Effects of hydrogen on the evolution of 4130 steel microstructure as a result of tensile loading

Neutron diffraction and Scanning Kelvin Probe Force Microscopy (SKPFM) were used to study the evolution of lattice strain, dislocation density, and phase partitioning of a ferrite/martensite pressure vessel steel (AISI 4130) that resulted from tensile loading in air and under three hydrogen pressures: 1.8 MPa, 9 MPa, and 18 MPa. Time-of-Flight (ToF) neutron diffraction patterns reveal a partitioning of the body-centered cubic (BCC) and body-centered tetragonal (BCT) phases as a function of applied strain and hydrogen pressure. A modified Williamson-Hall approach was used to analyze the broadening of Bragg peaks associated with BCC and BCT structures allowing dislocation densities of each phase to be extracted. No significant differences were observed in dislocation densities between in-air tests and those performed in hydrogen. However, lattice strain, particularly those measured along the loading direction, decreases with increasing hydrogen pressure once strained beyond the ultimate tensile strength. Moreover, a strain-induced loss in tetragonality resulting in greater BCC volume fraction observed for samples strained in air appears to be suppressed in the presence of hydrogen. Neutron diffraction results were compared with Scanning Kelvin probe force microscopy near the fracture surface. Contrasts in the contact potential differences measured at these surfaces are interpreted as the detection of BCC and BCT phases, which exhibit similar behavior as phase fractions determined from neutron diffraction results.