maximum. After that the mass does not remain homogeneous. A steel with 1.5 per cent. carbon, tempered Fig. 20.-CEMENTATION STEEL (1.57 per cent. carbon). Heated to nearly 1050°, and quenched in ice-cold water. Polish-attack. V x 1000 diameters. at 1050° (point N of fig. D) in iced water, separates into two constituents (fig. 20). One of the two, desig nated by the name austenite (this is the clear part of the figure), possesses the unexpected property of being scratched by a sewing-needle: the hardness diminishes then with the amount of carbon, when this proportion exceeds a certain limit. If we retard the quenching until the temperature has fallen below BEC but is still above AE, there will naturally be found in the chilled metal the constituents which have already been isolated during the slow cooling before the commencement of the quenching. Thus, steel with 0.30 per cent. carbon, quenched at 720° C. (point O of fig. D), will show white ferrite at the side of striated martensite (fig. 21); the steel with 1.24 per cent. carbon, quenched at 735° (point P of fig. D), will contain cementite, in relief, which appears black under the conditions of illumination adopted, as compared with the martensite which surrounds it (fig. 22). Below AE the structure is fixed, chilling commenced from a point in this region remaining without effect. Bronzes. The knowledge of the bronzes is much less advanced than that of steels. The complete curve has been determined by Mr Stansfield1 (fig. E). Below are shown together the curves which represent the variations, in a function of the chemical composition, of several of the physical properties :—the electro-motive force (after Laurie), the electric conductivity (after Lodge), the calorific conductivity (after Calvert and 1 Proc. Inst. Mech. Eng., p. 269, plates 41 to 43 (1895). Johnson), the hardness (after Martens), induction (by the Hughes Balance), tenacity (after Roberts-Austen), Fig. 21.-FORGED STEEL (0.30 per cent. carbon). Heated to 900°, and quenched at 720°. Polish-attack. x 1000 diameters. and the elongation (after Thurston). The majority of these curves are borrowed from Mr Stansfield. Many of them seem to indicate the existence of the definite compounds SnCu, and SnCu,, to say nothing of isolated Fig. 22.-FORGED STEEL (1.24 per cent. carbon). Heated, and quenched at 735°. Polish-attack. V x 1000 diameters. interruptions. Behrens, by the study of the microstructure, has arrived at the idea of definite compounds 1 Proc. Inst. Mech. Eng., pp. 70-91. |