Evaluation of the WRC 1992 diagram using computational thermodynamics

Abstract

Ferrite content of a stainless steel weld metal is a vital parameter for ensuring that the microstructure and thus also corrosion and mechanical properties are adequate. A rapid way of estimating ferrite content and thus microstructure is by using weld metal composition and a Welding Research Council (WRC)-1992 diagram. The ferrite-forming alloying elements are estimated to a chromium equivalent, the austenitic to a nickel equivalent and the ferrite content is estimated by help of isoferrite lines. This diagram was derived by empirical work on a large number of commercial and laboratory stainless weld metals covering a wide alloy span. The present paper uses computational thermodynamics, i.e. Thermo-Calc, to evaluate the influence of temperature and key stainless alloying elements. The alloying range for duplex stainless weld metal was studied and the WRC 1992 diagram was compared with results from calculations. The chromium and nickel equivalents were evaluated and coefficients for Mo, Nb, C, N and Cu assessed. The results showed that while the coefficients for Mo and N proved accurate, the values for Nb, C and Cu would require an adjustment. Alternative diagrams with adjusted chromium and nickel equivalents and iso-ferrite lines in both ferrite numbers and volume percent were suggested.