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Aviation History
1917
1917 - 0433.PDF
MAY 3, 1917. those made in medium carbon steel. The screw maker should have tendered accordingly in the first place, and not waste time in grumbling, and what is far more important, in failing to give the output that he had promised. With regard to bolts required in mild steel, or rather what is so termed in aeronautical manufacture (35 tons ultimate), very great trouble has been experienced in certain directions. A large quantity of steel exists in this country which has a sulphur and phosphorus content in excess of 0.1 per cent. Such steel is well beloved by the screw maker, it cuts like cheese, does not wear out the dies, and allows a well-cut thread to be produced ; on the other hand, it is absolutely dangerous I/LIGHT is most essential for the correct working temperatures" to be observed and also that the right amount of work is done on the metal. The chief cause of failure with stampings in alloy steel has been, due to the overheating of the steel In some cases this can be rectified by heat-treatment, but in the majority of cases the stamping is spoilt. The authors con- sider that far too little attention is given by the stampers to the metallurgical side of the problem. f-\ In many cases all they seem concerned with is to get as many stampings out as possible, irrespective of any special Fig. 27—Nickel chrome bar split during machining. Fig. 25. Fig. 26. Fig. 25.—Bright drawn hexagon bar, internal roak, no . external indication. (Mild steel.) Fig. 26 Nickel chrome bar cracked during rolling. for aeronautical purposes (it is not intended to criticise it for other uses). It is possible to keep down the sulphur and phosphorus content in crank steel to 0.03 per cent., though steel makers have asked a little latitude in this direction, which has been granted., but which it is very gratifying to note, is seldom if ever required. On the other hand, with the milder steels, a latitude for war production purposes of sulphur and phosphorus up to o .06 per cent, has been allowed. Military specifications for the milder steels formerly did not include an analysis, neither did they include an impact test. It has now been found necessary to include either one or the other. It is obvious that a limit on the chemical content of sulphur and phosphorus will eliminate the dangers attending the use of steel high in these constituents. It is also interesting to note that an impact test of even 15 ft. lb. on the Izod machine will have the effeft of keeping these constituents Fig. 28.—Izod specimens from cracked nickel chrome bars. precautions which should be taken. The limits of the steel should be better realised, and it must be recognised that it is not possible for these alloy steels to be handled in the same manner as mild steel. ••• Stampings to be machined all over and designed to approxi- mately finished external dimensions are almost impossible to jig, and the result is that although the stampings are absolutely to drawing and capable of being machined, it is commercially impossible to guarantee a correct jigging so as to avoid scrap. In the earlier days the designers had apparently one point of view when they issued stamping drawings, namely, that of obtaining stampings requiring a minimum of machining. An example occurred with connecting rods. This caused a great deal of trouble, and it was not until the radii were more than doubled and the stampings generally thickened up so as to Fig. 29. Fig. 30. Fig. 31. Fig. 29.—Stamped hollow head valve in alloy steel, showing flow of grain round head. Fig. 30—Similar valve to Fig. 29, turned from solid bar. JFig 31 Alloy steel valve stamping ; flow of metal in head not very satisfactory. low. With a percentage in excess of 0.06 practically no impact figure is obtained with this steel as used commercially. The use of alloy steels for drop stampings has led to many disastrous results. As previously pointed out, these steels are delicate, and require most careful manipulation. The stamper, when using mild steel, ran very little risk of burning it, and experienced no difficulty in making it fill the dies. It has taken a long time for the majority of stampers to recognise that alloy steels cannot be treated as easily, that it give what might be termed easy " flow lines " that difficulties were avoided. An example of a different nature occurs in the stamping of valves. The requirements are not altogether understood by the stampers or designers. In certain cases very satis- factory stampings have been produced, but in the majority of cases quite the reverse. The essential condition in these stampings is that the flow of the metal should follow the contour of the head of the valve 433
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