A Thermodynamic Study of a Constitutional Diagram for Duplex Stainless Steels

Abstract

Arc welding involves joining two metal plates by a welding pool, which is kept molten for short periods due to an electric arc. This weld metal has, sometimes, a different composition from the plates, due to a filler metal. Arc welding is a non-equilibrium process. Technological approach, nevertheless, uses empirical tools to predict important features of the joints. In stainless steel welding, the Schaeffler diagram, and other constitutive diagrams, as the WRC 1992 diagram, are used to predict the resultant microstructure in the welded joint. The composition input is calculated by Chromium and Nickel equivalents expressions. Schaeffler diagram is used to evaluate weldability of stainless steels. Some people adapt the W and N alloying elements in Schaeffler diagram to predict the weldability of duplex stainless steels (DSS). Present work shows how these technological tools can be understood as results of phase equilibrium in key isotherms of the Fe-Cr-Ni system, used in all constitutive diagrams. The validity and limitation of the Chromium and Nickel equivalents expressions were discussed based on the effect of up to two alloying components (X1 = Mo or W and X2 = N) on the alpha/gamma phase equilibrium, as estimated by calculations in system Fe-Cr-Ni-X1-X2 that are typical DSS alloying elements. This allows extending these Chromium and Nickel equivalents expressions based on thermodynamics and to identify vulnerabilities for two important problems in DSS welding: Chromium nitride and sigma phase precipitation. Originally, Schaeffler’s diagram does not consider the effect of W and N in the (γ/α + γ) equilibrium field.