Study on cleavage fracture mechanism in the heat-affected zone of X80 GMAW automatic girth weld

Embrittlement of the heat-affected zone (HAZ) of girth weld has large effect on the service safety of high strength pipelines. In the present study, the cleavage fracture mechanism of X80 automatic gas metal arc welding (GMAW) girth weld joint was investigated. Microstructure of the HAZ, fracture morphology, cleavage fracture initiation and propagation as well as corresponding crystallographic features were characterized. The results showed that microstructure in the HAZ of X80 GMAW girth weld mainly consisted of lath-like bainite (LB), with localized coarse granular bainite (GB) and small fraction of continuously distributed martensite-austenite (M-A) constituent along prior austenite grain boundaries. Initiation of cleavage fracture was usually induced by decohesion of M-A constituent from the matrix. Cleavage cracks preferentially propagated through coarse GB in transgranular form. High angle boundaries (HABs) generally distributed along lath boundary of LB and grain boundary of GB, while density of HABs within coarse GB was low. Hence, cleavage cracks could not be retarded when they transgranularly propagated through coarse GB. Localized coarse GB and continuously distributed M-A constituent are also accounted for toughness scattering of X80 GMAW girth weld.