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.

INTRODUCTION

Super duplex stainless steels are widely used in the petrochemical industry because of its good properties. Their high corrosion resistance and good mechanical properties are due to a high content of alloying elements and the high chromium content is especially important for the corrosion resistance. However, the wrong heat treatment to these metals can lead to embrittlement of the materials through intermetallic phases like sigma and chi phase. This can happen during fabrication, welding processes and prolonged exposure to high temperatures during their service lives. Even small amounts of these phases can be responsible for catastrophic service failure of components (Ong et.al (2018) and Børvik et. al. (2010).

A way to investigate the intermetallic phases is using EBSD (Electron Backscatter Diffraction) in the SEM (Scanning Electron Microscope). EBSD is well suited for crystallographic analysis like determination of orientation of individual crystallites, phase identification and characterization of grain boundaries. The aim of this project is to investigate a super duplex stainless steel alloy, UNS 32760 exposed to different heat treatments with EBSD for phase analyses, and EDX (Energy Dispersive X-ray Spectroscopy) for elemental analyses. The project is done in collaboration with Equinor ASA.

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