Computational thermodynamic analysis of secondary phases in super ferritic stainless steels

Super ferritic steels belong to a family of steels with Cr content greater than 25% and which also have Mo in their compositions. They were first developed for use in heat exchangers and marine environments. The increase in Mo content in austenitic stainless steels increases the corrosion resistance in mediums that are rich in naphthenic acid or other sulphur complexes. This present work is part of a study for adapting the composition of commercial super ferritic steels by increasing Mo content in the alloy so they can be used in petroleum plants that refine petroleum oils rich in sulphur compounds. We studied the precipitation of phases for experimental compositions of Fe–25Cr–XMo–YNi (X = 5% and 7%; Y = 2% and 4%) with Nb and Ti added. Calculations from the computational program Thermo-Calc were used for the simulation of the phase diagrams and the weight fraction of the phases present. The results from the experimental compositions were compared with those obtained for commercial UNS S44660 super ferritic stainless steel. The stability temperature of the ferritic phase was determined and the possible intermetallic phases were identified in accordance with the temperature variation and chemical composition.