Carbide and hardness development in the heat-affected zone of tempered and postweld heat-treated 2.25Cr-1Mo steel weldments

The temper-bead and the conventional weaving, multipass welding procedures have been developed to eliminate postweld heat treatment (PWHT) of creep resistant 2.25Cr-1Mo steel. Both procedures aim to refine and temper the heat-affected zone (HAZ). The temper-bead procedure resulted in a martensitic HAZ and a homogeneous fine-grain size. Hardness was not decreased by high heat input weaving fillout passes. Upper bainite developed in the conventional weaving HAZ, although grain refinement was inhomogeneous and some martensite was present. However, the conventional weaving procedure appears to produce a lower as-welded hardness than the temper-bead procedure. The carbides within the temper-bead HAZ aged more rapidly than those in the conventional weaving HAZ as a result of the initial martensitic temper-bead HAZ microstructure. Previous work indicated that the temper-bead HAZ toughness decreased after 1000 h of tempering at 538 degreesC. This correlated with the coarsening and the agglomeration of the carbides. The maximum hardness occurred within the as-welded coarse-grained HAZ. The PWHT resulted in the greatest decrease in hardness and also reduced hardness variability throughout the HAZ. The hardness decreased to postweld heat-treated values after approximately 1000 h at 538 degreesC and softening was associated with the precipitation and coarsening of acicular carbides and the development of coarse grain-boundary carbides.