Effect of Post-Weld Heat Treatment on the Microstructure of Plasma Arc Welded DP600 Steel

The effect of isothermal post-weld heat treatment on keyhole plasma arc welded DP600 steel was investigated using optical microscopy, scanning electron microscopy and analytical transmission electron microscopy. The isothermal heat treatment had no effect on the allotriomorphic, Widmanstätten and acicular ferrite in the fusion zone (FZ). The martensite lath structure showed complete recovery in FZ, base metal (BM) and heat-affected zone (HAZ) with the formation of coarse cementite particles of up to 200 nm size in subcritical HAZ. The coarse grain HAZ showed little change in bainite and had tempered martensite and transformed retained austenite. The fine grain HAZ consisted of bainite, tempered martensite and coarsened preexisting cementite. The recovered BM had tempered martensite and reduced dislocation density. Hardness was found to be the lowest in subcritical HAZ, which is attributed to complete tempering of martensite, formation of large cementite particles and disappearance of dislocations.