10 Transactions of the Society. 



various degrees of hardness, and it is upon this special property that 

 the great value of steel depends. Although there is no well defined 

 line of demarcation between high carbon steel and white iron, yet the 

 former has a much wider range of hardness than the latter when 

 submitted to suitable thermal treatment. 



The changes in the hardness of a steel are accompanied by a 

 corresponding change of structure. Take, for example, the ordinary 

 process of tempering. Steel is generally tempered by two processes : 

 (1) hardeniDg by quenching in water, oil, or mercury ; and (2) 

 re-heating the hardened steel to a given temperature and plunging 

 in water, or allowing to cool in air. Fig. 10 represents the quenched 

 structure of the steel shown in fig. 5 ; it is composed of a system of 

 interlacing crystalline fibres, and is known as " martensite," after 

 Prof. Martens, of Berlin. The structure of martensite is developed 

 by a prolonged etching with an infusion of liquorice, or by an attack 

 of alcohol containing J p.c. of nitric acid. When a quenched steel 

 is tempered, the interlacing crystalline fibres disappear, and the 

 structure becomes granular. The character of the tempered structure 

 varies greatly with the temperature and the time the steel is re- 

 heated. No name has yet been assigned to the tempered structure. 

 Fig. 11 shows the structure of a steel which has been re-heated to a 

 straw colour. 



In practice, the workman tempers steel by watching the various 

 colours assumed by the surface of the metal during the progress of 

 the operation, and when the proper colour makes its appearance the 

 object is suddenly cooled. These tints, some of which are extremely 

 brilliant, are probably occasioned by films of oxide corresponding 

 with considerable exactitude to the degree of heat to which the metal 

 is exposed, and they consequently serve as a tolerably accurate guide 

 in determining the hardness which the object will acquire on being 

 cooled. Although this method is often wonderfully accurate, it must 

 be borne in mind that the colours will appear even when the metal 

 has not been quenched, so that the tint alone is not indicative of a 

 good result. This may, however, be easily determined by the Micro- 

 scope. As the time and intensity of the re-heating increases, the 

 structure more and more resembles that of pearlite, so that it is 

 quite possible to ascertain the quality of the temper from the micro- 

 structure alone. 



The hardening of a steel by quenching is not merely due to a 

 change in the condition of the carbon, but also to the molecular trans- 

 formation of the iron, which may exist in a soft or a state, and a hard 

 or 7 state, the latter being produced, in the case of a high carbon steel, 

 above 800° C. Therefore, in order to produce hard steel, the metal 

 must be quenched above this temperature. Should the temperature of 

 quenching be rather low, the structure, instead of consisting wholly of 

 martensite, will be found to contain another constituent known as 

 " troostite " (from Troost, the chemist). Troostite almost invariably 



