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Aviation History
1920
1920 - 1155.PDF
NOVEMBER 4, 1920 • '" -. . SOME NOTES ON AIRCRAFT STEELS AND THEIR INSPECTION By Brigadier-General R. K. BAGNALL-WILD, C.M.G., C.B.E. THE seventh meeting of the Cambridge University Aeronau- range of safe tempering temperatures as possible. For£ ttr.T 7 TK *? f °n ^CtOb!,r 20> I92°- The Chairman' example, a steel of the following composition :_Nickel, 3.5 to ild SHTvfi Ilectu^, .^adier-General R. K. Bagnall- .3.0, carbon .3, chrome 1.0, for 55-60 tons will give a wideild said that there was little need for an introduction as ranee of irtivsical nmnpr+ip, TH W + <** ^^ !Wild, sai introduction, asthe General was already well known to most of those present as head of the A.I.D. and as past-chairman of the RoyalAeronautical Society, to which the C.U.Ae.S. was now affiliated. Following are some extracts from the paper read by Brig -Gen. Bagnall-Wild. The subject of steel may be considered under three headings : production, research and use. Thesenotes deal with the use of steel and are limited to its use for aeronautical purposes. In the early days of aeronauticsthere was a great tendency to demand steel which would give an ultimate tensile strength of over 100 tons per sq. in., andwith but little elongation. These steels were especially popu- lar in France, particularly for crankshafts and connecting-rodsof engines. Then came the welding craze. This process was nearly killed for ever as far as aeronautics were concerned inthat many early failures were caused by lack of knowledge on the part of the user. About this time alloy steel stampingwas introduced for connecting various spars" and struts, and affording anchorage for ties. Great attention was given tocrankshaft steel and many very valuable steels were evolved. Then came the War. Very little money had been previouslyspent on aeronautics ; research and production to new designs were then embarked upon to an undreamed-of extent. Thewhole of the resources of the Allies became available, and as a result we have the aeroplane of today, but we have not yetfinished. It must be borne in mind that the aeroplane of today is the outcome of the War, built for war ; we have notyet settled down to the commercial machine. During the War we wanted the best fighting performance and we wereprepared to pay for it. Commercially we want the best indus- trial performance at the minimum cost, and we have nowarrived at a stage where for commercial aviation the pounds, shillings and pence question bears a large share in the total Carbon, not over 0.35.Nickel, not over 4.00. Chromium, 0.5 to 1.30.Sulphur, not over .04. Phosphorus, not over .04. range of physical properties. The lecturer then showed aslide of the Air Board Specification K.I for crankshaft forgings or stampings. This specification is used to a considerableextent for 6, 8 and 12-cylinder engine crankshaft steel. Air Board Specification K.I.Chemical Composition. Physical Tests (Tensile). Yield ratio, . 75.Yield point, 50 tons sq. in. Ultimate stress feo to 70 sq. in.Elongation, 17.0 per cent. Reduction of area, 40.0 percent. Impact by 120 ft. lb. Izod,35.00 ft. lb. Brinell, allowable variationover shaft, o. 2 mm. Diagram No. I is a tempering chart for nickel-chrome steel,illustrating the effect on the mechanical properties of the steel of tempering at various temperatures after hardening. Thehatched band show the range of tempering temperatures within which the requirements of specification K. 1 with regardto the mechanical properties may be satisfied. Diagram 2 is a tempering chart for 3 per cent, straight nickel steel. Themuch narrower band within which the requirements^ of specification K.i may be met will be noted. The chart No. 1is a tempering chart for steel having the composition given above. It will be observed that there is a band within whichths specification requirements are completely met, and allowing a tolerance of ± 10 per cent, on the specificationfigures, the band becomes one of considerable width,* but merges of course at its boundaries into dangerous regions.It is evident from this diagram that if under stress of practical conditions inaccuracy of heat-treatment occurs, and theactually desired physical properties are not obtained, yet the TEMPERING CHHRT ANALYSIS of STEEL ' CARBON- 030°' NICKE.L . Z TEMPERING CHART 2:5% NICKCL -22 % CARBON 100 20O 3OO -tOO SQO 6OO TVO TEMPERATURE °C consideration governing design. In the early days of theWar one of the most serious problems was the' quantity production of a light-weight power plant. The solution ofthis problem depended largely upon the production of high- tensile steels of absolute uniformity of quality. The problemof the manufacture of crankshafts proved to be the crux of the whole position. As previously stated, the majority ofendeavours were concentrated on what may be termed very special alloys. In 1914 these were only made in smallquantities, when suddenly bulk production was embarked upon. In other words, these special steels, which were for-merly made in crucibles, a few pounds at a time, were then turned out from open-hearth furnaces at about twenty tothirty tons at a time. For other than experimental or very special purposes Istrongly advocate the use of a steel which gives as wide a I0O $00 5OO Aoo Soo 600 TEMPERATURE °C 7oo steel may be quite usable and, with a reasonable factor ofsafety, suitable. Hair Cracks. Considerable trouble has been experienced owing to theprevalence of hair cracks in some steels, and valuable research was made in order to overcome this trouble. The difficviltywith the hair crack is that there is no ready method of ascer- taining its depth. The presence of hair cracks leads one tobeheve that there may be other incipient cracks, or rather lack of adequate cohesion between the particles and crystallites,which may develop into cracks, and especially when the part is subjected to stress reversal with a high periodicity. Temper Brittienessis a condition which may be produced by slow cooling from the tempering temperature, in most nickel-chrome steels whichhave first been properly hardened. Thus a specimen of a "57
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