FM 1-506 Fundamentals of Aircraft Power Plants ... - Survival Books

WWW.SURVIVALEBOOKS.COMFM 1-506applications, are also being used in turbione wheels, shafts,spacers, and other parts. Their use is somewhatrestricted because of cost and because of their requirementfor critical materials.Cobalt-Base Alloys. Colbalt-base alloys formanother important group of high-temperature, highstrength,and high-corrosion- resistant metals. They containlittle or no iron. These alloys are used in afterburnerand other parts of the engine subjected to very hightemperatures.Chemical Elements of AlloysThe number of materials used in alloys is large.Some of the most commonly used elements are listedbelow.horsepower needed to drive the compressor. There isalso a severe temperature gradient (difference) of severalhundred degrees between the central portion of the diskand its periphery.Many metals which would be quite satisfactory atroom temperatures will lose much of their strength at theelevated temperatures encountered in the engine’s hotsection. The ultimate tensile strength of a metal at onetemperature does not necessarily indicate its ultimatetensile strength at a higher temperature. For example, atl000ºF Inconel X has an ultimate tensile strength ofapproximately 160,000 psi; and S 816 at the sametemperature has an ultimate tensile strength of 135,000psi. At 1500ºF their positions are reversed. Inconel X hasan untimate tensile strength of 55,000 psi; S 816 has anThe percentages of elements used partially determinesthe physical and chemical charteristics of the alloyand its suitability to a particular application, Temperingand other processes determine the rest. Three characteristicsthat must be considered are—High-temperature strength.Resistance to oxidation and corrosion.Resistance to thermal shock.High-Temperature Strength. The most highly stressedparts of the gas turbine engine are the turbine blades anddiscs. Centrifugal forces tending to break the disc varywith the square of the speed. For example, the centrifugalforce on a disc rotating at 20,000 RPM will be four timesthat at 10,000 RPM. Blades weighing only 2 ounces mayexert loads of over 4000 pounds at maximum RPM.Blades must also resist the high bending loads applied bythe moving gas stream to produce the thousands ofultimate tensile strength of 75,000 psi. The creepstrength, which is closely associated with ultimate tensilestrength, is probably one of the most important considerationsin the selection of a suitable metal for turbineblades. Engine vibration and fatigue resistance will alsohave some influence on the selection and useful life ofboth discs and blades.Many materials will withstand the high temperaturesencountered in a gas turbine engine (carbon columbium,molybdenum, rhenium, tantalum, and tungsten all havemelting points above 4000ºF). However, the ability towithstand high temperatures while maintainingreasonable tensile strength is not the only consideration.All of the following qualities must be taken into accountwhen selecting a particular metal:Critical temperature.Rupture strength.3-26