1919] 



on The Hardening of Steel 



485 



acid in the iron, and cutlers do sfcrengtlien it with alcaly of animals " 

 ^hence the use of snails. 



Though the practice and theories in the periods I have mentioned 

 were frequently fantastic, these early workers were quite right in 

 attaching importance to the liquid in which the steel was to be 

 quenched. To-day much simpler methods are used in the best 

 modern practice. If severe quenching is required, cold water or 

 brine is used ; if a lesser degree of hardening is necessary, hot water 

 or oil ; while in many cases the processes of hardening and tempering 

 are combined in one operation by plunging the tool in a bath of 

 molten metal such as lead. 



The rapid changes of temperature which occur when a bar of 

 steel is quenched in a liquid from a high temperature have been 

 studied experimentally by Le Chatelier, Benedicks and Lejeune, and 



iP85M 



Secorvds. 



their results utilized by McCance in calculating the theoretical curve 

 of quenching. Le Chatelier found that water gave a more rapid rate 

 of cooling than any other liquid, and concluded that the most 

 important property in determining the quenching power of a liquid 

 was its specific heat. Benedicks, by means of an automatic quenching 

 and photographically recording apparatus, obtained the most accurate 

 results which are available as to the temperature at the centre of 

 small round bars quenched in water. On the whole his conclusions 

 corroborated those of Le Chatelier, but he showed that the latent 

 heat is the most important property in determining the quenching 

 power of a liquid under practical conditions. 



The appended cooling curve (Fig. 2) deduced by McCance from 

 Benedicks' results shows what happened when a bar of steel containing 

 1 per cent carbon, 2 inches long and ^ inch in diameter, was quenched 



