Effect of Hydrogen on Tensile Shear Strength of Spot-Welded Ultrahigh-Strength TRIP-Aided Martensitic Steel Sheet

Abstract

In this study, the effect of hydrogen on the tensile shear strength of spot-welded transformation-induced plasticity (TRIP)-aided martensitic (TM) steel sheet was investigated. The tensile shear tests were carried out on a tensile testing machine using the spot-welded specimen which was spot-welded at the lapped portion of 30 mm×30 mm using specimen with dimensions of width of 30 mm and length of 170 mm at crosshead speeds of v=0.5 to 100 mm/min with and without hydrogen. The results were summarized as follows.

(1) The ultrahigh-strength TM steel without and with hydrogen charging possessed an excellent shear stress (τ_f) in comparison with the hot-stamped (HS1) steel. This might be attributed to the TRIP effect of the TM steel which exhibits the volume fraction of retained austenite (γ_R) of 1.52 vol% and low absorbed hydrogen concentration compared with that of the HS1 steel.

(2) The shear stress (τ_f) decreased with decreasing the deformation speed in the TM and HS1 steels with hydrogen whereas the shear stress (τ_f) was hardly changed by the crosshead speed in the HS1 steel without hydrogen. The decrease in the shear stress (τ_f) at slow strain rate might be caused by the occurrence of hydrogen diffusion to the crack initiation site and crack tip to accelerate the hydrogen embrittlement crack propagation.

(3) The hydrogen embrittlement crack was initiated at the heat affected zone (HAZ) due to the hydrogen diffusion and the hydrogen concentration at HAZ which is softer than its surroundings, so deformation is concentrated and HAZ becomes the origin of fracture, resulting in the stress concentration.