10 
MR. J. MUIR ON THE TEMPERING OF IRON 
It may be of interest to refer to the amount of stretcliing which occurred at 
the various yield-points shown in Diagram No. 2. At the first yield-point the 
permanent set was found to be 0'08 of an inch on the 4-inch length, at the second 
it was again 0’08 of an inch, at the third it was O'll, at the fourth 0'15, and at 
tlie fifth fracture occurred and only local yielding of about 0'07 of an inch Avas 
obtained. Although the extensions at the third and fourth yield-points Avere thus 
greater than those at the first and second, it is probable that theoretically there 
need have been no such difference. Had it been possible to remove the load from 
each little portion of the specimen as soon as the yielding Avhich occurs at a 
yield-point had spread throughout that portion of material, then probably the 
yielding at the third and fourth yield-points Avould not haA^e been different from 
that at the first tAvo. The extra extension at the higher yield-points was in all 
likelihood due to creejAing, whicli continued after the break-down AAdiich occurs at a 
yield-point had taken place. In fact the elongation obtained at the fourth yield- 
point shoAvii in Diagram No. 2 would have been greater still had not the load been 
removed shortly after the large yielding action had spread throughout the length 
of the specimen. Had this not been done, creeping Avould have continued, and 
jjrobably fracture AV(3uld have supervened, for previous experiments had shoAvn that 
there Avas considerable danger of fracture occurring Avhen a yield-point AA^as j^assed 
at a high stress.* 
The total elongation of the specimen Avhose history is given in Diagram 2 was 
thus 0‘49 of an inch, or rather over 12 j^er cent, on a 4-inch length. The breaking 
load Avas 59|- tons per sq. inch. Another specimen of this material which Avas 
annealed and tlien broken by the testing machine in the usual fashion, that is, 
Avithout alloAving intermediate recoveries of elasticity to take place, gave an ultimate 
strength of slightly over 44 tons per sq. inch, Avith an elongation of about 26 per cent, 
on a 4-inch length. 
Comparison of Two Materials. 
The folloAA'ing comparison of the material Avhose elastic properties have just been 
described, Avith that employed preAuously in the AA'ork on recovery from OA^erstrain, 
Avhich has been referred to more than once already, may be of interest. The chemical 
analyses of these tAvo materials Avere kindly supplied by Messrs. Edgar Allen and 
Co., Limited, Sheffield. They are as follows :— 
* “ On the Recovery of Iron from Overstrain,” ‘ Phil. Trans.,’ A, vol. 193, p. 35, 1899. 
