The overall objective of this study was to elucidate the processing structure property relationship associated with the heat-affected zone (HAZ) produced in an HSLA microalloyed steel during arc welding. Single pass submerged arc welds on a Nb-V microalloyed steel were made with variable heat input. The thermal cycle as a function of heat input and position in the HAZ was determined experimentally in the course of welding. In addition, weld simulations were produced for selected heat inputs and HAZ locations. The evolution of austenite and transformation product microstructure as well as the state of microalloy precipitation was monitored as a function of heat input and HAZ location primarily via light and electron microscopy on specimens from actual welds and simulation specimens. These observations were utilized to support efforts to model austenite microstructure evolution and continuous cooling transformation behavior in the HAZ. Charpy impact toughness testing was performed on actual weld HAZ specimens and specimens of selected simulation specimens. Impact transition curves were determined and the microsctructure through which fracture propagated was correlated with impact transition energies and fracture surface morphology determined via scanning electron microscopy (SEM).