Ductile-Brittle Transition Evaluation of Laser Welded Steel Metal by Means of Small Specimen Impact Test

Synopsis : Small specimen impact tests using miniaturized specimens with 1.5, 1.0 and 0.7mm square and 20mm length were carried out to evaluate the toughness of a laser welded metal which has been difficult to estimate accurately by the standard full or half size specimens due to a fracture path deviation from weld metal to base metal. Three kinds of steel welded metals with a quite same chemical composition were examined : submerged arc welded metal (Type S), laser welded metal on the submerged arc welded metal (Type L), and the submerged arc welded metal subjected to a thermal cycle with water quenching after welding (Type T). A reliable ductile-brittle transition temperature (DBTT) and fracture appearance transition temperature (FATT) based on the data partitioning method were obtained by using the miniaturized specimens with keeping correlations to those of the standard size specimen. The DBTTs and FATTs given by each size specimens showed a simple linear correlation, though lower DBTT and FATT were given by smaller specimens. The laser welded metal composed of an extremely fine acicular ferrite structure was estimated to have such an excellent toughness as 168K DBTT in conversion to the standard size specimen. The small specimen impact tests using miniaturized specimens were established as a useful method to estimate toughness in a small and narrow region of welded steels.