MAGNETIC MATERIALS IN RELATION TO STRUCTURE 



11 



strains in the lattice, and are believed, on this account, to be par- 

 ticularly harmful to the magnetic softness. 



The solubilities of the interstitial elements at room temperature have 

 been determined by a number of investigators. The most reliable 

 values, which, because of the difficulties involved, probably should be 

 considered tentative, place the room temperature solubility of carbon 

 at approximately 0.008 per cent;^ the solubility of oxygen at 0.01 per 

 cent; * of nitrogen at 0.015 per cent; ^ and of sulfur at 0.015 per cent.^ 



The solubilities of the interstitial elements vary with the tempera- 

 ture, increasing, in general, at higher temperatures. Varying solu- 

 bility provides the possibility of precipitation hardening, which, in the 

 case of iron, results in magnetic hardening. It is this latter type of 



Interstitial Elements 



Carbon 

 Oxygen 

 Nitrogen 

 Sulfur 



Iron Purification 



-+ + -H- 



1 l£_ 



Substitutional Elements 



Nickel 

 Cobalt 

 Silicon 

 Manganese 



a. Iron 

 Elimination of Interstitial Elements 



1. Addition of Sub- 

 stitutional Element 



C (Sol. in Iron) + Si - 

 C (Graphite) + Fe-Si 

 O + Si -* SiOj 



2. Vacuum Melting 



3. Hydrogen Treatment 



C +0 

 FexNy 



CO 



Fe + N2 



Fig. 2 — Schematic representation of methods of purifying iron for magnetic 

 purposes. The lattice structure shown is that for a-iron. The iron atoms are 

 located at the cube corners and at the center and are represented by the circles shown. 



hardening to which the magnetic aging of iron is attributed. This is 

 possible because the supersaturated phase will in most of these systems 

 precipitate the interstitial element at room temperature in a form 

 which produces severe strains with deleterious effects on magnetic 

 softness. Because of their harmful efTects, the purification of iron for 

 soft magnetic purposes has been in the direction of eliminating these 

 interstitial elements. The three methods of purification which have 

 been used are illustrated in the chart shown in Fig. 2. 



The earliest method used was the addition in the molten state of a 

 substitutional element which diminishes the harmful effect of inter- 

 stitial elements in the iron lattice. Silicon* is an element beneficial in 



* Messrs. Barrett, Brown, and Hadfield ' reported the magnetic properties of 

 iron-silicon alloys in 1900. More recent studies have been reported by Yensen.^ 



