11 - Materials

Abstract

The combustor liner and turbine blades are the most critical components in existing high-performance, long-life gas turbines. The extreme conditions of stress, temperature, and corrosion make the gas turbine blade a materials challenge. This chapter focuses on various problems and failures attributed to gas turbine blade metallurgy. Turbine blade alloys tend to indicate low ductility at operating temperatures. The ductility of a metal is affected by the grain size, the specimen shape, and the techniques used for manufacturing. As a result, surface notches are initiated by erosion or corrosion, and then cracks are propagated rapidly. A very common type of failure that blades in turbines undergo is “high cycle fatigue.” This type of failure is caused when the blade is subjected repeatedly to an unsteady load. This type of failure would be depicted by a chevron type of markings on the failed surface, near the trailing edge of the blade. Thermal fatigue of turbine blades is a secondary failure mechanism. Temperature differentials developed during starting and stopping of the turbine produce thermal stress. The cycling of these thermal stresses is thermal fatigue. The analysis of thermal fatigue is essentially a problem in heat transfer and properties such as modulus of elasticity, coefficient of thermal expansion, and thermal conductivity. The operating schedule of a gas turbine produces a low-frequency thermal fatigue. The number of starts per hours of operating time directly affects the blade life. The use of Ni-base superalloys as turbine blades in an actual end-use atmosphere produces deterioration of material properties. This deterioration can result from erosion or corrosion.