Effect of Heat Input on Microstructure and Toughness of Coarse Grain Heat Affected Zone in Nb Microalloyed HSLA Steels

Abstract

The influence of Nb on microstructure, mechanical property and the transformation kinetics of the coarse grain heat affected zone (CGHAZ) in HSLA steels for different heat inputs, has been investigated. When welded at higher heat inputs (100 — 60 kJ/cm), impact toughness values of the steel without Nb are much higher than those of the steel with Nb, and the lowest span is 153 J at 60 kJ/cm. But only a little higher values are observed at lower heat inputs (40 — 30 kJ/cm), and the highest span is 68 J at 30 kJ/cm. Dilatation studies indicate that continuous cooling transformation starting temperatures ( T s ) of CGHAZ for the steel with Nb are approximately 15 — 30°C which are lower than those of the steel without Nb at all heat inputs. For higher heat inputs, Nb in solid solution suppresses ferrite transformation and promotes the formation of granular bainite which has detrimental effect on impact toughness. For lower heat inputs higher Charpy impact energy values in the steel with Nb are associated with the formation of low carbon self-tempered martensite.