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Aviation History
1926
1926 - 0528.PDF
SUPPLEMENT TO FLIGHT JULY 29, 1926 THE AIRCRAFT ENGINEER with the use of suitable'and proved heating appliances. The relatively slow absorption of heat by Duralumin has already been dealt with at length in connection with the heat-treatment process. Naturally, the same limitations have to be borne in mind in connection with the heating of the metal for forging and stamping. There is no doubt that a good deal of the trouble that has been experienced by beginners in the art of forging Duralumin must be attributed to the inadequate time allowed for soaking the material and allowing it to attain the true forging heat throughout the mass of the piece that is being worked. In the preparation of Duralumin for forging, dry heating is strongly to be preferred. If the metal is heated, prior to drop forging, in a salt bath, it is practically inevitable that some quantity of salt still adheres to the metal when it is placed between the dies. The salt and the oil on the dies may interact chemically and produce an explosion. Such cases have actually occurred and considerable damage been done. It is admitted that salt-bath heating is probably the quickest method of raising Duralumin to the forging heat, but the danger attached to the operation makes it one which should not be employed. The methods of dry heating previously described are much slower but are definitely to be preferred, and it is quite safe to use any of them provided always that the requisite time is allowed. In order to give the material a full opportunity of soaking; and becoming heated uniformly throughout its mass, it is, obviously, necessary to provide very ample muffle accommodation, as otherwise there is a great likelihood of the work being given insufficient time prior to stamping because of a shortage in the supply of fully heated metal to the stamps. When Duralumin is being forged it is desirable to have an adequate supply of power. The metal wants "" hitting" hard. At the forging temperature the strength of Duralumin is approximately three tons per square inch. This may be compared with a strength of one and a-half tons per square inch for mild steel at its forging temperature, and of two tons per square inch for nickel chromium steel when at its forging temperature. These figures alone indicate that the power required to forge Duralumin is necessarily of a higher order than that required for forging steel, and Duralumin responds to heavy working much better than it does to light working. When the power supply is adequate. Duralumin can be made to flow quite freely, but when forging with an inadequate supply of power is attempted the material very frequently breaks up. When it is being worked between dies, Duralumin is found to flow in about the same way as the harder alloy steels. It is desirable, therefore, to arrange that the drop forgings shall be made with a very adequate radius in the corners. Sharp corners are undesirable in any kind of drop forging, and are particularly so in drop forgings made in Duralumin. Sweep- ing curves are greatly to be preferred, and the various parts should flow sweetly into each other as far as possible. On drop forgings the usual allowance for leave is about 7". One of the most pregnant sources of difficulty in the, manufacture of Duralumin stampings is the clipping opera- tion. Duralumin does not clip quite so cleanly or so easily as hot steel. It is essential, however, that the operation be carried out cleanly and accurate! ', or otherwise the stamping may be very definitely ruined bv the formation of a deep and fine crack or fissure running along the clipping bine of the drop forging. In order to avoid the formation of this defect, the flash on the stamping may be left a little full in width, so that any drag comes on the flash itself and not on the metal in the body of the stamping. A further and most important way of avoiding the trouble is to ensure that the clipping tools are thoroughly sharp, and that they register very accurately. If all these precautions are taken the stampings can be clipped hot with complete success. A Duralumin stamping or forging has necessarily to be heat treated after shaping, in order that it shall develop its maximum mechanical properties. In the heat treatment operation the necessity for giving an adequate time allowance in order to produce a uniform temperature throughout the mass of the article is as essential as ever. With large stamp- ings this question becomes one of great importance. Apart from it, there are no points connected with the heat treatment of forgings and drop forgings that require very special comment. A word or two, however, should be said res- pecting the mechanical properties of the drop forgings after they have been heat treated. When the heat treatment of Duralumin was being described in an earlier article, it was made clear that the production of the high mechanical properties for which Duralumin is noted was a function of the quenching and ageing operations, and that the essential point in this heat treatment was the cooling of the material from an elevated temperature at a sufficiently high rate to ensure that the high temperature structure was retained in the metal down to atmospheric temperatures. In other words, although the successful heat treatment of Duralumin requires various things, above every- thing else a certain minimum speed must be attained in the quenching operation. Xow the speed of cooling of a piece of metal heated to an elevated temperature and then immersed in a cooling fluid is not at all uniform. It is not uniform in many directions, but the one direction which is of present interest is the variation in cooling speed of different parts of the mass located differ- ently in respect of the outer surface of the metal. The metal composing the outside skin of the article naturally conies first into contact with the cooling fluid, and remains in contact with it during the whole of the quenching operation. The surface of the article, therefore, cools much more rapidly than any other part, and it is obvious in a general way that as the distance of any selected portion of the article from the surface increases, the rate of cooling of that part during quenching necessarily decreases. There may be a difference so great in a part of suitable dimensions that the outside will cool at a speed some hundreds of times greater than the rate at which the centre of the article is cooling. Obviously under such circumstances it is only a question of dimensions for the interior of the article to cool so slowly during the quenching that its rate of cooling is less than the minimum rate required to bring about a retention of the high temperature structure in the metal. In other words, it is clear that when the mass of the part reaches a certain figure, the outer layers will cool sufficiently rapidly to retain fully the high temperature structure whilst the centre of the mass will cool so slowly that its structure will approximately V>e in a condition similar to that of annealed Duralumin. After ageing, this difference in the rate of cooling shows its effects very markedly, and whilst the outside of a large part will possess the full tensile strength associated with Duralumin, the core of the part will only develop a lower strength, higher possibly than that of annealed Duralumin, but not so high as that of the fully treated metal such as is found on the outside of the article. With Duralumin it seems to be clear that the largest cylinder which will harden uniformly throughout its mass when treated in the ordinary way is one of about three inches diameter. In such a part the material farthest removed from the cooling surface is distant about one and a half inches, and it seems to be not unreasonable to take this as a general guide in the heat treatment of Duralumin forgings and drop forgings, and to arrange that where the maximum strength of the material is to be employed the parts should not be thicker than is indicated. In many instances a great deal of assistance towards equalising the eventual mechanical strength can be rendered by rough machining the article before the final heat treatment, thus reducing the mass of the article and bringing the parts most distant from the cooling surface within the effective range of the quenching operation. Naturally, if the Duralumin is not uniform in shape and cross-section it is possible that some portions of the article after heat treatment will be hardened to the centre, whilst other portions are only fully hardened for a portion of the thickness. This fact should be taken into account by the designer of the part so that due allowance can be made for a possible deficiency in strength in the thickest parts. It may be taken as a general guide that in aircraft forgings 25 tons maximum stress will be attained in the majority of places, but that where exceedingly thick parts exist the strength may fall to a value of about 21 tons per square inch. 464ft
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