Evolution and Coarsening of Carbides in 2.25Cr-1Mo Steel Weld Metal During High Temperature Tempering

Transformation and coarsening of carbides in 2.25Cr-1Mo steel weld metal during tempering at 700 oC for different time intervals ranging from 1 to 150 h has been examined by transmission electron microscopy and scanning electron microscopy. M3C carbides were observed in the as-welded specimens and when tempered the precipitates were mainly composed of M3C, M7C3 and M23C6 carbides. A sequence for corresponding carbide transformation during tempering with initial precipitation of M3C and followed by M7C3 and M23C6 has been proposed. The precipitation of M7C3 with higher Cr content is the main factor contributing to the decrease of coarsening rate of precipitates after prolonged tempering. The decrease of hardness in the tempered specimens agreed well with the prediction of the weakening of precipitation strengthening due to the coarsening of carbides.