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Naval Engineering Bulletin • June 2001 Hot Corrosion of Marine Gas Turbine Blades An Engineering Plant Component in a Hostile Environment By LCDR Andrew Fysh, RAN Introduction Gas turbine engines are in widespread use today, because of their combination of performance, lightweight, low maintenance requirement, and efficiency. Developed primarily for the aircraft industry, they have found more recent application in warships (and, more recently still, in commercial shipping). Many internal components of the gas turbine are subjected to temperatures well in excess of 500 0 C, necessitating careful design and in-service management to ensure reliability and to avoid expensive repairs. This report outlines the corrosion effects of these temperatures, in the marine operating environment, with specific reference to the General Electric LM2500 gas turbine in use in the Royal Australian Navy’s surface fleet. Development of Gas Turbine Hot Section Materials The Hot Section Environment The performance requirements of gas turbine engines has increased considerably since the Whittle engine of the 1930s, to the end that turbine entry temperatures have more than doubled. Of all turbine engine components, it is undoubtedly the high-pressure turbine (HPT) 1 blades, which operate under the most arduous conditions: • high temperature (up to 1000 0 C) • high rotational forces and direct stress • rapid temperature transients (especially in aircraft engines) • hot corrosive combustion gases • erosive particles entrained in the gases (combustion by-products such as carbon, airborne particles such as sand 2 and salt) The combination of direct stress and temperature encourages blade creep, while the rapid temperature transients ultimately cause thermal fatigue. It was these phenomena that first dictated material requirements for HPT blades. The Whittle engine commenced service using austenitic steel; this was found to have insufficient creep strength. Development of cobalt-based alloys followed, leading to the now widespread use of cobalt-based and nickel-based superalloys. Nickel-based superalloys can be routinely used at temperatures up to 0.8 times melting temperature, and can have a service life of up to 100 000 hrs at slightly lower temperatures. Nickel-Based Superalloys Table 1 shows the various alloying elements used in nickelbased superalloys for gas turbine hot-section components, and their principal characteristics. The composition of two common alloys is shown for comparison. Inconel® 718 is an earlier alloy (1960s), and is still used widely in the LM2500 engine for HPT rotor and stator structural components. René 80 (introduced by General Electric in 1980) has superior properties designed for use in the LM2500 first-stage HPT blades. In addition to changes in composition of nickelbased superalloys since their introduction, further improvements in properties have arisen from optimisation of melting, casting and forming methods, hot-working processes and heat treatments. In the pursuit of even greater engine performance efficiency, the stress/temperature environment in some engines is such that first-stage (and, more recently, second-stage) turbine blades also require air cooling, by convection through radial passages. Later blade stages, though less likely to 1 The ‘hot section’ components of a gas turbine engine are the combustor, turbine inlet nozzles, turbine rotor blades, and associated structure and assemblies. The HPT, consisting of one or more stages of nozzles and rotor blades, receives the hot pressurised gases from the combustor to provide power to drive the compressor section (enabling the engine to be self-sustaining after start-up). Downstream of the HPT is a propulsion jet (in aircraft engines), or a mechanically independent low-pressure turbine to drive a propeller shaft (marine engine) or alternator rotor (power generation). The HPT inlet temperature represents the hottest part of the gas turbine cycle. 2 While predominantly a problem for land-based or marine gas turbines, airborne sand particles have been known to exist at an altitude of 30 000 ft in the Middle East! 43
Naval Engineering Bulletin • June 2001 Table 1 - Common elements of nickel-based superalloys, and composition of typical alloys Element % in % in Characteristics Inconel® 718 René 80 Ni 50.0-55.0 Balance • principal constituent - superior mechanical properties and endurance for high high thermal stress applications. Cr 17.0-21.0 14.0 • forms protective scale of Cr 2 O 3 when heated in oxygen-rich environment (better for hot corrosion prevention than Al 2 O 3 , but is volatile above 95 0 C. • forms chromium carbides for strength at high temperatures • primary contributor to hot corrosion resistance. Fe Balance - • some alloys still contain high iron content for reduced cost. • less oxidation resistance as Fe 2 O 3 is less adherent oxide scale. Ti 4.75-5.50 5.0 • strengthens by forming y’ (gamma prime: Ni 3 (Al,Ti), Ni 3 Nb). Al 0.65-1.15 3.0 • Nb, Ti form strong carbides. Nb 0.20-0.80 - • Al forms stable Al 2 O 3 protective oxide scale (better for oxidation prevention than Cr 2 O 3 ). • more recent developments have seen decrease in Cr to optimise creep rupture resistance of y’ - Al compensates for loss of oxidation resistance from this reduction in Cr content. Mo 2.80-3.30 4.0 • provide solid solution strengthening at high temperatures. W (trace) 4.0 • form complex carbides. Ta ( n il) - Co 1.0 max. 9.5 • helps maintain strength at elevated temperatures (reduces solubility of Al and Ti in the Ni-Cr matrix). • greater solubility for carbon than Ni. C 0.08 max. 0.17 • forms carbides with some alloying elements, for improved microstructural strength. B 0.006 max. 0.015 • improve creep strength and ductility. Zr (nil) 0.03 • but, weldability can be adversely affected. Ca (nil/trace) - • improve workability. Mg (nil/trace) - • improve oxidation resistance. Y (nil/trace) - Hf (nil) - • added in recent years, in small amounts, mainly to cast alloys. • Improves ductility and strength at low/medium temperatures. • raises hot tear resistance in directional solidification. need cooling, may nonetheless contain radial holes to reduce weight. Given the high temperatures, at which modern turbines must operate to achieve superior power-to-weight ratio, this cooling must reduce metal temperatures to a level several hundred degrees below the local gas temperature. This results in an extreme temperature gradient across the blade (which can be reduced by film cooling techniques, in which air is passed through the blade wall via discrete holes to form an air film on the blade surface). Thus, in modern gas turbine engines, the hot section components are extremely light, hollow (thin-walled) sections of high-strength superalloy. Corrosion, if allowed to take 44
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Engineering - Royal Australian Navy

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