VIII. The crystalline structure of metals. (second paper.)

Abstract

The investigations described in the present paper deal principally with the phenomena of annealing. They form a continuation of the research described in the Bakerian Lecture for 1899 (‘Phil. Trans.,’A, vol. 193, 1900, pp. 353-377). In iron, steel, and brass these phenomena have been studied with the aid of the microscope by various workers, among whom Arnold, Charpy, Stead, and Roberts-Austen should be particularly mentioned. As a result of their labours it is well known that annealing is accompanied by a re-arrangement of the crystalline grains of the metal. Thus, when a piece of iron is strained in tension its crystalline grains become elongated in the direction of tension ; but when the specimen has been subsequently annealed by being heated to a bright red, all signs of such elongation disappear from the crystalline pattern revealed by the microscope. In fact it is not generally possible to find any definite connection between the crystalline pattern seen in the same specimen before and after annealing. In general, the pattern seen after annealing resembles that found in a similar specimen before it has been strained, but the scale and character of the pattern produced depend very much on the details of the annealing process, particularly upon the temperature applied, the time of its application, and the rate of cooling. Arnold and Stead have shown that prolonged annealing tends to produce large crystals in iron and steel. But even short exposure to a suitable temperature is well known to produce complete re­crystallisation, and it has been suggested that these changes occur at critical points corresponding to the “arrest-points” in the cooling of the metal. These arrest-points indicate evolutions of heat, and it is natural to suppose that they are evi­dences of re-arrangement of the structure of the metal.