Abstract
In situ synchrotron X-ray diffraction was performed on an X60 ferritic steel during fully-reversed strain-life cycling in 3.5 MPa hydrogen gas and in air using beamline 1-ID at the Advanced Photon Source (APS) at Argonne National Laboratory. The 1-ID beamline is equipped with both wide and small angle X-ray scattering (WAXS/SAXS) detectors, allowing for simultaneous data collection at length scales that probe lattice strain, dislocation density, and voids. These material properties can impact multiple proposed mechanism of hydrogen embrittlement such as, hydrogen-enhanced localized plasticity (HELP), hydrogen-enhanced decohesion (HEDE), and nanovoid coalescence (NVC). Therefore, understanding how the steel microstructure changes throughout the strain-life cycle in a hydrogen environment is of great interest among industry, materials scientists, and engineers alike.
A series of fully-reversed strain-life measurements in air and hydrogen gas were performed on X60 pipeline-equivalent plate steels at the National Institute of Standards and Technology (NIST) to establish the number of cycles to failure in both environments. A strain amplitude of 
= 0.01 was then selected for further investigation at the APS. A custom aluminum chamber, developed at NIST for X-ray and neutron beamline studies, was mounted on a 12 kN load frame on beamline 1-ID. X-ray diffraction patterns, in air and in hydrogen, were obtained throughout the gage volume of steel specimens during hold times at specific moments in the strain-life cycle. Measurements like this were repeated until just before failure occurred, which allowed for the propagating crack to be fully surveyed. Analysis of WAXS data reveals a distribution of lattice strain and dislocation density in proximity to the crack front at various cycle numbers in the material lifetime curve. These results are compared to previous ex situ measurements on strain-life tested steels and demonstrate a unique combination of capabilities to investigate multiple mechanisms that drive hydrogen-assisted crack growth in steel.