The Physicochemical Characteristic Study of Melts of Corrosion Resistant Nitrogen-Alloyed Steels

Physicochemical characteristics of the nitrogen-alloyed steel melt of the 04Kh20N6G11M2AFB grade (nitrogen content 0.47–0.49%) are investigated by using the torsional vibration method of a crucible with metal in the atmosphere, which provides a stable nitrogen content during the experiment (80% nitrogen and 20% helium). Using the method sensitivity to the aggregate state of the tested substance, at a heating rate of 0.0033–0.0050 K/s, the liquidus temperatures are experimentally determined for the 04Kh20N6G11M2AFB steel (1660–1666 K) and low nitrogen steel ([N] = 0.063%) with the identical content of other elements (1685–1690 K). These results make it possible to recommend the value of the coefficient –60 K/% [N] for the calculated assessment of nitrogen influence on the liquidus temperature of complex and high alloy steels. The viscosity of the 04Kh20N6G11M2AFB steel melt is relatively high ((11.5 ± 0.7) ×10–7 m2/s) in comparison with the traditional austenitic steels ((8.2 ± 0.2) × 10–7 m2/s) with a relatively small partial effect of nitrogen. A significant nonequilibrium of the structural state of the 04Kh20N6G11M2AFB steel melt is revealed by the high-temperature viscometry methods. The comparative analysis of polytherms and kinematic viscosity isotherms of the 04Kh20N6G11M2AFB steel melt and its low nitrogen ([N] = 0.063%) analogue makes it possible to conclude that the nitrogen presence in concentrations close to saturation plays a decisive role in the melt nonequilibrium level and low relaxation rate. This is confirmed by the results of special experiments on saturation with nitrogen of low nitrogen steel, in which a sharp increase in nonequilibrium of the melt structural state is recorded when nitrogen concentration in metal reaches the limiting values (0.45–0.50%). There is a potential possibility to increase and stabilize the operational properties of corrosion resistant nitrogen-alloyed steels due to reducing the melt’s structural state nonequilibrium by excluding the excess of limiting values of nitrogen concentrations for the chemical composition considered.