Hydrogen Effects in Prestrained Transformation Induced Plasticity Steel

Thermal desorption analysis (TDA) was performed on laboratory heat-treated transformation induced plasticity (TRIP) steel with 14.5 pct retained austenite (RA), ultimate tensile strength (UTS) of 880 MPa, and elongation to failure of 33 pct. Samples were tensile prestrained 5 pct at 253 K (–20 °C), 296 K (23 °C), and 375 K (102 °C) to generate different amounts of deformation-induced martensite, 10.5, 5.5, and 0.5 pct, respectively, prior to cathodically charging to a hydrogen content of 1 to 2 ppm. TDA was performed on charged samples to determine the location and strength of hydrogen trapping sites. TDA results suggest that dislocations were the main trapping sites in prestrained TRIP steel. The TDA peak intensity increased with prestrain, suggesting that the quantity of hydrogen trap sites increased with deformation. Tensile tests were performed on the four hydrogen-charged TRIP steel conditions. As confirmed with transmission electron microscope images, samples with more homogeneous dislocation distributions (i.e., prestrained at 375 K (102 °C)) exhibited greater resistance to hydrogen embrittlement than samples that included a high dislocation density adjacent to the formations of strain-induced martensite (i.e., samples prestrained at 253 K (–20 °C) and 296 K (23 °C)).