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.
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