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 i on implantation (PIII) can increase the wear resistance of austenitic stainless steel without losing its corrosion resistance. In this work, the effect of PIII treatment on the microstructure and the properties of an austenitic (X6CrNiTi1810, AISI 321) and a duplex austenitic-fer ritic (X2CrNiMoN2253, AISI 318) stainless steel has been studied and t he results compared. Three different treatment temperatures and treatm ent times were used. The microstructures were studied by optical metal lography and glancing angle X-ray diffraction (XRD). The formation of expanded austenite was observed in both steels up to treatment tempera tures of 400 degrees C. The ferrite in the duplex austenitic-ferritic steel was also transformed to expanded austenite. At 500 degrees C, a surface layer consisting of CrN was formed on the duplex austenitic-fe rritic steel whereas the modified layer on the austenitic steel was st ill expanded austenite with a small amount of CrN precipitation. Eleme ntal depth profiling by sputtered neutral mass spectrometry (SNMS) rev ealed a similar treatment depth for both materials up to 400 degrees C , which was a function of treatment temperature and time. A pin on dis c tribometer was used to determine the tribological behaviour. A chang e in the wear behaviour was observed and the wear depth decreased rela tive 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 degrees C and can be attributed to the formation of a CrN layer. Corrosion tests have shown that good corrosi on resistance was preserved up to 400 degrees C for both materials wit h only a small decrease being observed. This is due to nitrogen remain ing in solid solution without CrN-precipitation. At a treatment temper ature of 500 degrees C, the corrosion resistance decreased dramaticall y, especially for the duplex austenitic-ferritic steel where a layer o f CrN was formed. These results show the capability of PIII treatment to increase the wear resistance of these stainless steels without losi ng their good corrosion performance. This may allow the use of such st eels in applications where the poor wear resistance of the untreated m aterial would normally prohibit their use. In comparison to the austen itic steel, the duplex austenitic-ferritic steel performed better afte r PIII treatment. For an optimum surface treatment, it is necessary to consider the substrate material as well as the treatment parameters.