Abstract
The lattice stress-induced diffusion of nitrogen and hydrogen in austenitic stainless steel, taking place during nitriding in nitrogen/hydrogen plasma, is analyzed in the presented work. Stress-induced diffusion has an anisotropic nature and depends on the orientation of the crystal lattice. However, during simulations, it is not enough to take into account only the anisotropy of stress-induced diffusion, since this leads to contradictory results when comparing with experimental data. The problem is the surface concentration of nitrogen. Processes on the steel surface such as adsorption, desorption and heterogeneous chemical reactions are also very important. In the presented work, it is shown that these surface processes also have anisotropic natures, and it is very important to take this anisotropy into account during simulations. The influence of anisotropic surface processes on austenitic steel nitriding is analyzed in this study. It is shown that the nitrogen diffusion is anisotropic due to the effects of the anisotropic stress gradient and the anisotropic effects on the steel surface.
Highlights
Austenitic stainless steels (ASS) are often used as engineering materials in many industrial applications among which are food and chemical processing, the automotive industry, and some surgical implants due to very high corrosion resistance in many aggressive environments because of a passive Cr2 O3 surface film
The low-temperature plasma nitriding of ASS results in improved properties, such as a greatly increased surface hardness, wear resistance, fatigue resistance, and corrosion resistance [1,2,3,4,5,6,7,8,9,10,11,12,13]. Such a hardening process is related to the lattice distortion of the fcc austenitic phase (γ phase), leading to the formation of nitrogen-rich expanded austenite, with a hardness close to 1500 HV and no loss of corrosion resistance [6,14]
This work aims to study the impacts of the elastic anisotropy of ASS, as well as the anisotropy of the surface free energies, on the phenomenon of anisotropic nitrogen penetration during low temperature nitriding
Summary
Austenitic stainless steels (ASS) are often used as engineering materials in many industrial applications among which are food and chemical processing, the automotive industry, and some surgical implants due to very high corrosion resistance in many aggressive environments because of a passive Cr2 O3 surface film. The low-temperature plasma nitriding (performed at temperatures up to 450 ◦ C) of ASS results in improved properties, such as a greatly increased surface hardness, wear resistance, fatigue resistance, and corrosion resistance [1,2,3,4,5,6,7,8,9,10,11,12,13] Such a hardening process is related to the lattice distortion of the fcc austenitic phase (γ phase), leading to the formation of nitrogen-rich expanded austenite (γN phase), with a hardness close to 1500 HV and no loss of corrosion resistance [6,14]. Intensive studies on γN phase structure and formation have been undertaken by various researchers in the past several decades, and a comprehensive review of the scientific literature regarding the formation, characteristics and properties of the γN phase is presented
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