512 



Professor W. C. Eoherts- Austen 



[Feb. 6, 



of the property steel possesses, of becoming hard when it is quenched 

 from redness in a fluid which will abstract its heat with more or less 

 rapidity. 



The changes which take place at 855^ and 650° have to be 

 arrested, as it were, by rapidly cooling the mass of steel ; and if this 

 is done, the steel will be more or less hard according to the rapidity 



Fi«. .'). 



SCREEN 



with which the progress of the molecular change has been stopped. 

 It is, however, useless to attempt to harden steel if the temperature 

 of the mass has fallen below 650°. In " oil hardening " or cooling 

 a large mass of steel, like the " A " tube of a gun, which may be 30 

 feet long, great care should be taken to insure that the temperature 

 of the mass is as uniform as possible ; for, if part of the mass is 

 hotter than 650°, while part is colder, the oil will really be cooling 

 a mass of steel which is itself passing through various stages of 

 complex molecular change, and the operation of *' hardening " arrests, 

 as it were, the atoms in the midst of a conflict incidental to their 

 attempt to group themselves into one or other of the molecular 

 modilications of iron. By cooling a mass of steel which is not at 

 uniform temperature, stresses of great complexity and intensity are 

 set up, stresses that may greatly reduce the effective strength of the 

 gun.* The result is told in failures, by which many lives have been 

 sacrificed ; but I need hardly say that the Director-General of Ord- 

 nance is fully sensible of the national importance of studying the 

 behaviour of iron and steel at high temperatures, and, at Dr. Ander- 

 son's suggestion, the Institution of Mechanical Engineers appointed 

 a Committee, and have intrusted me with a large portion of the 

 inquiry. 



♦ * Internal Streesos in Cast Iron and Steel,' by Nicholas Kalakouteky, 1888. 



