Advanced Materials for Land Based Gas Turbines

The gas turbine (Brayton) cycle is a steady flow cycle, wherein the fuel is burnt in the working fluid and the peak temperature directly depends upon the material capabilities of the parts in contact with the hot fluid. In the gas turbine, the combustion and turbine parts are continuously in contact with hot fluid. The higher the firing temperature, higher is the turbine efficiency and output. Therefore, increasing turbine inlet temperature (firing temperature) has been most significant thrust for gas turbines over the past few decades and is continuing in pursuing higher power rating without much increase in the weight or size of the turbine. Firing temperature capability has increased from 800 A degrees C in the first generation gas turbines to 1,600 A degrees C in the latest models of gas turbines. Higher firing temperatures can only be achieved by employing the improved materials for components such as combustor, nozzles, buckets (rotating blades), turbine wheels and spacers. These critical components encounter different operating conditions with reference to temperature, transient loads and environment. The temperature of the hot gas path components (combustor, nozzles and buckets,) of a gas turbine is beyond the capabilities of the materials used in steam turbines thus requiring the use of much superior materials like superalloys, which can withstand severe corrosive/oxidizing environments, high temperatures and stresses. However, for thick section components such as turbine wheels, which require good fracture toughness, low crack growth rate and low coefficient of thermal expansion, alloy steels are extensively used. But the wheels of latest models of gas turbines, operating at very high firing temperatures (around 1,300-1,600 A degrees C), are made of superalloy, which offers a significant improvement in stress rupture, tensile and yield strength and fracture toughness required for the application.