Analysis of creep rates of tempered martensitic 9%Cr steel based on microstructure evolution

The creep rates of tempered martensitic 9Cr–3W–3Co–VNb steel with different carbon and boron concentrations were investigated with emphasis on the behavior in the transient creep region. Creep tests were carried out at 650 °C up to about 7 × 104 h and at 700 °C up to about 4 × 103 h. The precipitates were identified as M23C6 carbides and MX carbonitrides after tempering. The movement and annihilation process of excess dislocations in the transient region is controlled by not only the movement of dislocations in the matrix but also the annihilation of dislocations at lath and block boundaries. Sub-boundary hardening is shown to be the most important strengthening mechanism in creep of the present steel and is enhanced by fine dispersions of precipitates along boundaries. The reduction of carbon concentration to very low values promotes fine distribution of MX nitrides along boundaries and also eliminates M23C6 carbides. The addition of boron reduces the coarsening rate of M23C6 carbides along boundaries near prior austenite grain boundaries during creep.