Effect of Heat Input on Microstructure and Tensile Properties in Simulated CGHAZ of a V-Ti-N Microalloyed Weathering Steel

Abstract

The mechanical properties of a coarse-grained heat-affected zone (CGHAZ) are affected by welding thermal cycling with varied heat input (Ej), but its effect on tensile properties is rarely studied. In the present work, Ej = 15, 35, 55, 75 kJ/cm CGHAZ samples were prepared via GleebleTM (St. Paul, MN, USA) for a novel V-Ti-N microalloyed weathering steel. The tensile properties of CGHAZ with varied Ej were evaluated. The results indicated that mixed microstructures dominated by lath bainitic ferrite (LBF) and granular bainitic ferrite (GBF) were obtained at Ej = 15 and 35 kJ/cm, respectively, while a mixed microstructure composed of GBF, intragranular acicular ferrite (IGAF), and polygon ferrite (PF) formed at Ej = 55 and 75 kJ/cm, apart from martensite/austenite (M/A) constituents in each Ej condition. The above variation tendency in the microstructure with the increase in Ej led to coarsening of low-angle grain boundaries (LAGBs) and a decrease in dislocation density, which in turn resulted in a yield strength (YS) decrease from 480 MPa to 416 MPa. The mean equivalent diameter (MED), defined by the misorientation tolerance angles (MTAs) ranging from 2–6°, had the strongest contribution to YS due to their higher fitting coefficient of the Hall–Petch relationship. In addition, the increase in the average size (dM/A) of M/A constituents from 0.98 μm to 1.81 μm and in their area fraction (fM/A) from 3.11% to 4.42% enhanced the strain-hardening stress. The yield strength ratio (YR) reduced as the Ej increased, and the lower density and more uniform dislocations inside the ferrite led to a uniform elongation (uE) increase from 9.5% to 18.6%.