstitial hardening using low temperature carburisation. The austenite phase of the duplex stainless steel responds to low temperature carburisation in a similar manner as single phase austenitic stainless steels, forming 'expanded' austenite (also called S phase). The surface layer that forms on the ferritic portion of 2205 steel consists of a paraequilibrium carbide, a carbide with the same metal composition as the underlying ferrite. This two-phase case has about three times the Vickers hardness of non-treated material, an improved ultimate tensile strength and increased fatigue resistance, and much improved crevice corrosion resistance.