The influence of the crack morphology caused by delaminations in the used ferrite-pearlite steel on its fracture toughness

Fracture toughness of 17H1S steel (analogue to API 5L X52) of the reserve pipe and pipe operated for 38 years on the gas main pipeline was determined using the J-integral method to assess its serviceability for hydrogen transportation. Longitudinal and transverse relative to the rolling direction beam specimens with a single-edge fatigue crack under three-point bending loading were tested. For transverse specimens, the typical regularity of the lower value of J0.2 of the operated steel compared to its initial state is not fulfilled. The fractographic analysis of the static crack growth zone from the fatigue crack tip confirmed the presence of delaminations at the steel fracture surface, which were more pronounced for the operated metal and transverse specimens. The most intense delamination with a complex morphological crack growth was associated with crack front division into smaller areas of its active growth and branching. Such a feature was considered a factor of stress relaxation at the tip of a static crack and an increase in the steel fracture toughness.