Microstructural evolution during aging of novel superferritic stainless steel produced by the HIP process

Abstract

The present paper presents results on the effect of selected heat treatments on the phase stability of an experimental superferritic stainless steel grade produced via the hot isostatic pressing (HIP) process. Both deformed and undeformed specimens were subjected to a variety of heat treatments within the 650–950°C temperature range for aging times of 5 min–1000 h. The temperature region of microstructural consistency for the superferritic stainless steel grade that guarantees optimum mechanical and corrosion properties was determined. Intermetallic phases were found to precipitate under all chosen temperatures, but the length of heat treatment before precipitation occurred varied with temperature and amount of deformation. The formation of minor amounts of austenite was detected in cold deformed specimens heat treated at 850 and 750°C while for the undeformed specimens, such a presence was verified only for specimens heat treated at 850°C. Formation of austenite is always preceded by the formation of an intermetallic phase. It was confirmed as anticipated, that deformation increases the rate of intermetallic phase precipitation. Superferritic steels are natural candidate materials for naval applications and fabrication techniques, such as welding, involve elevated temperatures on components previously deformed during a shaping exercise, therefore it is vital that an assessment of microstructural consistency of this novel powder metallurgy superferritic steel is performed.