Dramatically enhanced impact toughness in welded 10%Cr rotor steel by high temperature post-weld heat treatment

10%Cr steels are being used in steam power plant due to their excellent properties at high temperatures. Dramatically enhanced impact toughness was obtained in the welded 10%Cr rotor steel after post-weld heat treatment, increasing from 5.0 J for the as-welded material to 37.5 J after being heat treated at 730 °C. Analysis of crystallographic structure showed that the microstructure was transformed from as-welded lath martensite to tempered martensite (treated at 670 °C to 700 °C), and then to high-temperature tempered martensite (treated at over 710 °C). With the increase of treatment temperature, the fractography varied from brittle fracture to a fracture mode with both brittle and ductile fracture features. A small amount of M7C3 (M = Fe, Cr) particles was found in as-welded metals, while large amounts of M23C6 particles were observed in those after being treated. It is proposed that the microstructure evolution from martensitic laths in the as-welded metals to a softer ferrite matrix with the dispersion of metal carbides led to higher toughness in the post-weld heat treatment materials.