Effect of Slow Strain Rates on the Hydrogen Migration and Different Crack Propagation Modes in Pipeline Steel

The slow‐strain‐rate test (SSRT) is the most commonly used method for evaluating pipeline steel in service environments. However, to accurately assess the sensitivity of steel to hydrogen, it is necessary to investigate the effect of different strain rates, taking into account microstructure‐influenced hydrogen migration. Herein, a hot‐rolled X70 pipeline steel sheet is investigated by a SSRT at different strain rates with and without synchronous hydrogen charging. The influence of the pearlite content and different strain rates on the hydrogen‐assisted crack propagation and hydrogen diffusion in pipeline steels is discussed. Using scanning electron microscopy, electron backscatter diffraction, transmission electron microscopy, and hydrogen microprinting, the hydrogen atoms are observed to be easily segregated at the ferrite/pearlite (F/P) interface without external stress. Under loading, a higher strain rate results in lower hydrogen permeation content in steel, penetrating into pearlite and interacting with its internal vacancies, leading to transgranular fracture initiating from pearlite. At a low strain rate, the F/P interface is more vulnerable to hydrogen degradation, leading to intergranular crack mode along the interface and increased tendency to form secondary cracks. Therefore, strain rate–induced crack initiation and propagation characteristics should be considered during the SSRT. Herein, slow‐strain‐rate test is used to investigate X70 pipeline steel at different rates and the influence of pearlite content, and the effect of strain rates is emphasized on hydrogen‐assisted crack propagation and diffusion. The results show that higher strain rates cause transgranular fracture, while lower strain rates cause intergranular crack mode along the F/P interface.