Pre-strain and hydrogen charging effect on the plastic and fracture behavior of quenching and partitioning (Q&P) steel

This study investigates the fracture behavior of plastically deformed quenching and partitioning (Q&P) steel under hydrogen conditions. The experimental results from slow strain rate tensile (SSRT) tests reveal a considerable decrease in elongation with increase of hydrogen traps induced by pre-strains. The effect of pre-strain on hydrogen susceptibility is analyzed through fractography and microstructure measurements. The relative reduction of area decreased from 0.389 at 0 % pre-strain to 0.181 at 12 % pre-strain with hydrogen. For hydrogen-free case, fracture surfaces show typical ductile fracture with shallow dimples by micro-void coalescence induced by phase transformation. For hydrogen-charged specimens, the fractography indicates quasi-cleavage and transgranular fracture at specimen center and brittle-like fracture can be analyzed by the hydrogen-enhanced decohesion. Additionally, the brittle fracture area is enlarged with pre-strain. The mixed fracture between the edge and center of cross-section can be explained using finite element modeling, which calculates the distributions of dislocation density and flux, hydrostatic stress during SSRT at different pre-strains. As an effect of pre-strain in the hydrogen-charged Q&P steel, micro-cracks initiate at transformed martensite with low level of pre-strain, whereas their propagation is blunted at ferrite interface. For specimens with pre-strains exceeding 8 %, the cracks initiate predominantly at the martensite, which propagate through deformed ferrite matrix with high dislocations and trapped hydrogens. Our findings shed light on a new metallurgical design for improving the resistance to hydrogen embrittlement in Q&P steel when pre-strain, hydrogen transport, and phase transformation are properly controlled.