32 BELL SYSTEM TECHNICAL JOURNAL 



gauss. The 1 per cent chrome steel has the advantage over the carbon- 

 manganese steel in that the desirable characteristics are produced by 

 oil quenching while the carbon-manganese steel is ordinarily water- 

 hardened. Water-hardening frequently results in cracked magnets 

 and consequently a higher proportion of rejections. 



Of slightly higher quality are the 3.5 per cent chrome and the 5 per 

 cent tungsten steels.* Typical values of coercive force are from 60 to 

 70 oersteds, and of residual induction from 9500 to 10,500 gauss. In 

 general, tungsten steel in the hardened condition has a higher residual 

 induction than the other magnet steels. Tungsten steel requires 

 water-quenching, while 3.5 chrome steel is oil-quenched. In addition, 

 chrome steel is a somewhat lower cost material. For these reasons, in 

 recent years it has been substituted to some extent in applications 

 where tungsten steel was formerly used. 



The best permanent magnet steel in commercial use is the 35 per cent 

 cobalt steel,^^ a complex alloy which contains, in addition to the cobalt, 

 tungsten, chromium and carbon. Typical values of coercive force and 

 residual induction for this material are 220 oersteds and 9500 gauss. 

 Although this material is decidedly superior in properties to the other 

 magnet steels, because of high cost, its use is limited to applications 

 where space curtailment and apparatus requirements eliminate the 

 cheaper steels. 



New Developments in Permanent Magnet Alloys 

 Within the last five years, there have been a number of publica- 

 tions ^°' ^^ describing new materials which have properties of interest to 

 engineers using permanent magnets. These materials are alloys with 

 no intentional carbon additions, and, hence, are a radical departure in 

 this field. The new magnet alloys solidify as alpha-solid solutions 

 which by suitable heat treatment at a lower temperature decompose 

 precipitating second phases. Contrary to the case with the iron- 

 carbon alloys, the alpha-solid solution undergoes no phase change with 

 decreasing temperature. Consequently the alloys have a coarse grain 

 while the hardened magnet steels have a fine grained structure due to 

 the intermediate phase change. 



The permanent magnet qualities, however, result from the same 

 type of metallurgical reaction that occurs in the carbon steels, that is, a 

 precipitation of a second phase which is dispersed throughout the 

 alpha-solid solution. The useful properties are secured by the usual 

 precipitation hardening treatments; either a quench from a high 



*A commendable detailed discussion of tungsten magnet steels is given by 

 Gregg in the recent byyk, Alloys of Iron and Tungsten, p, 212, McGraw Hill, 1934. 



