Micro-Behavior of Inclusions in X80 Pipeline Steel under Tensile Loading

Abstract

Specially designed SEM in-situ tensile tests have been conducted to trace the entire process of crack initiation and propagation induced by inclusions in X80 pipeline steel. The main inclusions are complex oxides containing aluminum, calcium and magnesium with round or oval shape, and their sizes are in the range of 3~20μm. Cracks initiate firstly in inclusions and also easily initiate among inclusions if they distribute in the steel as an inclusion chain. The size of an inclusion has little effect on the initiation stress of the first crack, which mainly depends on the yield strength of the steel. Once a crack initiates in X80 pipeline steel, it zigzag propagates along the direction of maximum shear stress that orients in about 45° to the loading direction till failure. In the process of crack propagation, the crack experiences propagation, anti-crack, bluntness and new micro-cracks initiation again and again. These valuable experimental results reveal the harmful effect of inclusions in micro-scale and are helpful for understanding failure mechanisms of the advanced steel components.