Prediction of toughness scatter in heat affected zones (HAZ) with local approach

Abstract

Carbon-manganese (C-Mn) steels are widely used in welded structures. For low temperature applications, it is essential to maintain a good toughness level in the welds. In this paper, the “local approach” for cleavage fracture is used based on the weakest link hypothesis. This kind of analysis has already been applied with success to homogeneous materials and in particular to the base metal and the weld metal of structural steels. By contrast, it has never been applied to the case of heterogeneous materials such as welds. The adopted test plan included four phases of increasing complexity. First, the approach was validated in very simple configurations, where it was possible to identify without ambiguities the mechanical properties of the different zones. Then more complex welds were studied. The first part of this study included measurement of mechanical properties and local criteria of the different zones [base metal and simulated heat affected zone (HAZ)]. In the second part, a brittle zone of well controlled dimensions was produced with an electron beam. The theoretical predictions match with experimental results. The third part was a validation attempt on a simple monopass weld. Single edge notch bend (SENB) specimens with a notch in the HAZ were tested in order to measure toughness ( K, J, CTOD). Results from the first part allowed comparison of experimental results with those predicted by the local approach. Although the real configuration of the electroslag welding (ESW) was more complex than had been expected, the local approach predictions were consistent with the experimental results. The objective of the fourth part was to study a multipass weld. As in the second part, the toughness measured on SENB specimens. The toughness in multipass HAZ decreases markedly at first and then very slowly when the percentage of coarse grains intercepted by crack front increases. The predictions agree fairly well with the experimental results. This study shows that local criteria allow a realistic prediction of the fracture toughness variation not only with temperature, but also as a function of brittle zones intercepted by the crack front. The scatter is also well predicted.