Microstructure Evolution and Mechanical Properties of HR3C Steel during Long-term Aging at High Temperature

Microstructure evolution and the changes in mechanical properties of HR3C steel during long-term aging at 650, 700 and 750 °C were investigated. The precipitated phases of the aging steel included M23C6 carbides, Z-phase and a trace amount of Nb(C,N). The M23C6 carbides were distributed mainly at the grain boundary, while Z-phase was mainly inside the grains. Amounts of both M23C6 carbides and Z-phase during the aging process increased with increasing aging period and temperature. Coarsening of M23C6 carbides was influenced significantly by aging time and temperature, while the size of the Z-phase was relatively less affected by the aging time and temperature, which had a steady strengthening effect. Coarsening of the M23C6 carbides was the main reason for the decline in high temperature yield strength during long-term aging at 750 °C. The M23C6 carbides were linked into a continuous chain along the grain boundary which accounted for the decrease of toughness during aging.