Elucidating the heat input on CGHAZ microstructure and its irregular effect on impact toughness for a novel V–N microalloying weathering steel

Abstract

This study investigated the effect of heat inputs on coarse-grained heat affected zone (CGHAZ) microstructure and impact properties of a novel vanadium (V) and nitrogen (N) microalloyed weathering steel via a Gleeble™ system, revealing the metallurgical essence of the steel suitable for large heat input (Ej) welding. The results indicate that in CGHAZ, the dominant microstructure was composed of lath bainitic ferrite (LBF) at Ej = 15 kJ/cm, granular bainitic ferrite (GBF) at Ej = 35 kJ/cm, and a mixture of intragranular acicular ferrite (IGAF), intragranular polygon ferrite (IGPF) and grain boundary polygonal ferrite (GBPF) at Ej = 55 and 75 kJ/cm, respectively, apart from martensite/austenite (M/A) constituents in each one. The area fraction, fM/A, and mean size, dM/A, of M/A constituents increased monotonously with the Ej. In addition, with increasing Ej from 15 to 35 kJ/cm, the fraction, fMTAs＞15°, of high-angle grain boundaries (HAGBs) with misorientation tolerance angles (MTAs) greater than 15° reduced, while the mean equivalent diameter, MEDMTA≥15°, of grains with HAGBs increased. However, as Ej further increased to 55 and 75 kJ/cm as the two large heat inputs, the precipitation of (Ti, V) (C, N) and thereby the heterogeneous nucleation of polygonal and acicular ferrite were promoted by a lowered degree of undercooling, resulting in the decreased MEDMTA≥15° and the increased fraction fMTAs＞15°. Accordingly, the impact toughness of CGHAZ was degraded with the increase of Ej from 15 to 35 kJ/cm, but enhanced with the increase of Ej from 35 to 75 kJ/cm.