Microstructure and impact toughness of the coarse-grained heat affected zone in titanium-vanadium microalloyed steel weldments.

摘要

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.