| The severity of toughness loss of the HAZ in titanium microalloyed steel weldments increases with the welding heat input. The present study investigated the coarse grained zone (CGZ) in Ti-V microalloyed steels to identify the microstructural factors affecting the impact toughness of the CGZ. Steels with a high Ti-low V (0.04 wt% Ti, 0.02 wt% V) and a low Ti- high V (0.02 wt% Ti, 0.08 wt% V) microalloying were compared to delineate the beneficial effects of vanadium.;Single pass welding at 5.5 kJ/mm (140 kJ/inch) was employed with a welding geometry appropriate to produce a nearly straight fusion line that would permit the evaluation of the impact toughness of the narrow CGZ. Charpy impact results revealed the CGZ in the high V steel to possess better toughness as compared to the low V steel. In terms of the impact transition temperature (ITT), the high V steel CGZ had a 34 J(25 ft-lb) ITT of ;Microstructural studies of the simulated material revealed that the microstructure comprised of proeutectoid ferrite, Widmanstatten side plates, and acicular/lath ferrite. However, the CGZ with microstructure matching the simulated condition was narrower in the high V steel. Analytical electron microscopy (AEM) of second phase precipitates showed that the precipitates in the high V steel were (TiV) carbonitrides while those in the low V steel were Ti carbonitrides with trace amounts of V, Al, and Fe. While the precipitates, ranging in size from 5 nm to several microns, displayed local nonuniformity in the population distribution, in the normalized condition, both steels revealed a distribution peak at about 15 nm. However, the high V steel had a higher volume fraction of particles smaller than 25 nm in size. While the particle volume fraction was little affected by the welding process, the population peak in the high V steel shifted to smaller particle sizes leading to a precipitate dispersion with increased potential to inhibit grain coarsening. |
