Embrittlement and aging at 475 °C in an experimental superferritic stainless steel with high molybdenum content

Abstract

Superferritic steels are characterized by high Cr content (exceeding 25 wt.%) with addition of Mo. They were originally developed for use in heat exchangers and marine environments. Current trend to use these alloys in the oil industry drives the research on effect of the increased Mo content on the microstructure of these steels. Mo increases the resistance to naphthenic corrosion. The α'(alpha prime) phase precipitation, typical in Fe-Cr system, causes Cr depletion in the ferritic matrix which decreases the corrosion resistance, toughness and increases hardness of steels. This phase is rich in Cr, has paramagnetic properties and occurs in the temperature range between 350 and 550 °C. The influence of increase of Mo content and Ni addition is investigated in the temperature range of α' phase precipitation in experimental Superferritic stainless steel FeCrMoNi with different compositions. An initial analysis was performed with Thermo-Calc® (TCFE6). Samples were subjected to aging treatment for different time and temperatures. Results of micro hardness tests, magnetic measurements (by means of ferritoscope) and corrosion test, were used to study the precipitation behaviour of α′ phase. The results were compared with solution annealed samples and results obtained from the Thermo-Calc® calculation.