Hydrogen-Induced Cracking Susceptibility and Hydrogen Trapping Efficiency of theWelded MS X70 Pipeline Steel in H2S Environment

Pipeline steels for sour oil and gas containing H2S generally suffer from either hydrogeninduced cracking (HIC) or sulfide stress corrosion cracking (SSC). Oil and gas containing high concentration H2S are noxious to pipeline steels because of the hydrogen- induced corrosion. In this study, HIC susceptibility of welded MS X70 pipeline steels was evaluated in NACE "A" solution at room temperature. Meanwhile, microstructure and regions near a HIC crack in the MS X70 base steel and its welded joint were analyzed through OM, SEM and EBSD. The hydrogen trapping efficiency was also investigated by measuring the permeability (J(infinity)) and the effective hydrogen diffusivity (D-eff). The results showed that both base metal and welded joint were highly susceptible to HIC and the later steel sample was more vulnerable than the former. This higher susceptibility could be primarily attributed to the following effects: the higher hydrogen trapping efficiency of bainitic lath microstructure in the welded joint; the more low angle grain boundary in the welded joint also made it easier to crack by improving the hydrogen trapping efficiency of high angle grain boundary; the less amount of coincidence site lattice grain boundary and Sigma 13b. Sigma 29b lead to higher HIC susceptibility by decreasing the resistance to crack of high angle grain boundary.