Plasma immersion ion implantation of stainless steel: austenitic stainless steel in comparison to austenitic-ferritic stainless steel

It has been shown previously in the literature that plasma immersion ion implantation (PHI) can increase the wear resistance of austenitic stainless steel without losing its corrosion resistance. In this work, the effect of PHI treatment on the microstructure and the properties of an austenitic (X6CrNiTi1810, AISI 321) and a duplex austenitic-ferritic (X2CrNiMoN2253, AISI 318) stainless steel has been studied and the results compared. Three different treatment temperatures and treatment times were used. The microstructures were studied by optical metallography and glancing angle X-ray diffraction (XRD). The formation of expanded austenite was observed in both steels up to treatment temperatures of 400 °C. The ferrite in the duplex austenitic-ferritic steel was also transformed to expanded austenite. At 500 °C, a surface layer consisting of CrN was formed on the duplex austenitic-ferritic steel whereas the modified layer on the austenitic steel was still expanded austenite with a small amount of CrN precipitation. Elemental depth profiling by sputtered neutral mass spectrometry (SNMS) revealed a similar treatment depth for both materials up to 400 °C, which was a function of treatment temperature and time. A pin on disc tribometer was used to determine the tribological behaviour. A change in the wear behaviour was observed and the wear depth decreased relative to untreated material. This was due to an increase in the surface hardness and a decrease in the coefficient of friction. The decrease in wear depth correlated with the thickness of the modified layer. The best results were found with the duplex austenitic-ferritic steel at a treatment temperature of 500 °C and can be attributed to the formation of a CrN layer. Corrosion tests have shown that good corrosion resistance was preserved up to 400 °C for both materials with only a small decrease being observed. This is due to nitrogen remaining in solid solution without CrN-precipitation. At a treatment temperature of 500 °C, the corrosion resistance decreased dramatically, especially for the duplex austenitic-ferritic steel where a layer of CrN was formed. These results show the capability of PIII treatment to increase the wear resistance of these stainless steels without losing their good corrosion performance. This may allow the use of such steels in applications where the poor wear resistance of the untreated material would normally prohibit their use. In comparison to the austenitic steel, the duplex austenitic-ferritic steel performed better after PIII treatment. For an optimum surface treatment, it is necessary to consider the substrate material as well as the treatment parameters.