Computational and thermodynamic simulation of a model super-ferritic steel containing 0.07%wt. of boron

ABSTRACT Super ferritic stainless steels are used in equipment such as heat exchangers and distillation towers operating under extreme temperature and pressure conditions and in environments containing high concentrations of chlorides, H2S and CO2 found in the new areas of oil and gas exploration on the Brazilian coast. A super ferritic alloy with composition based on commercial steels was developed for the present study, aiming to adapt the composition of super ferritic steels in order to improve them. The developed alloy contains 4.4 wt.% of nickel (Ni) to improve toughness, 25.7wt.% of chromium (Cr) so that the stability of the ferritic phase is maintained due to the addition of Ni, 4.6wt.% of Mo to increase the corrosion resistance. The differential for the study of this alloy was the addition of 0.07wt.% (700 ppm) of boron, aiming to increase hardenability, toughness and grain boundary stabilization. Thermodynamic calculations were performed using the Thermo-Calc software for the simulation of phase diagrams and the weight fraction of the phases present in the material. The results of the experimental compositions were compared with results from commercial steel UNS S44660. The results obtained an estimate of the stability temperature of the ferritic phase and determined the possible deleterious phases according to temperature variation and chemical composition.