Effects of Hydrogen on Tensile Properties at Slow Strain Rate of Ultra High-Strength TRIP-aided Bainitic Ferrite Steels

To clarify the effects of hydrogen on the deformation properties of TBF steel at low strain rate, the tensile properties of hydrogen-charged TBF steel were investigated by using slow strain rate technique (SSRT). Total elongation of TBF steel decreased due to the hydrogen absorption. The hydrogen absorption changed fracture surfaces of TBF steel from dimple fracture to quasi cleavage fracture. In addition, hydrogen-assisted cracks perpendicular to the tensile direction were observed at the prior austenitic grain, the packet and the bainitic ferrite lath boundaries although voids occurred at the lath boundary in non-hydrogen-charged TBF steel. In the case of the hydrogen-assisted cracks in hydrogen-charged TBF steel, transformed martensite is located near cracks. Transformation of retained austenite to martensite in the TBF steel was slightly accelerated during tensile deformation when the TBF steel was charged with hydrogen. When tensile strain was applied to the hydrogen-charged TBF steel, hydrogen was evolved at the high temperature region of peak corresponding to vacancy clusters generated by the intersection of dislocation in the hydrogen evolution curve. Thus, deterioration of ductility of hydrogen-charged TBF steel was caused by the acceleration of transformation of retained austenite because of the hydrogen absorption and the promotion of crack initiation at the prior austenitic grain, packet and lath boundaries due to the hydrogen diffusion from transformed martensite.