Fracture behaviour of C–Mn steel multipass MMA weld metals at −60°C in Charpy V testing

Abstract

By observation of the fracture surfaces and of appropriate metallographic sections of C–Mn steel multipass MMA (manual metal arc) weld metals and simulated weld specimens which were fractured at −60°C in Charpy V tests, it was found that the impact toughness of the specimen could be correlated with the length of the fibrous crack which was limited by unstable propagation of the cleavage crack; the latter could be initiated at a type of second phase particle transformed from carbon rich regions or non-metallic inclusions. The weakest zone in which the cleavage crack initiated was characterised by coarse grains of ferrite and the critical event which gave rise to unstable propagation of a cleavage crack was a crack in the ferrite grain larger than 30 μm cutting through the boundary and extending over the specimen. On the basis of these results, a model of the fracture mechanism is proposed and the effect of Mn content on increasing toughness is explained using the model.MST/647