Physicochemical characteristics of melts of corrosion resistant nitrogen-alloyed steels

Abstract

Physicochemical characteristics of the melt of nitrogen-alloyed steel of 04Kh20N6G11M2AFB grade (nitrogen content 0.47 0.49 %) were investigated by the method of torsional vibrations of a crucible with a metal in the atmosphere providing a stable nitrogen content during the experiment (80 % nitrogen and 20 % helium). Using sensitivity of the method to aggregate state of the test substance, at a heating rate of 0.0033 0.0050 K/s, the liquidus temperatures were experimentally determined for 04Kh20N6G11M2AFB steel (1660 1666 K) and lownitrogen steel ([N] = 0.063 %) with identical content of other elements (1685 1690 K). This made it possible to recommend the value of the coefficient - 60 K/% [N] for the calculated assessment of the effect of nitrogen on liquidus temperature of complex and high-alloy steels. It is shown that the viscosity of the melt of 04Kh20N6G11M2AFB steel has a relatively high level ((11.5 ± 0.7) 10-7 m2/s) in comparison with the traditional ((8.2 ± 0.2) 10-7 m2/s) austenitic steels with a relatively small partial effect of nitrogen. A significant non-equilibrium of the melt structural state of this steel was established by high-temperature viscometry methods. Comparative analysis of polytherms and isotherms of the melt kinematic viscosity for 04Kh20N6G11M2AFB steel and its low nitrogenous ([N] = 0.063 %) analogue made it possible to conclude that the presence of nitrogen at concentrations close to saturation plays a decisive role in the level of non-equilibrium of the melt and low rate of its relaxation. It was confirmed as a result of special experiments on the saturation of low-nitrogen steel with nitrogen, during which a sharp increase in non-equilibrium of the melt structural state was recorded with achievement of nitrogen concentration in the metal of limiting values (0.45 0.50 %). The principal possibility of increasing and stabilizing the operational properties of corrosion resistant nitrogen-alloyed steels is indicated by reducing non-equilibrium of the melt structural state by eliminating the excess of nitrogen concentration limits for the considered chemical composition. © 2020 National University of Science and Technology MISIS. All rights reserved.