Effect of Microstructure on the Mechanical Properties and Fracture Toughness of API X65 Pipeline Steel in the Presence of Hydrogen

Abstract

This study investigated the influence of microstructure on the mechanical properties and fracture toughness of API X65 pipeline steel in the presence of hydrogen. In this study, electrochemical method was used for hydrogen charging and indentation technique was applied to obtain the fracture toughness. The results showed that in the presence of hydrogen, elongation (EL%), reduction of area (RA), ductile fracture percentage, and fracture toughness of all microstructures decreased. The microstructure of martensite (M) + bainite (B) + ferrite (F), had the highest hydrogen trapping and uptake (Capp) as 8.58 × 10–6 mol cm−3 and the lowest apparent hydrogen diffusivity (Dapp) as 5.68 × 10−10 m2 s−1; thus, the maximum decrements of 33% in fracture toughness, 40% in ductile fracture percentage, 47% in RA, and 35% in EL% were observed. However, the microstructure of ferrite (F) + degenerated perlite (DP) + martensite-austenite micro constituent (M/A), where the lowest value of 5.85 × 10–6 mol cm−3 for Capp and the highest value of 8.5 × 10–10 m2 s−1 for Dapp had the minimum decrements as 2% in fracture toughness, 10% in ductile fracture percentage, 4% in RA, and 7% in El%. According to the obtained results, depending on the type of microstructures, hydrogen-induced work softening or hardening were observed by decreasing or increasing the yield stress respectively.