Effect of ferrite on the hydrogen embrittlement in quenched-partitioned-tempered low carbon steel

This article proposes a novel method for the reduction of the hydrogen embrittlement susceptibility in a low carbon steel by the introduction of the ferrite phase into the quenched-partitioned-tempered (Q-P-T) specimen. Taking the Q-P-T specimen without ferrite as the reference, the CQ-P-T and QCQ-P-T specimens containing ferrite were obtained by intercritical annealing before the Q-P-T treatment and by pre-quenching before the CQ-P-T treatment to further refine the microstructure. The susceptibility of the three kinds of specimens toward hydrogen embrittlement was evaluated by the slow strain rate tensile test. The results showed that the elongation and area reduction of the three kinds of specimens continuously decrease with the greater electrochemical precharging time, but the three kinds of specimens exhibited different susceptibility index values. Fractographic observation showed that the primary fracture modes gradually evolved from typical dimple to quasi-cleavage, quasi-cleavage mixed with intergranular cracking and then complete intergranular manner, but the evolution rate of the QCQ-P-T specimen is the lowest while that of the Q-P-T specimen is the highest. It is suggested that the ferrite may reduce the stress concentration caused by the martensitic transformation and retard the crack propagation during deformation. Refining microstructure further improves the hydrogen embrittlement resistance by trapping more hydrogen atoms at the interphase boundary, lowering their density in the lattice.