Toward Understanding of Mechanical Property Degradation of Steel Welds in High-Pressure Hydrogen Transmission Pipeline – Role of Microstructure

An important component of the hydrogen infrastructure is the transportation of hydrogen from production site to usage area through high pressure (>1000 psi) pipelines. Exposure to high-pressure hydrogen can lead to severe mechanical property degradation in pipeline steel. The failures are mainly correlated to hydrogen embrittlement (HE) due to susceptible microstructures especially those encountered in the weld region including weld metal and Heat-affected Zone (HAZ). Fundamental understanding of the effect of weld microstructures on the resistance to hydrogen induced crack initiation and propagation is crucial for tailoring the filler metal chemistry and welding conditions for improving joint performance. In the present study, the microstructure-strength relationship in hydrogen is established for the high-strength X-80 pipeline steel girth weld. The mechanical testing is based on the in-situ spiral notch torsion test, where a specially designed load-frame is used to apply a pure torsion to a round bar with a spiral notch in the center. The load-frame containing the pre-stressed specimen is placed inside a hydrogen vessel charged to the required pressure. Strain gages attached to the load-frame are monitored for the crack initiation and growth in hydrogen. The testing result (i.e., threshold torque) is converted into the fracture toughness using a finite element model. The microstructure characterization involves the optical microscopy, SEM and micro-hardness mapping of fracture surfaces and weld regions. The microstructure consisting of various ferrite micro-constituents is quantified. Soft and hard zones are examined in details under SEM. It is found the soft zone is made of mostly coarsened ferrite while the hard zone consists of a mixture of bainite and martensite. The microstructure heterogeneity of X-80 pipeline steel weld is extracted based on the micro-hardness distribution.