Abstract The welding preheat temperature is shown to significantly affect the dynamic fracture behavior of a 12CR-1Mo steel (HT-9) weld metal. A decreased preheat, effecting a faster weld metal cooling rate, results in an increased upper shelf energy and lower ductile-brittle transition temperature with no charge in weld metal. SEM examinations indicate a decreased dendrite spacing and lower interdendritic segregation with a faster cooling rate. It Is evident that the varying interdendritic ferrite content and morphology and the dendrite spacing, both controlled by the cooling rate, play a significant role on the weld metal dynamic fracture behavior. The results suggest the lowest preheat compatible with good welding practice be used for maximum benefits to weld metal fracture mechanics.