Relationship between microstructural features in pipeline steel and hydrogen assisted degradation

Abstract

Abstract The contributions of various microstructural features towards hydrogen-assisted degradation in API 5L X80 pipeline steel have been investigated. The current study was conducted on specimens obtained from the top surface and mid-thickness layers. After microstructural characterization on the RD-TD plane of each specimen, the mechanical response was investigated with and without in-situ hydrogen charging; before probing the electrochemical behavior in hydrogen producing and non-hydrogen producing corrosive environments. Despite reduced tensile and yield strengths, the mid-thickness layer showed relatively greater resistance to failure under both testing conditions. The increased susceptibility to hydrogen damage at the top surface layer was linked to higher strength and lower ductility. Also, the microstructure at the mid-thickness region contained mainly acicular ferrite, while the top surface was dominated by bainitic-acicular ferrite. In addition, the crystallographic orientation of grains spread towards 〈111〉‖ND and 〈110〉‖ND texture fibers (specifically, 〈123〉‖ND) at the mid-thickness; unlike the top surface which had mostly 〈001〉‖ND textured grains (specifically, 〈013〉‖ND).