Nov, 14, 1889] 



NATURE 



35 



I have referred already, change in position with the 

 degree of carburization of the metal. It is useless to 

 attempt to harden steel by rapid cooling if it has fallen in 

 temperature below the point (in the red) «, and this is the 

 point of " recalescence " at which the carbon combines 

 with the iron to form carbide-carbon : it is called V by 

 Brinell. In highly carburized steel, it corresponds exactly 

 with the point at which Osmond considers that iron, in 

 cooling slowly, passes from the ^ to the o modification. 

 Now with regard to the point b of Chernoff. If steel 

 be heated to a temperature above a, but below b, it 

 remains fine grained however slowly it is cooled. If the 

 steel be heated above b, and cooled, it assumes a crystal- 

 line granular structure whatever the rate of cooling may 

 be. The size of the crystals, however, increases with the 

 temperature to which the steel has been raised. 



Now the crystalline structure, which is unfavourable to 

 the steel from the point of view of its industrial use, may 

 be broken up by the mechanical work of forging the hot 



Fig. II shows the way in which the tenacity of steel containing varying 

 amounts of carbon is increased by oil hardening,' while at the same lime 

 the elongation rapidly diminishes. 



mass ; and the investigations of Abel, of Maitland, and of 

 Noble, have shown how important " work" on the metal is. 

 When small masses of hot steel are quenched in oil, they 

 are hardened just as they would be if water were used as 

 a cooling fluid. With large masses, the effect of quench- 

 ing in oil is different. Such cooling of large hot masses 



' This was well shown in Prof. Akerman's celebrated paper on " Harden- 

 ing Iron and Steel," Joum. Iron and Steel Institute. 1879. Part ii. p. 501. 



appears to break up this crystalline structure in a manner 

 analogous to mechanical working. If the mass of metal 

 is very large, such as a propeller shaft, or tube of a large 

 gun, the change in the relations between the carbon and 

 the iron, or true "hardening" produced by sutrh oil 

 treatment is only effected superficially — that is, the 

 hardened layer does not penetrate to any considerable 

 depth, but the innermost parts are cooled more quickly 

 than they otherwise would have been, and the develop- 

 ment of the crystals, which would have assumed serious 

 proportions during slow cooling, is arrested. It depends 

 on the size of the quenched mass, whether the tenacity of 

 the metal is or is not increased, but its power of being 

 elongated is considerably augmented. This prevention 

 of crystallization I believe to be the great merit of oil 

 quenching, which, as regards large masses of metal, is 

 certainly not a true hardening process. 



There has been much divergence of view as to the 

 relative advantages of work on the metal, and of oil- 

 hardening, but I believe it will be possible to reconcile 

 these views, if the facts I have so briefly stated be 

 considered. 



The effect of annealing remains to be dealt with. In a 

 very compUcated steel casting, the cast metal probably 

 contains much of its carbon as hardening carbon, and the 

 mass which has necessarily been poured into the mould 

 at a high temperature is crystalline. The effect of an- 

 nealing is to permit the carbon to pass from the " harden- 

 ing" to the " carbide " form, and, incidentally, to break 

 up the crystalline stucture, and to enable it to become 

 minutely crystalline. The result is that the annealed 

 casting is far stronger and more extensible than the 

 original casting. The carbide-carbon is probably inter- 

 spersed in the iron in fine crystalline plates, and not in a 

 finely divided state. It would obviously be impossible to 

 " work"— thatis,to hammer— complicated castings,and the 

 extreme importance of obtaining a fine crystalline struc- 

 ture by annealing, with the strength which results from 

 such a structure, has been abundantly demonstrated by 

 Mr. J. W. Spencer, whose name is so well known to you 

 all in Newcastle. 



The effect of annealing and tempering is in fact very 

 complicated, and I can only again express my wish that it 

 were possible to do justice to the long series of researches 

 which Barus and Strouhal have conducted in recent 

 years. They consider that, annealing is demonstrably 

 accompanied by chemical change, even at temperatures 

 slightly above the mean atmospheric temperature, and 

 that the " molecular configuration of glass-hard steel is 

 always in a state of incipient change, ... a part of 

 which change must be of a permanent kind." Barus 

 says " that during the small interval of time within which 

 appreciable annealing occurs, a glass-hard steel rod sud- 

 denly heated to 300° is almost a viscous fluid." ^ Barus 

 considers that glass-hard steel is constantly being 

 spontaneously "tempered" at the ordinary temperature, 

 which, he says, " acting on freshly quenched [that is 

 hardened] steel for a period of years, produces a diminu- 

 tion of hardness about equal to that of 100^ C, acting for 

 a period of hours." 



The nature of the molecular change is well indicated in 

 the long series of researches which led them to conclude 

 that in steel " there is a limited interchange of atoms 

 between molecules under stress, which must be a property 

 common to solids, if, according to Maxwell's conception, 

 solids are made up of configurations in all degrees of 

 molecular stability.'' 



Barus and Strouhal attach but little importance to the 

 change in the relations between the carbon and the iron 

 during the tempering and annealing of hard steel. They 

 consider that in hardening steel the " strain once applied 

 to steel is locked up in the metal in virtue of its 



Fhtl. Mag , xxvi., 1888, p. 209. 



