286 THE AMERICAN MONTHLY [Oct 



color 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 toler- 

 ably accurate guide in determining the hardness which the 

 object will acquire on being cooled. Although this meth- 

 od is often wonderfully accurate, it must be borne in mind 

 that the colors 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 microscope. 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 transformation of the iron, which may ex- 

 ist in a soft state, and a hard state, the latter being pro- 

 duced, in the case of a high carbon steel, above 800° C. 



Therefore, in order to produce a hard steel, the metal 

 must be quenched above this temperature. Should the 

 temperature of quenching be rather low, the structure, in- 

 stead of consisting wholly of martensite, will be found to 

 contain another constituent known as "troostite" (from 

 Troost, the chemist). Troostite almost invariably occurs 

 in a matrix of martensite, or a mixture of martensite and 

 ferrite. Steels containing troostite are soft, so that sub- 

 sequent tempering will be useless. They are, however, not 

 so soft as steel containing pearlite. In ordinary steel the 

 pearlite develops at about 700°C. Should a specimen be 

 quenched after the formation of this constituent it will not 

 harden. In practice, steel is generally quenched at near- 

 ly 1,000°C, which has the effect of converting the whole 

 mass into martensite. 



To be able to determine the quality of the quenching of 



