Effect of Submicron‐Scale MnS Inclusions on Hydrogen Trapping and HIC Susceptibility of X70 Pipeline Steels

In this work, the non‐metallic inclusions contained in a trial X70 pipeline steels are characterized by optical microscopy, scanning electron microscopy, energy‐dispersive X‐ray spectrum, and transmission electron microscopy. Statistical analysis is conducted to summarize the size and shape of the inclusions. The hydrogen trapping and the resulting hydrogen‐induced cracking (HIC) susceptibility of the steels are tested. Density functional theory is used to calculate the binding energy of hydrogen at MnS inclusions, and the impact of MnS inclusions on hydrogen trapping and the HIC susceptibility is evaluated. It is found that the majority of submicron scale inclusions are MnS, which serve as irreversible hydrogen traps. The content of the trapped hydrogen can be effectively decreased by controlling the size of MnS inclusions below submicron scale and distributing the inclusions uniformly in the steel. As a result, the susceptibility of the steel to HIC is reduced.