Influence of the Crack-Starter Bead Technique on the Nil-Ductility Transition Temperature of ASTM A572 Grade 55 Hot-Rolled Plate

Abstract

An investigation of the nil-ductility transition (NDT) temperature of three heats of A572 Grade 55, hot-rolled steel plate, revealed a distinct and decided difference in behavior for specimens fabricated with a single-pass technique in comparison with those fabricated with the two-pass technique, which places the terminal weld crater at the bead center. The single-pass technique resulted in NDT temperatures 11°C (20°F) higher than those for the two-pass technique. Furthermore, the crack behavior was distinctly different at temperatures above the NDT, with the single-pass specimens showing progressively shorter cracks as the temperature was raised. This result is in contrast to that for the two-pass technique, which revealed no crack extension from the starter bead heat-affected zone (HAZ) above the NDT. The metallurgical reasons for the observed behavior were determined by utilizing optical microscopy, scanning electron fractography, and microhardness methods. The toughness of the various regions of the HAZ and the base metal were correlated with the microstructure and NDT behavior. The differences in the NDT temperature are related to a toughening of the HAZ by the two-pass technique. The effect of this toughened HAZ on crack propagation and crack fracture morphology from the crack-starter bead, through the HAZ, and into the base metal was evaluated.