PHILOSOPHICAL TRANSACTIONS. 
I. On the Tempering of Iron Hardened hij Overstrain.^ 
By James Muir, B.Sc., B.A., Trinity College, Cambridge (1851 Exhibition Research 
Scholar, Glasgow University). 
Communicated by Professor Ewing, F. R.S. 
Received July 11,—Read December 6, 1900. 
[Plate 1.] 
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 tempei'ing 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 fths 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 rooVoof^ of an inch. This instrument, 
which is shown attached to a specimen in the following illustration, is of a design 
* The work described in this paper was a continuation of that already described in a paper by the 
present author, “On the Recovery of Iron from Overstrain,” ‘Phil. Trans.,’ A, vol. 193, 1899. 
VOL OXtWIII.—A 300. B 23.1.1902 
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