1019] on The Hardening of Steel 501 



and copper, for instance, has been shown to have its atoms built up 

 in a ' face-centred ' cube, that is, a cube having an atom at each 

 corner, and one at the centre of each face. The effect of deformation 

 on such systems must be to alter the position of each atom relatively 

 to its neighbour, and permanent deformation must cause so much 

 alteration that the atoms along the planes of slip can no longer return 

 to their positions of equilibrium. This corresponds to the condition 

 of interstrain and results in a hardening of the material." * McCance 

 admits that the destruction of the crystalline arrangement does go so 

 far as to render the material in these planes of greatest movement 

 actually amorphous, but appears to deny that this amorphous phase 

 is hard. 



The important connection between rate of cooling and hardening 

 is well illustrated by the behaviour of alloy steels. If, for instance, 

 manganese is added to the steel it lowers the temperature of the 

 transformations and greatly decreases their velocities, so that with 

 sufficient manganese even at ordinary rates of cooling the structure 

 is martensitic, and the steel very hard and brittle. The addition of 

 even more manganese ultimately prevents any transformation taking 

 place, even when the steel has cooled to the ordinary temperature, 

 and a pure austenitic steel is the result. If the martensitic manganese 

 steel be water-quenched, not only is it not hardened but is actually 

 softened, this being due to the complete suppression of both trans- 

 formations and resulting in the production of an austenitic steel. 

 Similar results may also be obtained with nickel. 



The following experiments quoted by McCance are also important 

 as bearing out his contention that the main cause of the hardening 

 of steel lies in a change in the iron itself. A steel containing 

 o'6 per cent nickel and U'19 per cent carbon had a normal hardness 

 of 183, which on quenching in water from 1000° C. had risen to 444, 

 whereas a steel containing the same amount of carbon but no nickel 

 increased after the same treatment from 143 to 218. In other 

 words, whereas the carbon steel increased only 65 points, the nickel 

 steel was augmented by 259. As nickel does not form a carbide in 

 this range of composition, the state of the carbon is similar in both 

 alloys, consequently the increased hardness must be due to the 

 influence of the nickel on the iron, and McCance explains this by the 

 view^ that the iron is enabled to retain a greater degree of interstrain 

 by lowering the temperature at which the transformation of y to a iron 

 takes place. He rightly lays stress on the importance of internal 

 friction in retarding transformations and preserving metastable states. 



McCance's theory of hardening has been built up by acute reason- 

 ing on a solid foundation of well-established facts. As yet, however, 

 it cannot be regarded as constituting a comjjJete theory, because it is 

 intentionally non-committal with regard to the expression " inter- 



* Transactions of the Faraday Society, 1914, p. 53. 



