Microstructure and Hardness Change in High Temperature Service Depending on Mo Content in 2.25Cr-1Mo Steel Weld Metals

The microstructural characteristics and properties of 2.25Cr-1Mo weld metal were investigated at high temperatures by modifying the Mo content of commercial filler wires using the flux cored arc welding (FCAW) process. The microstructure of the as-welded specimens was composed of fine bainite and acicular ferrite, and the hardness and strength increased with increasing Mo content. Each weld metal was heat-treated at 400 degrees C, 500 degrees C, 600 degrees C and 700 degrees C for 24 h. The microstructure exhibited a tempered bainite structure after the heat treatment at 400 degrees C and 500 degrees C, whereas the bainite was slightly coarsened at 600 degrees C. Bulky ferrite with carbides that had transformed from the bainite dissolution was observed at 700 degrees C, and the hardness significantly decreased.