20 
MR. J. MUIR ON THE TEMPERING OF IRON 
hours, and then very slowly cooled by gradually reducing the gas supply while the 
specimen was kept in the furnace. The effect of prolonged annealing is shown in the 
further lowering of the j^rimary yield-point, in the diminution of the step by which 
the yield-jDoint is raised, and in the reduction of the amount of stretch which occurs 
at a yield-point. 
The Lowmoor iron, whose elastic properties have just been considered, was also 
examined by means of the microscope. A small piece was cut from each of the three 
specimens used to obtain the curves of Diagrams 4 and 5 ; these small pieces of 
material were polished, etched with dilute nitric acid (2 per cent, strength), and 
examined under a magnification of 100 diameters. Fig. I (Plate 1) shows the micro¬ 
structure of the material when in the condition as supplied by the makers. The 
gramdes were comparatively small, and slag was fairly uniformly distributed in small 
quantities all over the section. Fig. 2 (Plate l)^shows the structure of the iron which 
had been annealed at 750° C. for six hours, that is, this figure shows the structure of 
the specimen whose elastic j^roperties are illustrated at B, Diagram 5. The granules 
are shovm to be much larger than in the material in the condition as supplied ; the 
slag had consequently segregated into larger masses.* The sections in figs. 1 and 2 
were transverse to the length of the rod of iron. The photograph reproduced in fig. I 
was taken from a portion of the section where the granules were larger than the 
average, and where the slag was less thickly distri])uted. Fig. 2, on the other hand, 
shows by no means the largest granules found in the material after it had been 
annealed, but it should be stated that in some portions of the annealed material the 
granules were still found to be c[uite small. 
The specimen taken from the bar used to obtain curves A of Diagram 5, that is, 
the specimen of Lowmoor iron which had been annealed by heating to 750° C. for 
only a few minutes, showed granules larger than in the virgin material, but, taking 
the average, distinctly smaller than those shown with the material which had been 
subjected to prolonged annealing. This is, of course, in accordance with Arnold’s 
and Stead’s results. It should be remembered, however, that the structures of 
annealed specimens in particular were found to be by no means uniform, so that a 
complete survey of the sections had to be made in order to get a just comparison of 
the different granular structures. 
To return to the elastic properties of the Lowmoor iron, attention may be called to 
the fact that the ' permanent sets which occurred at the various yield-points are 
marked in both series of curves in Diagram 5, and also at the yield-points in 
Diagram 4. The total elongation of the specimen of Diagram 4, owing to the 
position of the fracture, could not be exactly measured, but it was pi’obably about 
12 per cent, on a 4-inch length. The breaking load was 31|- tons per sq. inch. 
* This change in structure produced by annealing has been shown by ARNOLD, “On the Influence of 
Carbon on Iron,” ‘ Proc. Inst. C.E.,’ 1895; and by Stead, “ The Crystalline Structure of Iron and Steel,” 
‘ Journ. Iron and Steel Inst.,’ 1898. 
