Fracture toughness of welds - effect of brittle zones and strength mismatch

The aim of this study is to examine the effect of the welding procedure, the heat input in particular, on the changes induced in the mechanical behavior of welds on an high strength quenched and tempered steel. Welds were performed by submerged arc welding, in a high quenched and tempered steel RQT 601 with 25 mm thick, using heat inputs of 2, 4.2 and 6.6 kJ/mm. COD/J specimens were extracted transversely to the welding direction and tested according to BS 7448-Part 1 and Part 2, at a temperature of -20 degreesC. The increase of heat input caused softening of the weld metal and heat-affected zone, due to microstructural changes and microstructural coarsening. A decrease in toughness was observed in the weld metal and heat-affected zone with the increase of heat input. The experimentally obtained value of the dimensionless factor m, relating J with COD, is similar to that predicted by the equations established in the literature. A general trend for the decrease of J and COD toughness with the increase in the size of the brittle zone sampled by the fatigue crack front is observed, though some scatter is always omnipresent. (C) 2004 Elsevier B.V. All rights reserved.