Abstract
In this study, the effect of hydrogen on the spot-welded tensile shear strength of transformation-induced plasticity (TRIP)-aided martensitic (TM) steel sheet was investigated. The tensile shear tests were carried out on an Instron-type universal testing machine using the tensile shear specimen which was spot-welded at the lapped portion of 30×30 mm2 using specimens with dimensions of width of 30 mm and length of 170 mm at crosshead speeds of 0.5-100 mm/min without and with hydrogen. The results were summarized as follows.
(1) The ultrahigh-strength TM steel without and with hydrogen charging possessed an excellent tensile shear stress (τf) in comparison with the hot-stamped (HS1) steel. This might be attributed to the TRIP effect of the TM steel which exhibits volume fraction of retained austenite of 1.52 vol% and low absorbed hydrogen concentration compared with that of the HS1 steel.
(2) The τf decreased with decreasing the deformation speed in the TM and HS1 steels with hydrogen, whereas the τf was hardly changed by the crosshead speed in the HS1 steel without hydrogen. The decrease in τf at slow strain rate might be caused by the occurrence of hydrogen diffusion to crack initiation site and crack tip to accelerate hydrogen embrittlement crack propagation.
(3) The hydrogen embrittlement crack was initiated at heat affected zone (HAZ) due to the hydrogen diffusion and hydrogen concentration at the HAZ which is softer than its surroundings, so the deformation is concentrated and HAZ becomes the origin of fracture, resulting in the stress concentration.
