Hybrid and duplex plasma surface alloying of ferritic-Austenitic duplex stainless steel

Duplex stainless steels are of great technological importance as they possess high corrosion resistance and high mechanical properties as well as low cost. However, their low hardness and poor tribological properties have limited their wider applications in severe wear conditions. This research is directed at the response of ferritic-austenitic duplex stainless steel to plasma surface alloying with both carbon and nitrogen simultaneously (i.e. hybrid carbonitriding) and sequentially (duplex alloying treatment). It has been found that both hybrid and duplex plasma surface alloying treatments can produce a similar dual-layer case consisting of a surface nitrogen alloyed layer followed by a carbon alloyed layer; the dual-layer case is harder and thicker than the single layer produced by typical plasma nitriding or plasma carburising under the same temperature and time. After plasma surface alloying, while the original austenite grains in the duplex stainless steel can fully transform to S-phase, retained ferrite grains, expanded with interstitial solution alloying elements, carbon, nitrogen or carbon+nitrogen, were detected by TEM and XRD. The dual-phase S-phase/ferrite (expanded) structure will decrease the corrosion resistance of duplex stainless steel.