IMPACT FRACTURE BEHAVIOR OF X80 PIPELINE STEEL

Pipeline steels have been widely used in a long-distance transportation of large amounts of crude oil or natural gas under high pressure due to their high transportation efficiency, low energy loss and production cost. For high pressure gas transmission pipelines made from high-strength steels an important problem is to know their fracture behavior in a long running process. In this paper, fracture toughness of X80 pipeline steel was measured by V-notch Charpy impact test at -20 and -60 degrees C. OM, SEM, EDS and EBSD were used to analyze its fracture mechanism. Experimental results show that the maximum impact load is slightly influenced by temperature, but with the decrease of temperature, crack forming work, crack propagating work and energy absorbed by crack arresting decrease significantly. During fracture process, the intensive tensile stress in the sample would induce the deformation bands formed around the fracture surface and grains elongated along the direction vertical to the main crack, but original austenite grain boundaries are hardly deformed, it would cause stress concentration and then produce intergranular fracture. Generally, brittle second phase particles could act as crack sources under intensive internal stress. In the crack propagation process, grains near the main crack are deformed and elongated tempestuously, resulting in their breaking and new grains with high angle grain boundary (HAGB) formed. Therefore, the grain size decreases and the fraction of HAGB increases in the crack propagation region. Temperature has great influence on the plastic deformation behavior of pipeline steel during facture process. At -20 degrees C, the tested steel has good plasticity and the grains are refined during deformation, but at -60 degrees C, they are difficult to deform and refine. Consequently, the grain size near crack propagation zones in the sample impacted at -20 degrees C is much smaller than the original grain size, but in -60 degrees C impacted sample the grain sizes has little difference.