What is the creep-resistant steel used for?

Molybdenum has been a key alloying element in creep resistant ferrite steels operating at temperatures up to 530°C. The main applications of creep resistant steel are in power plants and petrochemical plants, where steam turbines require large forgings and castings, and pressure vessels, boilers and piping systems require tubes, plates and accessories of all kinds.In addition to high temperature creep strength, other material properties such as hardenability, corrosion resistance and weldability are also important. The relative importance of these properties depends on the specific application of the material. For example, large turbine rotors need steel with good hardenability, and power plant piping systems must be weldable. Even so, the alloys used in these different applications all use the same principles to improve creep strength.

Molybdenum in solid solution can reduce the creep rate of steel very effectively. When used at high temperatures, molybdenum slows the agglomeration and coarsening of carbides (ostwald’s ripening). Quenching and tempering produce a microstructure composed of upper bainite, resulting in the best results in high temperature strength. For coal-fired power plants, the efficiency of subcritical generating sets is less than 40 percent. Future ultra-supercritical (USC) plants are expected to be more than 50 percent efficient, reducing the carbon dioxide emissions per kilowatt-hour of electricity produced by almost half. Creep resistant ferrite steel is still commonly used in power plants, oil refineries and petrochemical plants worldwide. Components include seamless tubes for hot water boilers and superheaters, boiler drum, collector, pumps and pressure vessels for high temperature purposes, and steam turbine spines over 2 meters in diameter and over 100 tons in weight. This steel can be classified as C-Mn steel, Mo steel, low alloy C-RMO steel and 9-12% Cr steel.