502 Professor H. C. H, Carpenter [March 7^ 



strain." As he points out, the theory of interstrain action is in itself 

 an inquiry, and that inquiry has yet to be undertaken. 



The second of the most modern theories of hardening was put 

 forward by Professor Edwards and myself. ''• We started from the 

 position that steels are not the only alloys which can be hardened by 

 quenching in water from moderately high temperature. Many alloys, 

 in fact, are known which possess this property. The difference is one 

 of degree and not of kind. Steels, in fact, are simply the higher 

 members of a group of different alloys which pospess the same charac- 

 teristics in varying degrees. Further, all the alloys which behave 

 in this way have similar constitutions at high temperatures, and 

 undergo precisely similar changes as they cool to atmospheric tem- 

 perature. They undergo decomposition at certain critical tempera- 

 tures w^hich are marked by evolutions of heat. It is generally 

 admitted that the hardening produced by quenching any of these 

 alloys is in some way related to the effect of quick cooling upon these 

 critical heat changes. Within certain limits the effect of varying the 

 rates of cooling upon the Ar^ change in steels and other similar 

 changes is well known. As the rate of cooling is increased, the tem- 

 perature at which the changes occur is progressively lowered, and 

 simultaneously the thermal magnitude is decreased. It is therefore 

 in agreement v^ith established facts that, as the rate of cooling is 

 increased, there is an increased tendency to prevent the critical change 

 taking place, and to keep the heat of the change suppressed. The 

 careful experiments of Professor Benedicks show that when the 

 highest quenching velocities are attained there are no indications of 

 any heat at all being evolved. Logically, therefore, these facts appear 

 to point to the view that the hardening is connected with the sup- 

 pression of the hetit of the inversion. If this be the case, energy has 

 to be brought to bear upon the specimen during the quenching 

 process in order to overcome the tendency for the inversion to take 

 place. This energy is considered to be brought to bear in two 

 ways : — 



1. By the sudden contraction of the outer shell or envelope of 

 the specimen. 



2. By an internal molecular contraction of the mass which is con- 

 nected with the solution or osmotic pressure of the dissolved carbon 

 in the case of steel. 



This explanation, so far as it goes, is a perfectly general one, 

 applicable to all alloys which may be hardened by quenching. 



The next step in the argument is as follows : — In the case of all 

 alloys hardened in this way when the heat change is suppressed, 

 severe internal stresses are set up which exceed the elastic limit, and 

 consequently cause an internal straining of the material. That, in the 

 case of steels, the metal is internally strained appears to us well 



* Journal of the Iron and Steel Institute, No. 1, 1914, pp. 138-177. 



