Toward understanding of mechanical property degradation of steel welds in high-pressure hydrogen transmission pipeline - Role of microstructure

An important component of hydrogen infrastructure is the transportation of hydrogen from production site to usage area through high-pressure (>1800 psi) pipelines. Exposure to high-pressure hydrogen may lead to severe mechanical property degradation in pipeline steels and their welds. These failures could be correlated to hydrogen embrittlement (HE) due to susceptible microstructures in the weld region including weld metal (WM) and heat-affected zone (HAZ). Fundamental understanding of the effect of microstructures on the resistance to hydrogen induced crack initiation and propagation is crucial for designing filler metal chemistry and welding conditions for improved performance. In the present study, the weld microstructure of various pipeline steels ranging from X-52 to X-80 studied using advanced characterization techniques. The microstructure characterization involves the optical microscopy, electron backscatter diffraction (EBSD), and micro-hardness mapping of WM and HAZ. The microstructure consisting of various ferrite micro-constituents is quantified. Soft and hard zones are examined in details. It is found the soft zone in HAZ is made of mostly ferrite while the hard zone in WM consists of grain boundary ferrite and acicular ferrite. Possible mechanisms for forming the heterogeneous microstructure distribution are discussed. Mechanical testing based on spiral notch torsion test for quantifying the fracture toughness degradation in hydrogen is going. The effect of weld microstructure heterogeneity on mechanical property is discussed based on the preliminary testing results.