1919] on The Hardening of Steel 499 



tare at which chilling begins, {b) It may set up contraction strains 

 within the cooled mass ; these strains may be wholly or in part either 

 within or beyond the elastic hmits. When the strains exceed the 

 elastic limit, and permanent deformation occurs, then hardening may 

 result as in (3)." * 



One other fact must still be mentioned. Charpy and Grenet have 

 shown that in iron containing very suiall amounts of carbon, an 

 expansion occurs on passing the Arg point — that is, from y to f3 iron ; 

 and further that in steels of tool-steel composition the separation of 

 iron carbide from austenite on cooling is also associated with an 

 expansion. 



I pass now to consider three of the most modern theories of 

 hardening ; two of these were enunciated simultaneously and inde- 

 pendently in May, 1914, the third in November of the same year. 



The first of these is due to Dr.McCance. As already mentioned, he 

 showed that when the carbon reaches about • 7 per cent a maximum 

 hardness is obtained in the quenched state, and this value is not 

 exceeded in any pure carbon steel. He reasons that if the hardness 

 were the direct consequence of carbon in solution it would be propor- 

 tional to the amount of carbon dissolved ; that it is not so shows 

 that the action of carbon is indirect, and that the hardening element 

 is the iron itself. In the annealed condition the ball hardness of this 

 steel is 174, while in the hardened condition it is 713. 



Holding as he does the view that (3 iron is not a definite allotrope 

 of the metal, but merely a iron which is non-magnetic from purely 

 thermal causes, McCance considers that at Arg y iron passes directly 

 to a. He explains the liberation of heat at Ao as being due to the 

 rapid change in specific heat which necessarily accompanies the ferro- 

 magnetic transition. He points out that since a iron loses its magnetic 

 properties above a certain temperature, it might be suggested that 

 7 iron, which at higher temperatures is non-magnetic, might itself 

 become magnetic at lower temperatures. Hadfield has shown, how- 

 ever, that non-magnetic manganese steel (y iron) is not transformed 

 even at the temperature of liquid aii- ; and McCance reasons that as 

 no treatment can make austenitic steel magnetic which does not 

 increase the specific volume at the same time, this points conclusively 

 to its transformation to a iron, since the thermal magnetic transition 

 makes no appreciable alteration in the specific volume His theory 

 of hardening is based on the following considerations and reasoning : — 

 Since steels, which at the temperature of quenching are non-magnetic, 

 are magnetic in the hardt:^ned state, this change must have taken 

 place during the time taken by the quenching ; therefore some of the 

 original y at any rate must have changed to the magnetic a condition. 

 Measurements of the electrical resistance, however, show that the 

 carbon still remains in solution ; therefore under the conditions of 



* Transactions of the Faraday Society, Beilby, November, 1914. 



