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Aviation History
1928
1928 - 1168.PDF
the 2|-in. by 20 gauge, were required in 13-ft. lengths in many cases. Probably the ideal method would have been a vertical furnace from which the tubes could either be lowered or raised for air hardening, but the expense of such a furnace, capable of a temperature of 850° C, would have been prohibitive, and consequently other means of preventing distortion had to be found. The furnace already available, which was specially designed for blueing and heat treatment of tubes to aircraft specifica- tions, is heated by coal gas and has a mechanical side feed and discharge apparatus along its entire length of 20 feet. From the very beginning it was found that the tubes assumed an oval shape if heated to 850° C, while stationary on the floor of the furnace, and that the charging apparatus damaged the tubes when discharging for air hardening. The difficulty was overcome in the following manner. The charging apparatus was dismantled from the furnace, leaving a level carborundum floor, over which the tubes were rolled one at a time by two men equipped with hooked rods threaded in each end of the tubes. It took from five to ten minutes for the tubes to attain a temperature of 850° C. They were then lifted by means of the hooked rods and immediate! placed on the level floor near the furnace and continuously rolled until cold. By this method the tubes were kept reason- ably straight and comparatively round. In order to obtain the mechanical properties required (65-ton proof stress) the tempering was done at 400° C, after which the tubes were rolled on the furnace floor as before. The following dimensional limits were required : straightness within 600/length and round within ± 2 per cent, of the diameter. The roundness was obtained by very careful " reeling " and pressure. The straightening, however, was not so simple, as the tubes usually took a fairly sharp set near the ends. It was found that the tube collapsed before it could be straightened, and to overcome this difficulty it was necessary to load the entire tube with resin, which when cool resisted the crumpling effect of straightening. After straightening, the resin had to be melted out, but occasionally difficulty was experienced by the resin catching fire and in Three forms of Reynolds Aircraft Tubing in various sizes : Oval, streamline and square sections. effect tempering the tube below the required strength. In such cases the whole process of hardening, tempering, loading and straightening had to be done again. Thanks to the long experience of the Reynolds' Tube Company, many of the difficulties first encountered have been overcome, and the production of such tubes as a commercial possibility is in sight. Needless to say, however, the cost of the operations must necessarily make the price of such tubes high. Some two years ago tubes were produced experimentally from steel having a higher manganese content than is usually employed in plain carbon steel for the manufacture of tubes. Two steels, both containing 1 -5 per cent, manganese, but with varying carbon content, were used, the one having a maximum carbon content of 0-50 per cent, and the other 0-30 per cent. The steel usually employed for the manufacture of seam- less tubing contains about 0-60 per cent, manganese with a carbon range from 0-15 per cent, to 0 -55 per cent., according to the mechanical properties required. Since the effect of manganese in a steel is much the same as carbon, superior mechanical properties can be obtained. Reckoning on the fact that for each 0-01 per cent, of manganese present the tensile strength is increased by 200 to 400 lbs. per square inch, it will be seen that to increase the manganese content from 0 • 60 per cent, to 1 • 5 per cent, will increase the tensile strength by approximately eight tons per square inch. Take then a steel containing a suitable carbon content for welding, which the Air Ministry put at 0-30 per cent, maxi- mum, and increase the manganese content to 1 -5 per cent., we have a tube giving mechanical properties equal to B.E.S.A. DECEMBER 20, 1928 Specification T.5, without in any way detracting from its welding properties. As a matter of fact, the higher man- ganese content has improved the welding -properties of the steel, since the presence of manganese prevents the formation Some special forms of Reynolds Tubing of iron oxide during welding, as well as slightly reducing the melting temperature of the steel. The latter point is an important detail with which users should acquaint their welding operatives, since the art of good welding is not to overheat the metal It will, however, be quite apparent when welding that this steel does flow more readily than the usual low carbon steel. It is desirable, though not essential, that the " feed," and also other tubes and fittings being welded, are of the same chemical composition, the material now being obtain- able in the form of sheet, strip and wire. Tubes made from this steel are now covered by Air Ministry Specifications D.T.D. 89 A and D.T.D. 113. Tubes to the original Specification D.T.D. 89 are now divided into two classes. Specification D.T.D. 89 A covers all round tubes down to and including J-in. diameter, whilst D.T.D. 113 covers all non-circular tubes, and round tubes below i-in. diameter. The following table gives the comparative tensile strengths of these specifications compared with Specification D.T.D. 41, which had previously been the only specification for steel tubes suitable for welded structures. Specification Tensile before welding Tensile after welding (tons per sq. in), (tons persq. in.) 45 \ 35/ 25 Yield Break Yield D.T.D. 41 . . 28 30 17 D.T.D. 89 A. .. 40 D.T.D. 113 . . 30 It will be seen that the yield in tension after welding is 32 per cent, higher for the two new specifications. These new specifications have now been adopted as stan- dard by some aircraft constructors, the quantity of tubing supplied up to date being approximately a quarter of a million feet. The other steel containing 0-50 per cent, carbon with 1 -5 per cent, manganese is covered by D.T.D. 91 Specifica- tion, including an alternative composition now being drafted by B.E.S.A. to take the place of the present T.5 specification, on which it shows an increase of strength of 5 tons per sq. in. It is interesting to note that some of the first tubes to be made from this steel were used in the construction of the " nose " of R.101 by Messrs. Boulton & Paul, the size being 3J in. by 17 s.w.g., and were required to have a yield stress in tension of 55 tons minimum. It can be seen, therefore, that there is ample margin of strength on this steel to fulfil Specification D.T.D. 91, which calls for a yield of 45 tons per sq. in. 1074
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