490 



Professor H. C. H. Carpenter 



[March 7, 



Steels on the left-hand side of S consist, below 700° C, of a 

 mixture of iron and pearlite, whereas those to the right consist 

 of a mixture of iron carbide and pearlite. From this diagram it 

 will be seen why a tool steel containing O'l) per cent carbon has 

 been chosen for illustrating the theories of hardening. Such a steel 

 when cooled from a high temperature has only the one critical point 

 on cooling, namely, at S, and therefore furnishes the simplest case 

 for discussion. 



From a reference to the diagram it.wil 



be clear that if a steel 



Fig.4. EQUILIBRIUM DIAGRAM 



500. 



Li ^u icL 



CemAjiifLte: 



CpTTiPntih 



CementUe 



1 



isies.D.) 



CarboTL per Cent. 



tool is quenched from a red heat, say about 800° C, at the moment 

 of quenching the constituent present is austenite. The act of 

 quenching has for its object the rapid passage of the metal thr(»ugh 

 the change point S, and the prevention of the separation of the 

 single constituent austenite into the two constituents of pearlite. 



Coming now to the structures of pure y iron, y iron containing 

 dissolved carbon, pure /8 iron, and pure a iron, the following facts 

 have to be considered : — 



Pure carbonless y iron has never yet been obtained ; it is im- 

 possible by any quenching, however rapid, to bring down to the 



