Effects of Prestrain and Inner Stress on the Susceptibility of Hot Stamped Boron Steel to Hydrogen Embrittlement

Hot stamped boron steel parts with martensite and bainite combined microstructures are widely used in lightweight manufacturing of automotive bodies. The final products usually experience the manufacturing process, such as trimming or punching, which inevitably introduces inner stress and plastic strain. Hot stamped parts with complex microstructures and manufacturing effects exhibit some special characteristics of hydrogen embrittlement (HE) in service. In this paper, hardened boron steels with a total martensite content and different martensite-bainite combinations, called martensite or bainite dominant microstructures (MDM/BDM), are experimentally obtained by a flexible quenching tool, and the HE susceptibility considering the effects of prestrain and inner (tensile/compressive) stress is investigated based on slow strain rate tensile tests by developing hydrogen charging equipment under constant loads. SEM and EBSD analyses on hydrogen-inducted cracks (HICs) and thermal desorption analysis of hydrogen for different microstructures and strain/stress states are carried out to illustrate the HE mechanism. The HE susceptibility of the as-quenched specimen decreases with increasing volume fraction of bainite, with the fractograph exhibiting intergranular fracture in the MDM and the dimples feature in the BDM. Prestrain weakens the HE susceptibility in the MDM but raises it in the BDM due to the enhancement of hydrogen adsorption, promoting local plasticity in the evolution of the HICs. Inner stresses increase the HE susceptibility in the MDM, which lifts the hydrogen segregation at the grain boundaries to intensify the hydrogen-enhanced decohesion effect. Tensile stress intensifies the sensitivity to HE in the BDM. The promotion of hydrogen adsorption facilitates crack initiation. However, compressive stress has opposite effects on the HE susceptibility due to the improvement of the material ductility by reducing the hydrogen content in the specimen.