محمدعلی مهتدی بناب

The demand for pipeline steels has been increased in several recent decades. These steels carry oil and natural gas over long distances. In this research, the microstructure and texture of an API X70 pipeline steel in the middle and top layers have been characterized by using SEM and EBSD analyses. Then, the effect of different sour and acidic environments on the mechanical properties of pipeline steels and hydrogen-induced cracking susceptibility is investigated. Moreover, the effect of texture, mesotexture and inclusions on HIC crack initiation and propagation is studied at different conditions in acidic and sour environments. The results show that two types of detrimental inclusions, including CaS and Al2O3, lead to the failure of pipeline steels in harsh environments. Moreover, it is concluded that an acidic environment does not alter the nature of fracture in the mentioned steel, while electrochemical hydrogen charging causes a brittle fracture. The 〈111〉ǁND oriented grains are not observed around the HIC cracks while the grains mostly tend to orient in 〈100〉ǁND direction around these cracks. The martensite phase accumulated around the HIC crack is recognized as the weakest microstructure that could resist against HIC propagation. Finally, the effect of mesotexture on HIC crack propagation is studied. According to results, Σ11, Σ29a and Σ13b CSL grain boundaries indicate a high resistance to the HIC propagation.