Carbide development in the heat affected zone of tempered and post-weld heat treated 2,25Cr-1Mo steel weldments

In this work, welds were fabricated in thick section 2.25Cr-1Mo steel pipe using the temper bead and conventional weaving procedures. The development of alloy carbides in the heat affected zone of each weldment during tempering at 538 degreesC was studied and compared to those achieved by post-weld heat treatment at 725 degreesC. As-welded samples were tempered either at 538 degreesC for up to 4000 hours to simulate operating temperature exposure or at 725 degreesC for 3 hours to simulate post-weld heat treatment. Carbon extraction replicas were taken from the hear affected zone and analyzed using transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). The grain boundary carbide distribution, areal fraction, carbide species distribution and carbide chemistry were determined. In the as-welded condition, the major carbide species was M3C with minor amounts of fine, globular M2C. Also, M3C dissolved and M7C3, M23C6 and M6C developed during aging. Post-weld heat treatment over-aged the heat affected zone and coarse, globular carbides (identified as M7C3, M23C6, M6C and M2C) developed primarily at the grain boundaries, Only a small amount of fine, acicular M2C was observed. Carbides within the temper bead heat affected zone aged more rapidly than those in the conventional weaving heat affected zone as a result of the initial martensitic temper bead heat affected zone microstructure. Previous work had shown that the temper bead heat affected zone toughness decreased after 1000 hours at 538 degreesC. This correlated with the coarsening and agglomeration of the grain boundary carbides.