FACTORS AFFECTING MAGNETIC QUALITY 263 



trodeposition, and numerous operations such as punching, pressing and 

 spinning. In the commercial fabrication of ductile material it is common 

 practice to start the reduction in a breakdown or blooming mill (Fig. 13) 

 after heating the ingot to a high temperature (1200° to 1400°C). Large ingots, 

 of several tons weight, are often led to the mill before they have cooled 

 below the proper temperature. The reduction is continued as the metal 

 cools, in a rod or flat rolling mill, depending on the desired form of the final 

 product. When the thickness is decreased to 0.2 to 0.5 inch the material has 

 usually cooled below the recrystallization temperature. Because of the diffi- 

 culty in handhng hot sheets or rod of small thickness, they are rolled at or 

 near room temperature, with intermediate annealings if necessary to soften 

 or to develop the proper structure. In experimental work, rod is often 

 swaged instead of rolled. 



In recent years the outstanding trends in methods of fabricating materials 

 have been toward the construction of the multiple-roll rolling mill for roll- 

 ing thin strip, and the continuous strip mill for high-speed production on 

 a large scale. Figure 14 shows the principle of construction of a typical 4-high 

 mill ((a) and (b)), and of two special mills ((c) and (d)). In the 20-high 

 Rohn'^ mill and 12-high Sendzimir^ mill the two working rolls are quite 

 small (0.2 to one inch in diameter). These are each backed by two larger 

 rolls and these in turn by others as indicated. In the Rohn mill (c), power is 

 supplied to the two smallest rolls and the final bearing surfaces are at the 

 ends of the largest rolls. In the Sendzimir mill (d) the power is suppHed to 

 the rolls of intermediate size and the bearing surfaces are distributed along 

 the whole length of the largest rolls so that no appreciable bending of the 

 rolls occurs. The small rolls reduce the thickness of thin stock with great 

 efficiency, and the idling rolls permit the application of high pressure. 

 In the Steckel mill power is used to pull the sheet through the rolls, which 

 are usually 4-high with small working rolls. 



The continuous strip mill is an arrangement of individual mills such that 

 tlie strip is fed continuously from one to another and may be undergoing 

 reduction in thickness in several mills simultaneously. Figure 15 shows a 

 mill of this kind, used for cold reduction, with 6 individual mills in tandem. 



For magnetic testing numerous forms of specimens are required for vari- 

 ous kinds of tests; these include strips for standard tests for transformer 

 sheet, rings or parallelograms for conventional ballistic tests, "pancakes" 

 of thin tape spirally wound for measurement by alternating current, ellip- 

 soids for high field measurements, and many others. The various forms are 



■^ W. Rohn, Heraeus Vacuumschmelze, Albertis, Hanau, 381-7 (1933). 

 8T. Sendzimir, Iron and Steel Engr. 23, 53-9 (1946). 



