THE STRUCTURE OF METALS 



THE INFLUENCE OF MECHANICAL TREATMENT 



ON STRUCTURE 



By CECIL H. DESCH, D.Sc, Ph.D. 



The microscopic structures described in the former article 1 were 

 those of cast metals and of worked metals which had been 

 sufficiently annealed to cancel the effects produced by the 

 mechanical treatment to which they had been subjected. The 

 mechanical treatment, such as forging, rolling, pressing or 

 wire-drawing, to which metals are usually subjected influences 

 in a most important manner the microscopic structure as well 

 as the mechanical properties of the metal ; as numerous 

 relationships between these properties and the structure have 

 been established, the examination of worked metals is a highly 

 important branch of the metallographer's activity. The subject 

 offers a wide field for future research, on account of the diversity 

 of mechanical conditions that come under consideration and the 

 minute and elusive character of some of the internal structural 

 changes to which they give rise. 



One of the chief factors in determining structure is the tem- 

 perature at which the change of form of a metal by mechanical 

 means, such as rolling, is conducted. A mass of metal that is 

 forged or rolled at a bright red heat and then allowed to coolslowly 

 assumes a structure which is essentially that corresponding with 

 the annealed condition ; it may differ in several respects from 

 that of the same metal as cast, the difference being in the 

 arrangement of the micrographic constituents, however, not in 

 their nature or proportions. More rapid cooling may, of course, 

 disturb this equilibrium, as in the case of a cast alloy. On the 

 other hand, when the rolling or forging is carried out at a 

 considerably lower temperature, readjustment of the crystalline 

 structure may be impossible; the cold metal then exhibits 

 unmistakable evidence of the treatment to which it has been 



1 Science Progress, No. 25, p. 87. 

 194 



