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Aviation History
1919
1919 - 0682.PDF
FLIGHT by a special process and can be welded, punched, rolled and bent longitudinally and laterally without further heat treat ment. This steel should be tempered either below 1500 C. or above 450® C. if durability and capacity to resist shock and vibration are required. This remarkable phenomenon is set forth in the interesting monograph, Appendix II.. by Mr. Henshaw, of Kayser, Ellison. Between these temperatures the Izod impact test gives a low figure. Further properties of, this type of steel are set forth in Mr. Mackinder's monograph, Appendix III. It is a pleasure to pay a tribute to the patriotic way in which Mr. Kayser, of Kayser, Ellison and Co., Ltd., together with the associated firms pf Arthur Lee and Co., Ltd., and the Effingham Steel Co. and other manu facturers of Sheffield and district have tackled and solved successfully the problem of the evolution of metal strip suitable for aircraft construction. The strength of metal constructions appears to be governed more by the yield point than by the ultimate strength, hence attention should be paid to obtaining a good yield point. Straight steel strip can now be obtained up to 5 in. wide, having yield points of 100 tons, 80 tons, 60 tons or 40 tons. MAT 22, 1919 as 500° C. is not exceeded, in which case there is danger of the metal being burnt and reduced to aluminium. The parts should be left in the bath long enough to be heated uniformly to the temperature of the bath, the actual times allowed being five minutes in the case of small units up to several hours in the case of 3 in. bar. There is no danger in leaving it in the bath too long, providing the actual temperature of 500° is not exceeded. At the end of this time the article is removed from the bath and quenched out in water or non-acid oil. Immediately after heat treatment the metal is found to be in a very soft and plastic condition, and it may be pressed, forged, bent to the smallest radius, or in the case of drawn sections which have warped with the quenching, straightened without any fear of damaging the metal. The metal now commences of itself to harden up, and in about an hour's time has reached about 26 tons tensile strength, with an elongation of about 15 per cent. This process of hardening continues for a very long period, the strength slightly increasing at the expense of the elonga tion. Heating duralumin to 300° C. and then quenching out In ordering strip care should be taken to order the exact width required, since the process of splitting the strip in variably gives it a curve which can only be corrected by additional heat treatment. Duralumin. The second material available for metal construction is duralumin. This has many advantages over steel and a few disadvantages. Duralumin being three times lighter than steel it follows that the various members may be, weight for weight, three times thicker than steel members. Hence the difficulty in overcoming local failure due to local flexure is greatly reduced. Nevertheless, the perfection of a design can only be discovered by constructing it in steel, and it is thought that duralumin designs will be improved by adopting the principles which have proved to be the only feasible ones in steel construction. A prejudice appears to exist in certain quarters against duralumin owing to the fear of corrosion. It should be stated that duralumin, unlike aluminium, is not acted upon by sea water, or affected by atmospheric influences, and experience in connection with airship work shows that duralumin, if properly varnished, is not affected by corrosion under ordinary conditions of service. Nearly every case of corrosion so far experienced in actual practice has been traced back to incorrect heat treatment, or to cold working after heat treatment. The heat treatment of duralumin instead of being a disad vantage, as is often supposed, is in actual point of fact of the greatest assistance in the practical working of the metal. This heat treatment is carried out by placing the member to be treated in a bath containing a mixture of potassium and sodium nitrates heated to a temperature of 480° G. In practice there is a fairly wide margin of temperature so long anneals the metal, and it can then be -worked in any way desired. In order to give it back its strength it is necessary to heat treat it at 480° C, as explained above. In practice it is usual to anneal any duralumin part if a number of operations have to be performed, and on the other hand to heat treat before final operation if this is not of too prolonged a nature, taking advantage of its abnormal con dition immediately after heat treatment. An actual example of the value of this property of duralumin in slowly hardening up after heat treatment occurs when duralumin is used for rivets. If an attempt were made to rivet these up in the ordinary hard condition they would crack badly. By heat treating them, however, and using them immediately after this, a perfect job may be made while they are soft, and within a very short time afterwards they have hardened up to their full strength. If duralumin in the ordinary hard condition is bent to a small radius, there is, as would be expected, a probability of incipient cracks developing on the surface of the metal at the bend. These cracks are the places where the corrosion begins. If the metal was properly heat treated before such bending, no injury would be caused to the surface of the metal, and no trouble would be experienced from corrosion. Nearly the whole of the prejudice against the use of duralumin is due to ignorance as to its properties with and without heat treatment. Sections cannot conveniently be drawn thicker than .2 in. Above this thickness the sections are extruded. It is advisable to use only solid drawn sections for constructional purposes and designs should accordingly be modified to meet the limitations in the material. Constructional Designs. Having now described some of the properties of the materials available for metal construction it remains to 682
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