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Aviation History
1956
1956 - 0559.PDF
11 May 1956 559 Profound effects on the mechanical performance of an alloycan result from minute changes in its composition. This long- established fact is being put to advantage in the U.S.A. by perfec-tion of vacuum-melting techniques. The F. J. Stokes Machine Company and General Electric have independently evolved equip-ment and techniques for carrying out all operations, apart from final machining, in a controlled atmosphere, so allowing precisecontrol of reactive ingredients. Pratt and Whitney's "Waspaloy" is vacuum-cast and has been claimed superior to all other wroughtalloys in the region of 1,500/1,600 deg F. Now G.E. are going into pilot-production at 100,000 lb per year with even bettermaterial produced by vacuum melting. Undoubtedly the greatest series of turbine alloys are thosedeveloped and manufactured under the registered name "Nimonic" by the Mond Nickel and Henry Wiggin companies.The latest of diese high-nickel alloys is Nimonic 100, which is showing itself superior to all other blade alloys in the region900-980 deg C. An engine giving 5,000 lb-thrust with N.80A blading ought theoretically to give 5,500 lb-thrust with turbineblading of N.100. Apart from some increase in cost, the chief penalty attending the use of high-creep/high-temperature alloysis their difficulty of working. It is only natural that an alloy specifically prepared to retain its strength at elevated tempera-tures should be difficult to forge. In the U.S.A. thousands of turbojets have now been built inwhich all turbine blading is cast. If one is prepared to forego the good grain-flow associated with forging, one can use materialswhich do not become plastic until temperatures well outside present gas-turbine experience are reached. Ceramic materials,in particular, can be employed in gas temperatures as high as 3,000 deg F, and a whole range of synthetic materials known ascermets now bridge the gap between ceramics and metals, gener- ally with intermediate properties. Unfortunately most cast, or Generalized curves for a supersonic turbojet flying at Mach 2.5 at 4OJ0O0H at 100 per cent of design r.p.m. These curves, in com- parison with those on page 558, emphasize the changed character of all turbojets when in the high-supersonic range of Mach numbers. V3b 5 10 m 20 25 30 35~~4O 45 50 SPECIFIC THRUST(Ib perlb/sec airflow) sintered, materials of this type are markedly inferior to die bestmetals in resistance to thermal shock and impact. Much has been achieved by such firms as Ryan and Solar in developing ceramiccoatings, eidier to allow increased gas temperature or to permit the use of cheaper basic alloys. Such coatings are proving veryvaluable on lightly stressed components (especially canware) but seem to have very limited applications in turbine construction.Ceramic-coated turbine blades have been extensively evaluated and generally found wanting. Appreciable increases in gas temperature are being obtained byemploying blade materials based on titanium carbide. No British manufacturer has yet said much about this work—doubtless onsecurity grounds—but it is known that in America, using up to 30 per cent nickel as a binder, tensile strengths have been obtainedup to 68 tons/sq in at room temperature and no less than 18 tons/ sq in at 980 deg C. Kentanium, an American sintered titaniumcarbide, has been operated in the form of primitive turbine rotors for 100 hr at 30,000 r.p.m. in a gas temperature of 1,850 to 1,900deg F. Mention should also be made of German work, particu- larly with the WZ sintered hard-metal alloys. One of these, WZ12b, is claimed to have a 100-hour rupturing strength of 8.27 tons/sq in at 940 deg C and about 16 tons/sq in at 900 deg C. German engineers were also responsible for remarkable advancesin turbine performance during World War 2. To a considerable degree their work resulted directly from almost crippling short-ages of really good alloys, or alloying elements, so that the mass- production of usable turbojets for the Luftwaffe was largely acase of bricks-without-straw. Today the gas-turbine manufac- turer suffers under no such handicaps, yet some of the Germanlines of development are still being extensively exploited. Most important of all these developments is that of coolingturbine blades. This can be done by various means, some of which blanket the surface, some cool the centre of each blade,while other methods extract heat from all parts of the blade. In several of die early German gas turbines the turbine blading wasmanufactured from wrapped and welded sheet, the interior of each blade being cooled by an air blast. Modern cooled blades, how-ever, are frequently cut from solid bar or machined from a forged or extruded blank. The manufacture of cooled blades was recently described byHenry Wiggin and Company, Ltd., in the following terms: — "The increased strenpt' , at high temperatures, of