Effect of vanadium alloying on the phase composition of super duplex steel

Abstract

The application and advantages of a class of duplex steels (austenitic-ferritic type steels) as a structural material are described. The influence of nitrogen as an alloying element in steels of this class is considered, as well as the peculiarities of structure formation and different types of compounds in the crystallizing melt of the considered compositions of chemical composition. The effect of the concentration of nitrogen, which, when introduced into the steel for alloying, can be in the form of gas filling pores and cracks, the element of introduction in the solid solution based on iron and nitride phases is described. The latter two forms of nitrogen presence in steel are responsible for posi-tive effects on the physicochemical properties, providing in some cases an advantage over traditional stainless steels. Using FactSage software (version 6.4), work was carried out to determine the effect of the main alloying elements of UNS 32750 super duplex steel on its phase composition. As the concentration of the main alloying elements (Cr, Mo, Mn) increased, it was found that in the nitrogen concentration interval of 0.24‒0.35 % (wt.) during melt crystallization there is no zone of existence of purely austenite and ferrite mixture, and the steel cools with formation of metal nitrides in the austenite and ferrite mixture. It can also be noted that this mixture of phases is located at the heat treatment temperatures of this steel 1025‒1125 °C. The proposed composition together increases the temperature existence interval of ferrite during crystallization by an average of 30 °C, which will provide greater nitrogen solubility in the melt. The alloying of this composition with vanadium shifts the formation zone of α + γ + M2N mixture towards less nitrogen concentration, and in the temperature interval below 800 °C an intermetallide phase is formed, which is a compound of CrVN type.