I. On the tempering of iron hardened by overstrain

It is well known that iron hardened by overstrain—for example, by permanent stretching—may have its original properties restored again by annealing, that is, by heating it above a definite high temperature and allowing it to cool slowly. Experiments to be described in this paper, however, show that if iron hardened by overstrain be raised to any temperature above about 300°C., it may be partially softened in a manner analogous to the ordinary tempering or “letting down” of steel which has been hardened by quenching from a red heat. This tempering from a condition of hardness induced by overstrain, unlike ordinary tempering, is applicable not only to steel but also to wrought iron, and possibly to other materials which can be hardened by overstrain and softened by annealing. The experiments about to be described were all carried out on rods of iron or steel about ⅜ths of an inch in diameter and 11 inches long, the elastic condition of the material being in all cases determined by means of tension tests. The straining and testing were performed by means of the 50-ton single-lever hydraulic testing machine of the Cambridge Engineering Laboratory, and the small strains of extension were measured by an extensometer of Professor Ewing’s design, which gave the extension on a 4-inch length of the specimen to the 1/1th of an inch. This instrument, which is shown attached to a specimen in the following illustration, is of a design slightly modified from that of the larger extensometer fully described by Professor Ewing in a paper “On Measurements of Small Strains in the Testing of Materials and Structures.”