Effect of Separation on the Fracture Surface of Pipeline Steels with Ferrite-Bainite Dual Phases During Drop Weight Tear Test

The effect of separation on the fracture surface of ferrite-bainite dual-phase pipeline steels during a drop weight tear test (DWTT) was investigated via microstructural analysis and fracture surface examination. Two specimens (ST1 and ST2) were designed to have a dual-phase microstructure, with different ferrite and bainite fractions, by controlling the reduction ratio and holding time in the ferrite-austenite two-phase region during the thermomechanical process. Notably, ST2, which had a longer holding time for the retained austenite to absorb carbon, exhibited a harder bainite but softer ferrite phase compared to ST1. The greater hardness difference between those phases in ST2 induced the strain incompatibility frequently evident at the phase boundaries, resulting in lower ductility during the tensile test. Owing to the strain incompatibility, ST2 also generated more separation over a wider temperature range during DWTT. At low temperatures, where the brittle fracture was prominent, it was observed that ST2 generated separations on its fracture surface, whereas ST1 did not. These separations formed a local shear lip around themselves, thereby obstructing cleavage fracture propagation from the notch. Hence, it was confirmed that ST2 had a higher DWTT shear area than ST1 over a temperature range near the ductile-brittle transition temperature (DBTT). As a result, the DBTT of ferrite-bainite dual phase steel could be improved by increasing the amount of separation during DWTT.