Nitriding of austenitic stainless steel (ASS) at moderate temperature (around 400 °C) increases significantly its hardness (6-7 times) and wear resistance (about 3 orders of magnitude) without loss of corrosion resistance. The similar results were obtained in plasma nitrided Co-Cr alloys (for example, the surface hardness increases about 5 times). Using these processes it is possible to obtain nitrided layer 10-15 micrometers thick (for austenitic stainless steel) and about 10 micrometers (for Co-Cr alloys) with high nitrogen content (20-30 at.%) and observed nitriding rates are several orders of magnitude higher than those measured at the near-equilibrium conditions. Another peculiarity is that the nitrogen depth profiles in nitrided ASS and Co-Cr alloys exhibit plateautype shapes slowly decreasing from the surface, followed by a rather sharp leading edge, which cannot be explained by the classical diffusion models. Furthermore, the experimental results (of different research groups) show that plasma nitriding processes (at moderate temperatures) are accompanied by changes of stainless steel (and Co-Cr alloys) volume, since interstitial nitrogen causes an expansion of the crystal lattice of the solid matrix (internal stresses are observed). In our previous work the nitriding mechanism based internals lattice stress induced diffusion (barodiffusion) was proposed. The model explains many experimentally observed phenomena. However mechanisms of nitriding are not clear. Unexplained is the anisotropy of nitriding which is observed not only in hexagonal lattice but in cubic also. In addition, the experimentally was found, that hydrogen percentages in the nitrogen-hydrogen gas mixture has effect on the nitrogen transport mechanisms in plasma nitrided ASS. All these phenomena have a significant effect on the nitrogen diffusion mechanisms in the steel and Co-Cr alloys So, the main objective of the work is to describe mathematically and analyze the main mechanisms of nitrogen penetration taking into account the anisotropy of diffusion and internal stresses and influence of hydrogen.
Projects funded by the Research Council of Lithuania (RCL), Projects carried out by researchers’ teams
Scientific articles – 4 units.
Period of project implementation: 2017-09-01 - 2020-09-30
Project coordinator: Kaunas University of Technology