Cracking Failure Analysis of X70 Pipeline Steel Weld

The weld defects were detected by Nondestructive Testing Technology(NTT). The microstructure and microhardness were examined by optical microscope(OM), scanning electron microscopy (SEM) equipped with EDS and Vicker microhardness tester to identify the reasons of cracking failure in X70 pipeline steel weldment. As a result, the microstructure in the root layer and filling layer of the weld presents ferrite and pearlite while it is ferrite and bainite in the covering layer, but the morphology and distribution as well as the content of ferrite and pearlite in the root layer are different from which in the filling layer; the cracks initiate at the root of the weld and propagate towards filling layer, and the C, O, S segregation is serious, which contribute to the initiation and propagation of the crack in the weld; the hardness decreases sharply around the crack tip, which exhibits lower strength.