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Aviation History
1926
1926 - 0300.PDF
SUPPLEMENT TO FLIGHT 44 THE AIRCRAFT ENGINEER APRIL 29, 1926 What change of properties does occur is, however, of the nature of an improvement rather than the reverse, and the instability of Duralumin is represented by a process that results in a continuous improvement of the metal. For all practical purposes, however, it is clear that Duralumin has become stable after a period of ageing extending over, say, one week, and it is quite safe for any user of the metal to assume that the properties which Duralumin possesses after ageing for this period can be relied upon for the life of the material, provided that it is not subsequently re-treated in any way. In the above tables, the highest quenching temperature of Duralumin has been quoted as 500° C. When judged by the resulting mechanical properties, this quenching temperature is a perfectly satisfactory one. Certain practical considera- tions, however, make it desirable not to heat Duralumin to quite such a high temperature. In actual practice, it is found to be much more desirable to quench the metal from a tem- perature between 480° 0. and 490° C. So far as the mechani- cal properties of the metal are concerned, the difference produced by lowering the quenching temperature from 500° C. to 485° C. is negligibly small, but by employing the lower quenching temperature, certain definite advantages can be secured. The choice of the actual quenching tem- perature is naturally influenced by a variety of practical considerations. In the first place, it is clearly desirable that the temperature employed shall be one which produces the desired mechanical properties. In the second place, it is an axiom with respect to heat-treatment processes that they should be carried out at the lowest convenient temperature. Thirdly, with regard to Duralumin, it has to be remembered that over-heating the metal can be accomplished fairly easily and is accompanied by dangerous consequences. It has already been stated in regard to the first point that a quenching temperature of 485° C. produces mechanical properties which are, to all intents and purposes, as good as those secured by quenching from 500° C. Respecting the second point, it is important to notice that 480° C. represents just about the minimum temperature from which Duralumin can be completely hardened with any safety. If the tempera- ture of quenching is lowered below 475° C. the full mechanical properties are not obtained, and the use of such temperatures must be condemned. This consideration, therefore, fixes the quenching temperature at 480° C, or above. The upper limit of the quenching range is, as usual, determined by other considerations than the mechanical properties of the finished article. That the danger zone in the heat treatment of the metal is very easily approached will be appreciated when it is realised that incipient melting of Duralumin occurs at a temperature of 545° C. The metal does not become wholly molten at this temperature, but certain constituents melt, and, of course, if anything of this kind occurs, the material is entirely ruined. It also will be appreciated, since the heat treating temperature range is comparatively near to the melting range of the metal, that the material is exceedingly soft when heated to the quenching temperature. If the metal is heated to the upper part of the quenching range, it is softer than when heated to the lower end of the range, and. therefore, is more liable to severe distortion. Articles of an irregular shape, under these circumstances, will naturally tend to warp and may go right out of shape, even before quenching has occurred. The third danger which is encountered by heating to the higher end of the range is that of blistering in a marked degree. There is no need to enter particularly into details of the causes of blistering, and it is sufficient to notice that the higher the temperature to which Duralumin is heated, the more this trouble is likely to be encountered. The formation of blisters is, of course, something to be avoided carefully, and to this end it is obviously desirable to utilise as low a quenching temperature as possible. The fourth disadvantage of over- heating is connected with the mechanical properties of the material. If Duralumin is heated to and quenched from temperatures much above 500° C. it will (after ageing) have a maximum stress not appreciably different from that pro- duced by quenching from a temperature of, say, 485° C, but the metal will be distinctly more brittle. Here, again, the treatment of Duralumin is very parallel to that of steel, and the known effects of seriously over-heating steel are to a considerable extent reproduced in the treatment of Duralumin. Over-heated Duralumin is inferior in elongation and in toughness when compared with material that has been quenched from the correct quenching temperature. All these considerations, therefore, lead to the fixing of a narrow range within which Duralumin can be safely and profitably heated prior to quenching. The low side of the range is fixed by the mechanical properties at an absolute minimum of 475° C. The upper point of the range is fixed by the dangers of over-heating at a temperature of about 500° C. A prolonged experience of the treatment of Duralu- min leads definitely to the conclusion that the metal should be heated prior to quenching to a temperature between 480° and 490° C. Briefly, then, the heat-treatment of Duralumin intended to produce the optimum mechanical properties consists in heating the metal to a temperature between 480° C. and 490° C. followed by rapid cooling, in turn followed by ageing for a period of about four days. As a result of this treatment, the material possesses a maximum stress of approximately 26 tons per square inch. Material that has been treated in this manner is referred to as " normal " by the makers. If the material is required to be soft, it can, of course, be annealed, and the test figures given in Tables 1 and 3 indicate that the softening can be carried out quite satisfactorily by re-heating the metal to a temperature between 300° and 400° C. The most suitable temperature range for annealing the material is between 350° and 380° C. Between these temperatures Duralumin loses entirely the effects of previous heat-treat- ments, and also previous cold working. If it is heated below 35(1° C, the material is not as soft as it might be, whilst if it is heated over 380° C. it commences to harden again to a small degree. Naturally, this last hardening is to a consider- able extent a function of the rate of cooling from the annealing temperature. If the material was cooled slowly enough, even from a temperature of 450° C, it would be quite soft. In practice, it is difficult to ensure that the material shall cool sufficiently slowly to be entirely and permanently soft, even when cooling from a temperature of 400° C, and it is desirable, therefore, to utilise the temperature indicated, namely, from 350° to 380° C, for this purpose. When annealing Duralumin the mass of the article should be taken into account to some extent. With a large mass, which will naturally cool fairly slowly, it is comparatively safe to heat the metal to a temperature near to 400° C. Thin parts of small mass are better treated at the lower end of the annealing range, that is, round about 350° to 360° C, thereby reducing the liability to slight hardening. Even in parts of small mass, the hardening that may occur is only slight, and does not show itself in the metal, except after fairly prolonged standing. If the metal, after cooling from the annealed temperature, is utilised immediately, there is no appreciable danger in annealing it anywhere within the recommended annealing range. If, however, material in thin sections is allowed 1o cool in the ordinary way, and then not worked at all for three or four weeks, it will be found to have hardened quite appreciably in this time. Parts of a larger mass, treated in the same way, will not have hardened appreciably, simply because of the very much lower rate of cooling. Another point which emerges from a consideration of the heat-treatment of Duralumin is that of the properties of the metal immediately after quenching. The detailed results given above indicate that the metal immediately it is quenched is quite soft, and, in fact, that it has mechanical properties very similar to those of annealed metal. This softness only persists for quite a short time, but whilst this condition does exist it can be utilised very effectively. During the period that quenched Duralumin remains soft—that is, within, say, an hour or two of quenching—the material can be worked with just as much facility as if it was annealed. In some respects, the material appears to work even better in the quenched condition than in the annealed state, and it is frequently verv convenient to utilise this period of softness for carrying rmf aViQ-ninrc anrl H/ifnrinaHrm nf x^rinilK lrinHs. Whilst theout shaping and deformation o various kinds. W7hilst 260/*
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