Relationship between Microstructure and HIC Susceptibility of X60 and X100 Pipeline Steels

HIC resistance testing of X60 and X100 pipeline steels was performed according to the NACE standard and analyzed by metallographic microscopy and scanning electron microscopy (SEM). The results show that HIC sensitivity of X100 steel was much higher and the amount of cracks in X100 steel was more than that in X60 steel. Tiny cracks were formed on the interface of pearlite/ferrite in X60 steel, and were formed in bainite phase of X100. Thick cracks were formed owing to iron carbide and Al-O-Ti inclusions in X60 steel, and due to the combined effect of baintie and inclusions in X100 steel. However, the grain size of X100 steel was smaller. As a result, thick cracks in X100 steel were much thinner than those in X60 steel. In addition to iron carbide and calcification of Al-O-Ti inclusions, Mn-Ca-Mg-Si-O-S type composite inclusions appeared in X100 steel and the enrichment of Mo had little influence on cracks.