Annealing behaviour of nitrogen implanted stainless steel

The phase formation and annealing behaviour are reported after nitrogen PIII in two austenitic stainless steels, DIN 1.4541 (X6CrNiTi18.10) and DIN 1.4571 (X6CrNiMoTi17.12.2), differing only in the Mo content. The treatment parameter were pulses of 10 μs and 40 kV for a nitrogen dose of 4×1018 cm−2 at a temperature of 380°C. In 1.4541, without Mo, the modified layer is limited to 1.5 μm while the 2 wt.% Mo-containing 1.4571 exhibits a layer thickness of 15 μm. No difference in the hardness (corrected for the layer thickness) and the wear behaviour was observed. Using X-ray diffraction (XRD) a lattice expansion between 5 and 11%, was observed, not correlated with the nitrogen content (as determined by glow discharge optical spectroscopy — GDOS). After annealing at 400 and 425°C for different times, the lattice constant is slightly reduced, albeit no phase transformation was observed in 1.4571. In 1.4541 at a temperature of 425°C a strong reduction of the lattice expansion, coupled with an inward diffusion of the nitrogen was observed. No mechanism explaining this behaviour is available at the moment.