Low temperature plasma surface alloying and characterisation of a superduplex stainless steel

Because of their superior mechanical properties and high corrosion resistance in aqueous environments, duplex stainless steels are the material of choice for equipment used in the offshore gas and oil industry. However, wear in chloride containing water (i.e. corrosion wear) is considered as a major concern. During the past decade, a new low temperature plasma surface alloying technique has been developed to achieve combined improvements in hardness, wear resistance and fatigue properties of austenitic stainless steels, without impairing their corrosion behaviour due to the formation of S-phase. However, few researchers have applied this technique to duplex stainless steel (DSS) and no work has been reported for super DSS (SDSS). In this paper, SDSS 2507 has been DC plasma nitrided under a range of treatment conditions. Energy dispersive X-ray spectroscopy (EDX), Glow discharge Spectrometry (GDS), X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterise the microstructure of the surface treated layers. Mechanical properties were evaluated using microhardness, electrochemical corrosion, dry and corrosion wear tests. A dense super hard surface layer was produced uniformly across all the samples. The austenitic grains formed an S-phase and the original ferritic region formed e-nitride. The surface hardness, the load bearing capacity and dry wear results, increased with increasing treatment temperature; the samples treated under lower temperatures exhibited the best corrosion and corrosion wear resistance.