500 Professor H. C. H. Carpenter [March 7, 



quenching the transformation of the iron from the y to the a state 

 can take place independently of the change in the state of the carbon 

 from one of solution to that of precipitation. Thus, these two trans- 

 formations, which on slow heating and cooling take place simultane- 

 ously, can behave as independent reactions, although in a limited 

 sense. " For, though martensite may be considered as an enforced 

 solution of carbon in a iron, it must not be forgotten that there is 

 still some y iron present with the carbon, which is absolutely necessary 

 to maintain its solubility, and any change in the state of the carbon 

 causes this transformation of y iron to a. It might be on this 

 account he suggested that the iron is chemically combined with the 

 carbon, but an examination of the curve connecting the loss of 

 magnetic saturation intensity with the carbon content shows that the 

 y iron increases much more rapidly than the amount of carbon, which 

 points to purely physical influences." '• McCance then considers the 

 cooling of a hypo-eutectoid steel, that is, one containing rather less than 

 • 9 per cent carbon. As he points out, its complete transformation 

 into a iron and cementite involves two distinct transformations : 

 (a) a change in the state of the iron from y to a ; (^) a change in 

 the state of the carbon. The first of these can take place indepen- 

 dently of the second, but when the second takes ph^ce it necessarily 

 involves tlie first. There are therefore three possible conditions : — 



1. Both transformations are completed, producing pearHtic steel 

 which is soft. 



2. Both transformations are suppressed. This has never been 

 achieved with any pure iron carbon alloy, and the addition of a 

 certain amount of manganese is necessary, as Maurer has shown. In 

 this case pure austenite results, and this, while harder than the steel 

 in the pearlitic condition, is nevertheless softer than a hardened steel. 



8. When {a) takes place, but {h) is suppressed, then the resulting 

 structure is martensitic, and, according to McCance's theory, the 

 a iron formed under this condition is interstrained and very hard. 

 Case 3 then is the important one from the hardening standpoint, and 

 to be possible it is necessary that a difference should exist in the 

 respective velocities of transformation of {a) and (&), so that with an 

 appropriate rate of cooling the slower transformation can be sup- 

 pressed, while the faster still takes place. On McCance's view then, 

 it is interstrained a iron which is the cause of the hardening of steel. 

 He does not, however, accept Beilby's theory of a hard vitreous 

 amorphous phase. His view of interstrain is as follows : " Metals in 

 their normal unstrained condition are crystalline, that is, they are 

 composed of an orderly arrangement of their constituent atoms 

 arranged in a space-lattice which conforms witli whatever symmetry 

 the crystals possess. Successful attempts have been made by the aid 

 of X-rays to determine the actual position occupied by the atoms, 



* Transactions of the Faraday Society, 1914, p. 56. 



