12 
MR. J. MUIR OX THE TEMPERING OF IRON 
Microscopic Examination of Steel. 
The two steels whose elastic properties have just been compared, and whose 
chemical analyses are given above, v^ere also examined by means of the microscope. 
Tin •ee methods of examination were adopted, the first being tlie ordinary method by 
etching a polished surface with dilute nitric acid. 
A smootli surftice was prepared 1)y means of commercial emery j^aper, the final 
polishing l:)eing done by wet rouge contained on a piece of chamois leather stretched 
over a rotating disc. All or nearly all th_e fine scratches left on tlie surface of the 
steel by the emery paper liaving been removed l)y the rouge, the surface was washed 
and dried, and then etched with dilute nitric acid (0‘1 per cent, strength). 
Figs. A and (Plate 1) show, under a magnification of 150 diameters and with 
vertical illumination, the appearance (after polishing and etching in this manner) of 
transverse sections of the F-inch steel rod used in the experiments described in this 
paper. Fig. A shows the structure of this steel when in the condition as supplied by 
the makers, that is when in the condition having the elastic properties illustrated by 
Diagram 1. Fig. B shows the structure of the same steel after annealing l^y heating 
for a few minutes to 750° C., that is, fig. B shows the structure of the steel after 
it had been brought by annealing into the condition having the elastic properties 
illustrated by Diagram No. 2 of this paper. 
Tlie photographs from which figs. A and B have been reproduced were taken three 
or four weeks after the specimens had been prepared and etched, so that the surfaces 
had become slightly tarnished. The tarnish, however, seemed only to emphasise the 
distinction between the two constituents, the “ ferrite ” and the “ pearlite.” 
A comparison of figs. A and B shows that by annealing at 750° C. the structure 
of the |-inch steel rod had been considerably altered. After annealing the steel was 
much coarser grained than when in the somewhat hardened condition as supplied by 
the makers. This change produced by annealing on the dimensions of the micro¬ 
structure of steel has been often observed before.! Figs. A and B are given here 
for the sake of comparison with the change in elastic properties illustrated by 
Diagrams 1 and 2 of this paper. It had been thought that this change in elastic 
properties could be entirely accounted for on the supposition that the bars left tlie 
rolling mills at a comparatively low temperature (say a dull red heat), and had so 
become hardened by a species of overstrain. The microscopic examination illustrated 
* Reproduced from photographs. The author is indebted to JMr. Roseniiain, of St. John’s College, 
for photographing the micro-sections prepared for the paper, and also for information as to the methods 
of polishing and etching. 
t For example, by Osmond, “ JMethode generale pour I’Analyse micrographique des Aciers au Carbone,” 
‘Bulletin de la Soc. d’Encouragement,’ Mai, 1895 ; by Arnold, “ The Influence of Carbon on Iron,” 
‘ Proc. Inst. Civ. Eng.,’ December, 1895; l)y Stead, “The Crystalline Structure of Iron and Steel,” 
‘ Journ. of Iron and Steel Institute,’ 1898. 
