Refinement of the expression for calculating the limiting solubility of nitrogen in duplex steels

A description of a class of duplex stainless steels (DSS) is given, the influence of nitrogen as an alloying element in the steels under consideration, as well as the features of structure formation and various types of compounds in the crystallizing melt of the considered compositions of chemical composition are considered. The influence of the limiting nitrogen concentration in the metal as a limiting factor for obtaining defect-free ingots is described. A dependence has been established for predicting the limiting concentration of nitrogen in UNS S32750 steel, which is within the grade content of this element. However, it is noted that the temperature indicator to be used in this dependence raises questions. In various sources, researchers suggest different options - the recoil temperature of nitrided ferroalloys (1580-1600 °С), the temperature of metal release from the furnace or casting (1520-1580 °С), or the temperature of the beginning of crystallization (1430-1450 °С). However, such differences in the temperature used to calculate the temperature figure give completely different results for the ultimate solubility of nitrogen. To determine the temperature index to be used in the expression, mathematical calculations of the limiting nitrogen concentration in the temperature range 1450-1600 °С in increments of 30 °С were performed. For comparative analysis, compositions of different researchers have been selected, in the works of which both successful experiments on melting compositions of duplex steels alloyed with nitrogen and obtaining defective ingots (gas bubbles) have been reported. It has been established that the use of the existing expression is relevant for the temperature of 1480 °С, as opposed to the temperatures proposed by various researchers. Only when the condition that the calculated nitrogen solubility index at this temperature is higher than the actual concentration in the steel is met, is it possible to obtain a defect-free duplex steel ingot.