NITROGEN STEELS AND HIGH NITROGEN STEELS. INDUSTRIAL TECHNOLOGIES AND PROPERTIES

Abstract

Nitrogen pressure can be a basis for the most general classification of steel, alloyed by nitrogen. Nitrogen steels are made under normal pressure, high nitrogen steels are made under pressure that is higher than atmospheric in special units. Nitrogen, as well as carbon, also strengthens austenite, but increases thermal stability of austenite, has the smaller sizes of ions and high solubility in γ- and α-phases. The result is smaller size of nitrides, smaller superficial energy, their higher strengthening effect and possibility of simultaneous increase in durability and corrosion resistance of austenite. The article considers the mechanisms of nitrogen influence on properties of steel, thermodynamics and kinetics of steels alloying with nitrogen, critical concentration of nitrogen and influence of nitrogen on properties of steel. There is no uniform balanced database and thermodynamic model now. Therefore any choice of values of equilibrium constant and parameters of interaction according to tabular data reduces the accuracy of calculations of nitrogen solubility in steel. In the circumstances it is better to use experimental data for concrete alloy from original works. At the choice of data it is necessary to be guided by the following control values: KN = 0,044, A ≥ 600. The nitrogen solubility in liquid metal, in α- and γ-phases is significantly various. Critical nitrogen concentration Nk, which excess leads to formation of bubbles and interstices at steel solidification, depends on composition of steel. Now the acceptable results when determining critical nitrogen concentration, can be received from the following condition: during the whole time of solidification the nitrogen content in residual liquid has to be less its equilibrium with the general pressure in the system of content in liquid metal at the same temperature T. Examples of nitrogen and high nitrogen steels, including steels with special properties, such as corrosion-resistant in bioactive environments, bactericidal steel, alloyed according on the scheme C + N steels, are given.