Glow-discharge nitriding of AISI 316L austenitic stainless steel: influence of treatment temperature

Nitriding treatments of austenitic stainless steels can be performed only at relatively low temperatures in order to avoid a decrease of corrosion resistance due to chromium nitride formation. These conditions promote the formation of the so-called S phase, which shows high hardness and good corrosion resistance. In the present paper, the influence of the treatment temperature of glow-discharge nitriding process on the microstructural and mechanical characteristics of AISI 316L steel samples was evaluated. Glow-discharge nitriding treatments were performed at temperatures in the range 673-773 K for 5 h at 103 Pa. The modified surface layer of the nitrided samples consists mainly of the S phase and, according to metallographic technique analysis, it seems to be essentially a modification of the austenite matrix. All the nitrided sample types show a peculiar surface morphology due to both plasma etching during nitriding and the presence of slip steps and relieves at grain boundaries, the latter features presumably due to the formation of the nitrided layer. X-ray diffraction analysis shows that for the samples nitrided at temperatures up to 723 K, besides the S phase, small chromium nitride precipitates are present at the surface, while using higher treatment temperatures both chromium (CrN) and iron (γ'-Fe4N) nitrides precipitate along the grain boundaries and in the middle of the grains, and their amount increases as treatment temperature increases. High hardness values (from ∼1450 to ∼1550 HK0.01, depending on nitriding conditions) are observed in the modified layer with a steep decrease to matrix values. Preliminary corrosion resistance tests, carried out in 5% NaCl aerated solution with the potentiodynamic method, show that with the used treatment parameters a substantial improvement of corrosion resistance can be achieved when glow-discharge nitriding treatments are performed at temperatures in the range 703-723 K.