EBSD Characterization of the Microstructure Evolution in (σ, χ)-containing SDSS Subjected to Long Term Heating

Abstract

Super duplex stainless steels (SDSS) are used in oil wells with high temperature and high pressure in the oil and gas industry due to the good combination of high corrosion resistance and excellent mechanical properties. But SDSS are susceptible to formation of brittle intermetallic phases like sigma- and chi-phase at high temperature which gives a significant decrease in the mechanical properties of the steel. The SDSS material heat treated at 900°C for eight minutes and another treated at 750°C for four hours were examined by use of electron backscatter diffraction (EBSD) in SEM. The SDSS samples containing secondary phases were heat treated at 300°C over a period of four weeks. The use of SDSS is limited to temperatures below 250°C, mainly due to spinodal decomposition of alloying elements in the austenite and ferrite. The presence of the unwanted alloying element rich chi- and sigma-phase does not seem to influence the microstructure during long term heating at microscale. The mechanical properties, however, and especially the ductility, decreases drastically after the long term heat treatment when intermetallic phases are present.