Effect of Welding Heat Input on Simulated HAZ Microstructure and Toughness of a V-N Microalloyed Steel

Abstract

Abstract This study investigates the correlation among the welding heat input, microstructure and impact toughness of the simulated coarse grain heat-affected zone (CGHAZ) of a V-N microalloyed steel which contained 140 ppm of nitrogen using Gleeble simulation technique, light microscope, electron microscope and Charpy-V-Notch (CVN) testing. CVN toughness estimated at −20°C supported that a relatively low energy input is recommended for welding of low carbon V-N microalloyed steel, according to a fact that an impact energy as high as of more than 180j attained in the CGHAZ with heat input less than 25Kj-cm−1, while a CGHAZ with impact energy less than 80j was obtained at a heat input more than 40 Kj-cm−1. Quantitative metallography of samples showed that CVN toughness decreased with the increase of the amount of grain boundary ferrite (GBF). Quantitative metallography of two-stage electrolytically etched samples showed that the averaged size and ratio of lath martensite-austenite (M-A) constituent increased with increasing heat input, while the linear density and amount of M-A decreased. It is indicated that the toughness of CGHAZ was severely dependent on the characteristic parameters of M-A and the amount of GBF. Microhardness of M-A constituents estimated by nanoindentation technique were much higher than that of the neighboring matrix. Severer brittle cracking susceptibility can occur in the CGHAZ with higher heat input based on Chen et al's suggestion that the stress concentration and triaxiality of the neighboring matrix are increased by the hard phase particles such as M-A constituents.