The energy absorbed in the cold working of metals

It is well known that when ductile metals or alloys are subjected to plastic deformation at room temperature, i. e ., when they are subjected to “cold-working,” considerable quantities of heat are liberated. It has further been suspected for a number of years, and has more recently been experimentally established by the work of Taylor and Farren, that the heat thus generated is less than the equivalent of the mechanical work expended upon the metal. A certain quantity of heat, therefore, is absorbed or becomes latent in the metal as the result of changes which it undergoes during the process of plastic deformation. The amount of heat thus absorbed is small, and is probably of little practical interest, but for theoretical reasons, an accurate determination of the amount of heat which becomes latent in this manner is of importance. One of the present authors became interested in this question some ten years ago on account of the important bearing which a knowledge of the amount of heat which becomes latent in this manner would have upon the theory, first put forward by Beilby, that a certain proportion of metal which undergoes plastic deformation becomes converted from the crystalline into an amorphous condition. Some unpublished results which came to his knowledge at that time suggested that the amount of such latent heat was considerable, and an experimental effort to determine this amount was, therefore, begun. In the earliest attempts a testing machine of the usual type used for engineering purposes was employed, and an endeavour was made to measure the heat generated in a small block of metal, of known dimensions and properties, when compressed by a definite amount. Subsequently, in view of the experimental difficulties encountered in working with small compression pieces, large bars of metals strained in tension were employed. It was found that reasonably satisfactory thermal measurements could be obtained, but with the appliances then employed it was not possible to measure the amount of mechanical work applied to the test piece with sufficient accuracy. While this work was in progress, the results of the investigation of Taylor and Farren were published. These investigators had overcome most of the difficulties which had been encountered, and had succeeded in obtaining results of considerable accuracy. Further experimental work involving the use of tensile test pieces and testing machines was therefore abandoned, and another line of attack was adopted. Although the results of Taylor and Farren are of the greatest interest, it was hoped that other methods of attacking the problem might make it possible to obtain still higher degrees of accuracy. It is obvious that in a tensile test piece the amount of mechanical work which can be done, and the degree of plastic deformation which can be applied to a piece of metal, are very much limited. For instance, even the most ductile sample of metal will usually break before it has been stretched plastically to twice its original length, whereas in such a process as rolling or wire drawing, very much larger amounts of plastic deformation can be applied. The application of a larger amount of plastic deformation also involves the expenditure of a much larger amount of power, and it was hoped that this would make it possible to measure the power used to a higher degree of accuracy. An investigation was therefore begun for the purpose of measuring the heat evolved in the process of wire drawing. This process appeared to be particularly promising for the purpose in question, since the work is applied in the form of a prolonged steady pull which should be capable of accurate measurement and control. Further, the process of plastic deformation takes place in a very small space within the die itself, and this can be immersed in the calorimeter by means of which the total amount of heat generated can be measured. Unfortunately the results obtained have not entirely justified these expectations. The measurement of the mechanical work done in wire drawing has proved to be a matter of much greater difficulty than was anticipated, mainly because it has been found that the resistance encountered by the wire in passing through the die is not sufficiently uniform to allow of the maintenance of a steady and easily measured tension. It will be seen below, however, that this difficulty has been to a large extent overcome by the experimental devices adopted. A more serious difficulty has been that, in such a process as wire drawing, although the amount of heat generated can be made as large as desired by the use of great lengths of wire, the rate at which this heat is generated is not very high. The result is that the rise of temperature in the calorimeter is very gradual, and that the numerous corrections which apply to calorimetric measurements of this sort become of relatively very great importance owing to the long time over which an experiment has to be extended. It is this purely calorimetric difficulty which has served mainly to set a limit to the accuracy attainable by this method in its present form.