Diffusion and Critical Hydrogen Content of Carbon Steels with Different Strengths and Microstructures

Four thermomechanically rolled carbon steels with different strength levels and microstructures, namely S355M, X65M, S960M, and S1100M, were selected, and their critical hydrogen contents H_crit were determined. H_crit is the hydrogen content where brittle fracture of the steel can occur. The H_crit values for the four carbon steels are 2.03 (S355M), 0.91 (X65M), 0.32 (S960M), and 0.53 ppm (S1100M). Bainitic carbon steel S1100M outperforms lower-strength tempered martensitic steel S960M in terms of resistance to hydrogen embrittlement. All other steels follow the trend that a higher strength of steel results in a lower resistance to hydrogen embrittlement. The root cause for the beneficial behavior of bainitic steels is discussed.