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BELL SYSTEM TECHNICAL JOURNAL 



strains in a fabricated metal by annealing proceeds only slowly at low 

 temperatures (up to 600°C for most ferrous metals) without noticeable grain 

 growth or change in grain orientation, and is designated recovery. The prin- 

 ciple change is a reduction in the amplitude of internal strains, and this can 

 be followed quantitatively by X-ray measurements. Near the point of com- 

 plete relief distinct changes occur in both grain size and grain orientation, 

 and the material is said to recrystallize. At higher temperatures grain growth 

 increases more rapidly. The specific temperatures necessary for both re- 

 covery and recrystallization depend on the amount of previous deformation, 

 as shown in Fig. 33. Special orientations are also present in fabricated mate- 

 rials after recrystallization, and some of these are listed in Table VI and illus- 

 trated for nickel in Fig. 32 (b). 



As an example of the dependence of various magnetic properties on direc- 

 tion, Fig. 34 gives data of Dahl and Pawlek'^ for a 40 per cent nickel iron 



Table VI 

 Preferred Orientations in Drawn Wires and Rolled Sheets, Before and After Recrystalliza- 

 tion, and in Castings (Barrett^^) 

 The rolling plane and rolling direction, or wire axis, or direction of growth, are designated 



alloy reduced 98.5 per cent in area by cold rolling and then annealed at !■ 

 11(X)°C. After further cold rolling (50 per cent reduction) the properties 

 are as described in Fig. 35. 



The mechanical properties ordinarily desirable in practical materials are 

 those which facilitate fabrication. Mild steel is often considered as the 

 nearest approach to an ideal material in this respect. Silicon iron is limited 

 by its brittleness, which becomes of major importance at about 5 per cent 

 silicon; this is shown by the curve of Fig. 36. Permalloy is "tougher" than 

 iron or mild steel and requires more power in rolling and more frequent 

 annealing between passes when cold- rolled, but can be cold-worked to smaller 

 dimensions. If materials have insufficient stiffness or hardness, parts of 

 apparatus made from them must be handled with care to avoid bending 

 and consequent lowering of the permeability. If the hardness is too great 

 the material must be ground to size. This is the case with some permanent 

 magnets. 



" O. Dahl and F. Pawiek, Zeits. f. Metallkunde 28, 230-3 (1936). 



