The present paper summarizes several findings in relation to the effect of bead application on the nil-ductility transition (NDT) temperature (NDTT) of drop-weight test specimens.
A series of tests simulating the heat-affected zone (HAZ) under the bead have proved that the overlap of the bead placing in standard specimens exerts a quench and temper effect on the specimen material and produces high toughness values in the HAZ. This HAZ works as a barrier against crack propagation and causes the NDTT to be lower.
It was also found that the toughness of the HAZ depends on the welding conditions, the configuration of the overlapping of the two-pass beads, and the chemical composition of the material.
Further, some embrittled materials were deembrittled by the thermal effect of the welded beads. This problem is specific to temper-embrittled material, and the bead application results in a lower apparent NDTT.
The above-mentioned phenomena are intrinsic to low-alloy and alloy steels. Since the drop-weight test was originally applied to carbon steels, the test method should be reevaluated based on the new findings.