Experimental observations of nucleation and crack growth paths of hydrogen-induced cracking in pipeline steel

The microstructural features that favor hydrogen-induced cracking (HIC) in internal, middle, and external layers of pipeline steels were investigated. The electrochemical cathodic charging was used to induce HIC, and straight beam ultrasonic inspection was identified in which steel layers HIC nucleate and grow. The cracked plates were examined by optical microscope and scanning electron microscope (SEM). The hydrogen microprint technique (HMT) was used to visualize the site of high hydrogen concentrations. Electron backscatter diffraction (EBSD) was used to analyze the role of crystallographic texture in HIC pathways. The results revealed that HIC nucleation and growth are influenced by the segregation degree and inclusion contents, which are higher in the internal surface layer of our studied steels but not consistently so in the middle thickness layer. Further, HIC nucleated in cuboidal and spinal inclusions and propagated along ferrite/pearlite bands with high misorientations. The HMT results showed that hydrogen is present in pearlite colonies, grain boundaries, and non-metallic inclusions. The results indicated that HIC is not solely controlled by the crack geometry.