Cyclic strength of turbine blades made of cast nickel-based alloys

Abstract

We study the cyclic strength and durability of service-exposed turbine blades made of and ÉP539LM nickel alloys, some of them with a protective coating, upon a long operating time in gas turbine compressor sets. The blades of alloy are found to have the highest cyclic strength. The cyclic strength of uncoated IN-738 blades is about 10% higher than that of coated blades.Subject of Research. The development of modern gas turbine engineering faces the ever increasing gas stream temperatures, densities, and velocities. Also, there is always a problem of contaminated air intake. All these factors affect the turbine lifetime. This in turn requires frequent preventive repairs which are quite expensive. Therefore, to increase lifetime and improve reliability of gas turbines is a task of great significance. Advances along this line are achieved owing to the development of new heat-resistant alloys and efficient anticorrosion coating processes to protect hot duct components, mainly blades and vanes, against the corrosive action of combustion products.Application of protective coatings on gas turbine airfoils is now a generally recognized way of increasing their durability and operating reliability. Conventional thermodiffusion coatings (aluminizing, alumosiliconizing, chromosiliconizing) turned out to be of little help because of the increase in gas flow temperatures, thermal stresses, specific loads, and were replaced by new heat-resistant materials. The repair of blades of gas turbine compressor sets (GTS), in particular MS-3002 or GTK-10I, involves, in addition to mechanical removal of all corrosion defects, the airfoil coating repair by argon-arc welding and high-temperature brazing.The blades for GTK-10I turbine compressor sets are usually made of IN-738. Recent trends in Ukraine are toward the use of ZMI-3U or ÉP539LM alloys which have similar physical-mechanical properties. It is known that multicomponent CoCrAlY or NiCoCrAlY protective coatings are applied on blade airfoils by electron beam physical vapor deposition [1, 2]. The advantage this process offers over the others is that elements with significantly different pressure (vapor tension) can be evaporated simultaneously from one and the same source at a melt temperature of 1500°C.After a blade foil has been coated, it is then subjected to a final heat treatment -aging at 845°C for 16-24 hunless an outer ceramic layer is applied on it. In the case of protective cermet coating, the blades are put through an aging process after the deposition of a strengthening ceramic layer up to 90 μm in thickness. For uncooled GTS blades the ceramic layer serves primarily for erosion and corrosion protection of the inner corrosion-resistant binding MCrALV coating by preventing its direct contact with combustion products. The strengthening ceramic coating improves the durability of blades of GTS turboprop engines. 0039-2316/07/3902-0109