Fracture and Crack Propagation Behavior of 560 MPa Microalloyed Pipeline Steel under Different Cooling Schedules

Abstract

Three kinds of pipeline steel with different microstructures were fabricated by varying cooling schedules during thermo-mechanical controlled processing (TMCP). Charpy impact property of the pipeline steels were obtained, and the fracture and crack-arrest mechanisms were further studied. The results indicated that the steels were classified into two kinds according to their microstructures, the mixture of acicular ferrite (AF), quasi-polygonal ferrite (QF), granular bainite (GB) and small fraction of degenerate pearlite (DP), and the mixed microstructure of AF and GB, respectively. The processed steel with microstructure of AF and GB exhibited more excellent low-temperature toughness and crack-arrest properties with upper shelf energy of ~281 J and energy transition temperature of ~-76°C. The mixed microstructure (AF + GB) possessing smaller effective grain size hindered the propagating of crack and consumed large amount of energy during fracture. The effective grain size of microstructure was the dominant factor controlling low-temperature toughness and crack-arrest properties of pipeline steel, which increased the high-angle boundary length per unit area and further increased the crack propagation energy during fracture.

The Evolution and Distribution of Microstructures in High-Energy Laser-Welded X100 Pipeline Steel

“…The smaller the Σ value, the higher the frequency of occurrence. The more atoms there are at the grain boundary in the coincident lattice site, the denser the arrangement of grain boundary atoms; thus, there are fewer vacancies on the grain boundary and less adsorption of impurifying elements by the grain boundary, which improves the corrosion resistance and grain boundary strength [31,32,33].…”

“…The microstructure of the laser-welded joint in X100 pipeline steel was mainly composed of ferrite, and its twin system was {112}<11true1¯> [30,33]. The EBSD analysis results of X100 laser welded joints from the BM to the WELD are shown in Table 3.…”