March 7, 1919] The Hardening of Steel 489 



carbon, it is found that the temperature of the Av^ charge is lowered, 

 that the Aro charge is unaffected, and that a third critical point makes 

 its appearance at about 700° C, this last-named point being of small 

 magnitude, but completed within a very narrow range of temperature. 

 This is known as Ar^ Taking next an alloy with • 35 per cent carbon, 

 the position of Arg is still further lowered, Ar., still remains the same, 

 while Arj appears as a much larger point. At about 0*45 per cent 

 carbon, Arg has been so much lowered that it coincides with Ar^ ; 

 in other words, at this composition y iron changes directly to a. 

 Arj is also observed, and has grown in magnitude. With the 

 gradual addition of carbon Arg and Ar^, w^hich have now become 

 one point, are still further lowered, while Ar^ increases in magnitude. 

 Finally, at about 0*9 per cent carbon, all three points coincide at 

 one temperature, namely, that of the Ar^ change about 700° C. It 

 is obvious from the foregoing that while the points Arg and Ar2 

 are characteristic of iron, the point Ar^ is characteristic of iron plus 

 carbon — namely, steel. 



The interpretation of the critical points thus briefly outhned has 

 been rendered possible by the combined study of the foregoing 

 method with the microscope, which has revealed the structural 

 changes accompanying these heat evolutions, controlled by a con- 

 sideration of the requirements of the phase rule, and elaborated into 

 what is known as the Iron-Carbon Equilibrium diagram. This 

 diagram (Fig. 4), whose co-ordinates are temperature and concentra- 

 tion, defines W'hat, at any given temperature and concentration, are 

 the constituents present in an iron-carbon alloy. In so far as know- 

 ledge of these is required in connection with the hardening of steel, 

 attention may be confined to the left-hand portion of the diagram 

 between the limits of concentration to 1*5 per cent. The area 

 AG- S E defines the limits of existence of y iron, from which it will 

 be seen that it can exist from 1500'-700° C, and from up to 

 1 • 8 per cent carbon. This constituent is also known as austenite. 

 Within this area the carbon present in all steels is dissolved, and the 

 evidence, both chemical and physical, indicates that it is combined 

 with, as well as dissolved in, the iron as a carbide of iron. Below 

 700° C. only a iron is stable, and in a normally cooled steel almost 

 the whole of the carbon is precipitated as a crystallized carbide of 

 iron, FcgC. At the point S, corresponding to the composition 

 0*9 per cent carbon, we have the simplest case possible, for the 

 austenite is resolved on cooling below it into a mixture of a iron and 

 iron carbide, FcoC. This mixture separates in characteristic alternate 

 plates with a laminated appearance, and is called pearlite from its 

 analogy to mother-of-pearl. It consists of about 86 "5 per cent iron 

 and 13 '5 per cent FcgC, on the assumption that no carbide is dis- 

 solved in the iron. This assumption is not strictly speaking true, 

 but the amount dissolved is so small that for practical purposes it 

 may be neglected. 



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