The interaction of hydrogen with retained austenite in automotive steel grades

Abstract

This work investigates the effect of a constant load on hydrogen diffusion through transformation induced plasticity (TRIP)-assisted and quenching and partitioning (Q&P) steel containing metastable retained austenite by combining electrochemical hydrogen permeation and thermal desorption spectroscopy. Furthermore, a comparison is made with ferrite/martensite dual phase (DP) steel, which serves as a reference material not containing any retained austenite. Material samples are placed at different external loading conditions, ranging from 50% to 125% of the yield stress. The permeation transients indicate a different response between TRIP steel and Q&P steel. In the latter, hydrogen diffusion is delayed for all stressed conditions, even at stresses in the elastic regime, whereas TRIP steels follows the same behaviour as the reference DP steel with increased hydrogen diffusivity in the elastic regime and decreased in the plastic regime.From subsequent thermal desorption spectroscopy performed on the specimens after the permeation test, the hydrogen desorption spectra of Q&P steel samples show a high temperature peak which is not present in the spectrum of the unloaded sample. Thus indicating that the retained austenite is capable of trapping hydrogen in a loaded condition. Due to the difference in matrix surrounding the austenite phase, it is reasoned that the discrepancy in trapping behaviour is due to the internal stresses present in the austenite in Q&P steel and the higher hardness of the surrounding matrix compared to TRIP steel.