Effect of precipitate dispersion on secondary creep rate in martensitic stainless steels

The majority of thermal-electric power plants in the world are based on the Rankine cycle and utilize steam as the working fluid. The thermal efficiency of such plants is primarily determined by the temperature and pressure of the steam as it enters the turbine. There are great economic incentives to increase the efficiency, but to do so requires that the steam inlet temperature be raised, which, in turn, necessitates that the temperatures of the tubes and pipes containing the steam also be increased. In typical coal and oil-fired generating stations several grades of steel tubing, headers and piping are used to contain the steam as it travels from the hottest areas of the boiler (the superheater and reheater sections) to the turbine. It is typically the creep strength of these steels that defines the maximum steam temperature and pressure, and hence, the cycle efficiency. To quantify the influence of particle size and interparticle spacing on the secondary creep rate of martensitic steel, a new grade of 9.5% Cr steel was designed and a novel heat treatment was developed. Precipitate dispersions in the steel were systematically varied through thermal mechanical treatment while maintaining a constant particle volume fraction. Three different TiCmore » particle sizes were formed: 6.5 nm, 12 nm and 22 nm. Steady-state creep rates were investigated as a function of precipitate dispersion, applied stress and temperature (550 C, 600 C and 650 C). Test results indicate that by reducing the average particle size from 22 nm to 6.5 nm (and thereby also reducing the average interparticle spacing proportionally) the steady-state creep rate was decreased by about four orders of magnitude.« less