66 MODERN SCIENCE OF METALS 



a coarse structure. Fortunately it is possible to 

 carry the matter very much, further. The com- 

 bined use of the microscope and the pyrometer 

 in the study of metals has revealed the fact that 

 the structure of a given piece particularly in the 

 case of steel can be readily varied by the effects 

 of mechanical and thermal treatment; the whole 

 scale and character of the structure can be altered 

 by such means as heating to specific temperatures 

 followed by cooling at a specific rate. In this 

 way, it is possible specially to prepare our metals 

 not only to resist fatigue but also to meet many 

 other requirements. Once this has been under- 

 stood that the internal structure of metal governs 

 its physical properties, and that the internal 

 structure can be modified and controlled by methods 

 which the New Metallurgy has enabled us to 

 specify, we have the key to one of the most impor- 

 tant of modern metallurgical developments. In 

 iron and steel and more particularly in those 

 special steels, containing nickel, chromium and 

 vanadium, which are so largely used in the pro- 

 duction of guns, armour plate, and many other 

 forms of modern equipment, both military and 

 civil, this matter of heat-treatment has become 

 of paramount importance. The best and most 

 expensive types of steel which are employed for 

 such purposes become useless if wrongly treated 

 if heated or cooled or quenched from the wrong 

 temperature or if subsequently re-heated or "tern- 



