ANALYSIS OF METHODS FOR PREDICTING THE LIMITING NITROGEN CONCENTRATION IN DUPLEX STEELS

Abstract

The history of origin of the duplex steels (austenitic-ferritic type steels) and improvement of their manufacturing methods in the process of development of metallurgical technologies 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 chemical compositions. Influence of limiting nitrogen concentration in the metal as a limiting factor for obtaining defect-free ingots is described. The various methods are given, which makes it possible to predict the nitrogen content in the metal depending on its chemical composition, temperature and atmospheric pressure. These theoretical methods include calculations using the mathematical approach of interaction parameters, calculations using the concept of equivalent concentrations and empirical dependences. A calculation for the specific composition of super duplex steel has been performed and the most promising method for predicting the limiting nitrogen concentration has been established. In order to verify this method of calculation, the work was carried out to obtain experimental samples with different nitrogen contents by smelting pure materials in an open induction furnace. During the experiments, smelting of two compositions of superduplex steel with different nitrogen content was carried out. The results of the macrostructure and chemical composition study of the obtained samples correlate with the results of calculations according to various methods and the conclusion about the possibility of using a specific dependence, using in its basis the mathematical approach of the interaction parameters of the first and second order, when predicting the limiting nitrogen content as an alloying element in obtaining austenite-ferritic steels.