Impact of surface condition on sulphide stress corrosion cracking of 316L stainless steel

Abstract

The effect of surface condition on crack initiation in 316L stainless steel during laboratory testing in sour (H2S) environments for oil and gas applications has been investigated using the four-point bend test method. The main focus was on the effect of the degree of surface damage introduced during specimen machining and the influence of heat tinting to simulate the welding process. Detailed mapping of the surface of the four-point bend specimens before and after the tests revealed a greater tendency for pits to form at pre-existing mechanical defects than at inclusions. Perhaps surprisingly, pitting was initiated more readily on the finer ground surface. The effect of heat tinting was (i) to increase the pit density and (ii) to facilitate cracking, shifting the material from the pass to the failure domain. In all cases cracks initiated at pitting sites. A clear time dependence was observed in both the evolution of pitting and the transition from pit to crack during the four-point bend test, implying that the standard 30day test duration may not always be sufficiently conservative. Characterisation of pitting and cracking in the specimens using electron back-scatter diffraction (EBSD) and focused ion beam (FIB) milling revealed evidence of de-alloying local to the crack. The origin of the cracks could not be identified precisely but initiation in the thinned region of the metal caused by undercutting or intense localised dissolution along slip bands could both be important. Comparison of residual stress measurement using X-ray diffraction (XRD) and hole-drilling techniques demonstrated that near-surface residual stress measurement in austenitic stainless steels using XRD should be treated with caution due to the presence of a heavily deformed nanocrystalline layer on the surface arising from the machining process.