Crack Propagation Behavior during DWTT for X80 Pipeline Steel Processed via Ultra-fast Cooling Technique

The microstructure and crystallographic features of X80 pipeline steel of 18.4 mm in thickness, which was prepared by ultra-fast cooling (UFC) technique, were characterized by means of optical microscope, electron scanning microscopy, and EBSD technique. The performance of the steel by drop weight tear test (DWTT) was investigated in terms of the crack propagation and the morphology of fractured surfaces, while the mechanism related with the crack arresting ability of the steel resulted from UFC treatment was revealed. The results show that the microstructure of the steel is primarily composed of AF, CB, and M/A island, and the area fractions of AF and CB are ~83%, and ~17%, respectively. The effective grain size is ~3.5 μm, and the fraction of high-angle boundary is ~40.9%. The steel with higher fractions of AF and small-sized M/A island possesses smaller effective grain size, which is beneficial to the crack arresting ability of the steel. The mechanisms related to the enhancement of the crack arresting property may be ascribed to that the UFC promots the formation of AF, and increases the amount of small-sized M/A island by increasing cooling rate. Additionally, the variant selection during bainite transformation is weakened by UFC. Thus, the effective grain size is decreased, and the density of high-angle boundary is increased. © All right reserved.