Effects of Nb on microstructure and toughness of high-strength structural steels heat affected zone at high heat input

The influence of Nb on microstructure and toughness of heat affected zone (HAZ)in 610 MPa class high-strength structural steels have been investigated.At high heat input of 100 kJ/cm, HAZ width of two steels is all 7.5~8 mm, but coarse-grain heat affected zone (CGHAZ)width of the steel with Nb increases approximately by 1.5 mm compared to the steel without Nb.Impact energies values of CGHAZ within 2.5 mm from the fusion line for the steel without Nb is higher than 200 J, however, which are lower than 100 J for the steel with Nb.Weld thermal simulation indicates that impact toughness of the steel without Nb is far higher than the steel with Nb above peak temperature (T_p)of 1 200℃, and when lower than 1 200℃ the difference values of impact energies are reduced with the decrease of T_p.High peak temperature could result in the dissolution of niobium carbonitride precipitates.Nb in Solid solution may impede ferrite growth kinetics through a solute-drag effect on grain boundaries, and promote the formation of coarse lath bainite in CGHAZ.This study suggests that at high heat input Nb addition can deteriorate impact toughness of HAZ in high-strength structural steels.