A statistically designed series of vanadium and columbium microalloyed C-Mn HSLA steels was used for an investigation of heat-affected zone (HAZ) toughness in post weld heat treated (PWHT) multi-pass welds. The vanadium additions were in the range 0.005 to 0.097 Wt.% and the columbium additions were in the range 0.004 to 0.06 Wt.%. GMAW processes with welding heat inputs of 3kJ/mm and 5kJ/mm and post-weld heat treatments at 620°C for 2 and 10 hours were employed. A degradation of the HAZ toughness with additions of microalloy elements V and Cb in the as-welded and PWHT conditions was revealed. The 50 Joule (37 ft-lb) transition temperature (TT50J) for HAZs in all weld conditions correlated with maximum HAZ hardness. Increases in HAZ hardness and TT50J caused by PWHT were observed. Hence PWHT in some situations may not be beneficial for V/Cb microalloyed HLSA steels.

The randomly distributed precipitation of V and Cb carbides (V, Cb)C, including dislocation precipitation and matrix precipitation with particle sizes of 5-15 nm, is the predominant alloy carbide precipitate morphology in these steels. Banded morphology of (V, Cb)C precipitation is rarely observed in the HAZ. The volume fraction of (V, Cb)C precipitates increases as increasing V and/or Cb contents in the experimental heats. The volume fraction of precipitates also increases with increasing the PWHT time.