266 MAJOR A. E. OXLEY ON THE INFLUENCE OF MOLECULAR 



spontaneous magnetization is concerned, but the tenacity might even surpass that of 

 the chaotic -state since the transition ft y is accompanied by shrinkage. Such an 

 increase of tenacity was actually found by KOSENHAIN and HUMFREY.* 



The effects of prolonged heating on the plasticity of mild steel are interesting in 

 this connexion. EWING and RosENHAiNf have shown that the plasticity of a material 

 is caused by slips occurring on cleavage or " gliding " surfaces within each of the 

 crystalline grains, although the elementary portions of the crystals retain their 

 primitive form and the crystalline structure of the metal as a whole is preserved. In 

 the case of mild steel, exposure to a temperature of 1200 C. or higher temperatures 

 for several hours may cause the material to lose much of its plasticity, while some 

 specimens of soft iron after prolonged exposure at 700 C. to 800 C. (less than the 

 critical temperature) have been made brittle. These results are consistent with a 

 similarity of molecular configuration for temperatures above 1200 C. and below the 

 critical temperature (about 850 C.). On the other hand an exposure at 900 C. or 

 1000 C. (i.e., in the region of the paramagnetic state), followed by a slow or fairly 

 rapid cooling, induces considerable plasticity in the material, and this treatment may 

 even be used to remove brittleness originating from heating to the higher or lower 

 ranges of temperature mentioned above. As the plasticity is produced by slipping on 

 cleavage surfaces within the crystalline grains, this smalliiess of the grain structure 

 may, under stress, determine a molecular rotation. EwiNG and KOSENHAIN J have 

 shown that in some metals, in addition to slips or motions of pure translation, there 

 results a molecular rotation from strain which gives rise to twin-crystals. It is 

 interesting to note that the formation of twin-crystals is common in iron through the 

 y-range, but has not been observed in the ft and a ranges. 



As the temperature falls below 1400 C. there is some modification of the crystalline 

 cubic arrangement, resulting in a closer packing of the molecules, and accompanied by 

 an interlocking of the fine grains. Thus iron in the y-range (1400 C. to 900 C.) will 

 be paramagnetic. At a lower temperature than 900 C., this state is unstable and 

 another modification of the crystalline grouping occurs, accompanied by expansion, 

 thermal evolution, and the appearance of spontaneous magnetization. This latter effect 

 seems inconsistent with a more open packing of the molecules, but an analogy is found 

 in the case of water, where the molecular influence in the liquid just above C, is 

 small compared with that in ice just below freezing point, although the packing of 

 the molecules in the liquid state is closer than that in the crystalline state (see supra 

 p. 264). As the temperature is lowered the transformation progresses rapidly until 

 a point B is reached, after which the increase of magnetic quality is somewhat less 

 rapid. On continued cooling, the iron passes into the a-range where the magnetic 

 property is capable of attaining a saturation value. 



* 'Roy. Soc. Proc.,' A, vol. 83, 1909; 'Iron and Steel Institute,' No. 1, 1913. 



t ' Phil. Trans. Roy. Soc.,' A, vol. 193, p. 279. 



} See EWING, ' The Strength of Materials,' p. 47, 1906. 



