Effect of microstructure on hydrogen induced cracking in pipeline steel

Abstract

Pipeline steels used for sour service applications in the oil and gas industry are subjected to corrosive internal environments containing high partial pressures of hydrogen sulfide (H2S). During sour service, the presence of H2S allows for enhanced hydrogen ingress into the steel and subsequent internal cracking, a process called hydrogen induced cracking (HIC). The steels utilized for these applications are fine grained, thermomechanically processed (TMP) alloys with complex microstructures that can include microconstituents such as polygonal ferrite, quasi-polygonal ferrite, pearlite, martensite/austenite (M/A) microconstituents, non-metallic inclusions, and carbonitride precipitates. Steel cleanliness, inclusion shape control, and alloying strategies have been developed to mitigate HIC, yet it persists in higher strength steel grades, and would be relevant in other applications such as hydrogen pipelines. The primary objective of this study is to understand the interaction of HIC with the grain structures and secondary microconstituents in X52 and X70 steel alloys. Transgranular cracking was primarily observed in the X52 alloy, while both intergranular and transgranular cracking were observed in the X70 alloy. Misorientation distribution functions characterizing grain boundaries in the as-received material were compared to discrete misorientations measured across boundaries in cracked regions. Low angle boundaries (2 to 5) appeared the most often in the X52 material and cracked the most often. In contrast, low angle boundaries (2 to 5) appeared much less frequently in cracked regions in the X70 alloy than in the as-received microstructure. However, 5 to 25 misorientation boundaries appeared more frequently in cracked regions in both alloys. The boundary type with the highest multiples of a random distribution in the AR X70 material, 43.6 <100>, did not exhibit any cracking. These boundaries appear to be interphase ferrite/cementite boundaries in the AR X70 steel. It is hypothesized that these regions contain cementite precipitates that affect local hydrogen diffusion. Analysis of transgranular cracking revealed that the crack trace was found near both cleavage and slip plane traces at various locations along the crack length in both alloys, indicating that hydrogen could play an important role in the mechanism of transgranular cracking. In addition, M/A microconstituents occurred with equal frequency alongside cracks and in the AR X70 at the centerline, suggesting that M/A constituents are not more susceptible to cracking than other microstructural constituents.