The Influence of Microstructural Variations on Hydrogen Absorbance and Tensile Properties at Elevated Hydrogen Levels for TRIP-Aided Bainitic Ferrite Steels

The influence of microstructure on susceptibility to hydrogen embrittlement was investigated using a series of four experimental transformation-induced plasticity-aided bainitic ferrite (TBF) sheet steels. The TBF steels were designed to have similar carbon equivalent values (C_EQ) and tensile strengths despite having significant variation in carbon and manganese concentrations and microstructure. Quasi-static tensile tests were performed on the steels after various durations of electrochemical hydrogen charging in order to characterize susceptibility to hydrogen embrittlement. The microstructure of each steel was characterized using electron back-scatter diffraction and x-ray diffraction. In particular, microstructural attributes believed to potentially contribute to hydrogen retention, such as austenite (γ) and martensite/austenite (MA) island volume fractions, grain and phase boundary areas, and γ aspect ratio, were quantified. Variations in these microstructural components were compared to observed differences in hydrogen embrittlement susceptibility and hydrogen absorption behavior to identify the influence of key microstructural characteristics. Increased γ volume fraction and/or increased γ phase boundary area was found to have the dominant influence on increased hydrogen absorption and hydrogen embrittlement susceptibility. Increased γ aspect ratio was also found to have a minor influence.