Effect of hydrogen on the mechanical properties of X-70 pipeline steel in diluted NaHCO3 solutions at different heat treatments

Studies were carried out on the effect of hydrogen on the mechanical properties of X-70 pipeline steel at different heat treatment conditions. Heat treatments were carried out as-received, under water-quenched, water-quenched and tempered and water-sprayed conditions. The solutions used consisted of 0.1, 0.05, 0.01 and 0.005 M sodium bicarbonate (NaHCO3) for the slow strain rate testing (SSRT) technique and hydrogen permeation measurements at 50∘C. Hydrogen permeation increased with solution concentration and was maximum for the quenched steel. Pre-charging of specimens with hydrogen did not have any effect on the mechanical properties of the as-received or water-sprayed steel but it did in the quenched and the quenched+tempered steels. In all the heat treatments, the mechanical properties of the steel were seriously affected only in 0.005 M NaHCO3, the steel in the quenched condition being the most susceptible. The cracking mechanism is dominated by anodic dissolution, film rupture of the steel in the as-received and water-sprayed conditions. But it was dominated by hydrogen embrittlement for the quenched and the quenched+tempered steels.