606 Professor J. 0. Arnold [Jan. 24, 



shows the structure of nearly pure iron, containing • 89 per cent of 

 carbon. The mass now consists entirely of pearlite, a mechanical 

 mixture of 87 per cent of iron with 13 per cent of normal carbide 

 of iron, FcgC. The mass abrasion hardness of normal pearlite is 

 about 4-5, that is, between fluorspar and apatite on Moh's mineral 

 scale. 



We have next to consider the phenomena known as the hardening 

 and tempering of steel. 



The slide on the screen (Figs. 5 and 5a) shows very clearly the 

 beginning (region C), the progression (region B), and end (region A) 

 of the hardening of steel, that is to say, the transformation (during a 

 thermal amplitude of perhaps 3° C.) of the compound constituent 

 pearlite (21 Fe + FegC) to the micrographically amorphous constituent 

 hardenite, which corresponds to the empirical figures Fe.24C, in which 

 the carbide of iron, owing to the quenching, is trapped in some mole- 

 cular association with the whole of the iron. The constituent hardenite 

 has a hardness of seven on Moh's mineral scale — that is to say, it is as 

 hard as quartz, flint, or rock crystal. It is a little difficult to realize 

 how much the thermal capability of the mineral pearlite (with a hard- 

 ness of 4 • 5) to transform itself into the igneous rock hardenite (with 

 a hardness of 7) has contributed to the advance of civiHzation and to 

 the material well-being of the human race. But unfortunately it was 

 found that hardenite was thermally very nnstable, and that its cutting 

 powers were greatly limited by the fact that the heat of friction in 

 turning operations caused the hardenite to revert largely to relatively 

 soft pearlite at a blue heat, say 300^ C. This property naturally limited 

 the operations of engineers as to speed, as to traverse, and as to depth 

 of cut, and consequently as to the cost and rate of output of all the 

 engines and appliances necessary to our modern civilization. 



The slide on the screen shows in the black areas the evolution of 

 the latent heat of hardening, and consequently the transformatioL 

 of the quartz-like hardenite to soft pearlite. This change at about 

 250° C. acquires a marked increase in velocity, which reaches a 

 maximum at about 300° C. Here the soft pearlite becomes the pre- 

 dominant partner, and the cutting power of the mass has practically 

 vanished. 



About the year 1870 marked the first beginnings of an epoch in 

 cutting-steel metallurgy, which may be called the tungsten-chrome era. 

 Robert Forrester Mushet, at the Clyde Works, Shefiield, began to 

 mauufactiire on a considerable scale his " self -hardening steel." 

 Mushet had practically discovered that when carbon steel was alloyed 

 with a large percentage of tungsten, it, when cooled from a yellow 

 heat in a draught of air, was not only sufficiently hardened, but 

 that, owing to the fortifying action of the tungsten on the carbon, 

 the hardenite was thermally considerably more stable than that of 

 plain carbon steel. It is probable that in Mushet's early steels the 

 " breaking-down " point of the hardenite was raised to a temperature of 



