Effect of Nb on Weld Metal Toughness in Tank Car Steels

This study was undertaken to determine the effect of Nb on the toughness of welds made on a tank car steel in the stress-relieved condition. Three special laboratory heats were made with chemistry conforming to AAR TC128B and only the Nb content varying. These special ingots were hot-rolled to 12.5 mm thick. Submerged arc welds were made on the plates using parameters typically used in tank car fabrication, as well as other selected flux-wire combinations. One weld for each condition was stress-relieved and weld metal Charpy impact testing was carried out on the specimens in the as-welded and stress-relieved conditions. Of the several flux-wire combinations tested, only two were found to give acceptable stress-relieved weld metal toughness with the procedure employed at −30°F and −50°F and another at −30°F, only with the Nb-free base steel. The deleterious effect of Nb, especially in the stress-relieved condition and the consequent need to revise the TC 128B specification was demonstrated. Some possible mechanisms by which Nb could reduce weld metal toughness are discussed and future work is outlined.