die improvedalloys brings with it increased difficulty in hot-working, and extrusion has proved to be preferable to hammer-forging for theinitial working of ingots. Extrusion is particularly advantageous when the Ugine-Sejdurnet process of glass lubrication is used,and this procedure is now adopted as standard for the working of die Nimonic alloys. Cast billets are extruded to bar, which iseidier hot-rolled to a smaller cross section for machining or forg- ing to blade blanks, or used for forging in the extruded conditionafter machining of the surface for inspection. "Extrusion has also recently made possible the development ofa type of product which is of interest in the manufacture of cooled blades. Before extrusion, the billet is drilled with a number ofsuitably distributed holes which are filled with rods of a material specially chosen as having, at die extrusion temperature, deforma-tion characteristics which closely match diose of the alloy with which it is to be used, and also as being readily removable fromthe holes after extrusion. The plugged billet is extruded to approximate aerofoil form, and is finished by hot- or cold-rolling.The product is then cut to the required lengths, and the filler material is removed. Constant-section extrusion of the typedescribed is primarily suitable for stator blades, but further developments are expected to lead to use of the same process forproduction of rotor-blade forgings. No significant change in creep properties is produced by extrusion as compared with hammerforging, either in the actual extruded bar or in hot-rolled bar or blade forgings produced from it." The turbine blading can be cooled in order to permit the useof inferior materials or to raise the thermodynamic efficiency by increasing the top temperature, but by far the most importantadvantage is that it allows existing materials to operate in higher gas temperatures and so give greater specific power. On March16th we published a paper giving results of tests on single-stage blading (cooled stator and rotor) at the N.G.T.E. prior to 1953.It was concluded that an increase in gas temperature of 518 deg F could be allowed, compared with uncooled blades of similarmaterial. In a turbojet with a component efficiency of 90 per cent, a pressure ratio of 8 : 1 and a mass flow of 100 Ib/sec, thiswould raise the thrust from about 5,800 1b to over 7,800 1b, assuming odier factors could remain unchanged. In this earlyexploratory programme—the results of which have since been significantly improved upon—it was not found possible to preventincreases in the thermal stress borne by the blade. On the other hand, this resulted largely in a shift of the load to the cooler, andhence stronger, portions of the blade. Thermal shock was inves- tigated, many cycles being run in which the gas temperature wassuddenly reduced from 950 to 120 deg C, the blades remaining sound. Other forms of cooling include porous (sintered) cooling andeffusion cooling. The former method generally involves materials which are notoriously poor mechanically. The latter, however,can be very attractive, especially where die flow around the blade is laminar. Research suggests that effusion-cooled laminar-flowblades would require only 33 to 50 per cent of the cooling air flow needed by an internally cooled blade working under the sameconditions. The insulating action of the cooling air envelope can permit exceedingly high gas temperatures—higher, perhaps, thanare possible by any odier method at present in prospect. Prior to 1950 surprisingly good results were obtained from anearly Whitde engine in which die turbine blading was cooled externally by water sprays, of which three were mounted in eachof four symmetrically disposed nozzle guide vanes. With water equal to 0.2 per cent of the gas flow, die rotor-blade temperaturewas reduced from 1,200 deg F to less than 850 deg F. Several types of turbine have already been run with internal coolingsystems involving water or odier fluid such as liquid sodium. It is believed that American research in this field has resulted inturbines satisfactory for gas temperatures exceeding 2,500 deg F. Even light-alloy blades have shown promise with liquid cooling,owing to their excellent thermal conductivity. Several severe difficulties are being met in developing reliablecoo'ed turbines. A major problem is the achievement of maximum uniformity of temperature and reduction of thermal stresses inthe blades, discs and stator assembly. Another requirement is that the cooling medium should be scrupulously clean and freefrom solid particles which might choke passages of minute cross section. The latter consideration is somewhat less of a problemwhen the cooling medium is air. A degree of blade cooling is obtained as a by-product from somepatterns of afterburner. An example of such a system is that developed by the Canadian National Research Council, in which
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