Microhardness variations in HSLA-100 welds fabricated with new ultra-low-carbon weld consumables

Microstructural evolution resulting from gas metal are welding of HSLA-100 steel plate with an experimental ultra-low-carbon weld consumable, designat ed CTC-03, was investigated by transmission electron microscopy, optical mi croscopy and microhardness analyses. A color microhardness map was generate d from more than 1600 diamond indentation measurements taken across the tra nsverse cross section of the weldment, to make a direct correlation between the microhardness variations and the corresponding microstructures. Eight characteristic areas of the CTC-03/HSLA-100 weld were selected to study the corresponding microstructure by TEM and optical microscopy, and the micros tructures were then correlated with the microhardness map. The microstructu re throughout the fusion zone (FZ consists of a majority of lath ferrite wi th varying amounts (depending on the location) of untempered fine-lath mart ensite, some interlath retained austenite, and spherical oxide inclusions. The softest regions of the fusion zone are in curved white bands located at the lower portions of the heat-affected zones of the weld beads. The micro structure within these white bands is predominantly lath ferrite. The harde st regions in the fusion zone are located between the bead boundaries and t hese white bands, and contain significant amounts of fine untempered lath m artensite. The base plate heat-affected zone (HAZ) consists of a mixture of unrempered lath martensite and coarse autotempered plate martensite. The a s received HSLA-100 base metal exhibits a quench-and-tempered microstructur e with a majority of fine-lath martensite and significant amounts of coarse martensite. Despite its low heat input, the CTC-03 weldment exhibits hardn ess values comparable with those of the HSLA-100 base metal and similar to some weldments made With other filler metals. The microhardness map of this weld was compared to four other microhardness maps developed for welds mad e with other candidate ULC filler metals and/or different heat inputs. In a ll five weldments studied, the midsection of the HAZ of the base metal (i.e ., the region located about midway between the weld interface and the outer boundary of the HAZ) is the hardest region of the weld, regardless of base metal, filler metal type or heat input.