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ALLOYS AND COMPOUNDS OF IRON]
345
MAGNETISM


Honda and Shimizu (loc. cit.) have determined the two critical temperatures for eleven nickel-steel ovoids, containing from 24-04 to 7O'32 'f-1, of nickel, under a magnetizing force of 400, and illustrated by an interesting series of curves, the gradual transformation of the magnetic properties as the percentage of nickel was decreased. They found that the hysteresis-loss, which at ordinary temperatures is very small, was increased in liquid air, the increase for the alloys containing less than 30 % of nickel being enormous. Steinmetz's formula applies only for very weak inductions when the alloys are at the ordinary temperature, but at the temperature of liquid air it becomes applicable through a wide range of inductions. According to C. E. Guillaume* the temperature at which the magnetic susceptibility of nickel-steel is recovered is lowered by the presence of chromium; a certain alloy containing chromium was not rendered magnetic even by immersion in liquid air. Experiments on the subject have also been made by E. Dumont' and F. Osmond.” 9. Artovs AND COMPOUNDS or IRON

In 1885 Hopkinson (Phil. Trans., 1885, 176, 455) employed his yoke- method to test the magnetic properties of thirty-five samples of iron and steel, among which were steels containing substantial proportions of manganese, silicon, chromium and tungsten. The results, together with the chemical analysis of each sample, are given in a table contained in this paper, some of them being also represented graphically. The most striking phenomenon which they bring into prominence is the effect of any considerable quantity of manganese in annihilating the magnetic property of iron. A sample of Hadfield's manufacture, containing 12-36% of manganese, differed hardly at all from a non-magnetic substance, its permeability being only 1-27. According to Hopkinson's calculation, this sample behaved as if QI % of the iron contained in it had completely lost its magnetic property* Another point to which attention is directed is the exceptionally great effect which hardening has upon the magnetic properties of chrome steel; one specimen had a coercive force of 9 when annealed, and of no less than 38 when oil hardened. The effect of the addition of tungsten in increasing the coercive force is very clearly shown; in two specimens containing respectively 3-44 and 2-35% of tungsten the coercive force was 64-5 and 70-7. These high values render hardened tungsten-steel particularly suitable for the manufacture of permanent magnets. Hopkinson (Proc. Roy. Soc., 1890, 48, 1) also noticed some peculiarities of an unexpected nature in the magnetic properties of the nickel-steel alloys already referred to. The permeability of the alloys containing from 1 to 4-7% of nickel, though less than that of good soft iron for magnetizing forces up to about 20 or 30, was greater for higher forces, the induction reached in a field of 240 being nearly 21,700. The induction for considerable forces was found to be greater in a steel containing 73% of nickel than in one with only 33 %, though the permeability of pure nickel is much less than that of iron.

The magnetic qualities of various alloys of iron have been submitted to a very complete examination by W. F. Barrett, W. Brown and R. A. Hadfield (Trans. Roy. Dub. Soc., 1900, 7, 67; Journ. Inst. Elec. Eng., 1902, 31, 674).5 More than fifty different specimens were tested, most of which contained a known proportion of manganese, nickel, tungsten, aluminium, chromium, copper or silicon; in some samples two of the substances named were present. Of the very numerous results published, a few of the most characteristic are collected in the following table. The first column contains the symbols of the various elements which were added to the iron, and the second the percentage proportion in which each element was present; the sample containing 0-03 % of carbon was a specimen of the best commercial iron, the values obtained for it being given for comparison. All the metals were annealed.

A few among several interesting points should be specially noticed. The addition of I5'2 % of manganese produced an enormous effect 1 C.R., 1897, 124, 176 and 1515; 1897, 125, 235; 1898, 126, 738. 2 Ibid., 1898, 126, 741.

3 Ibid., 1899, 128, 304 and 1395.

See also ] Hopkinson, Journ. Inst. Elect. Eng., 1890, 19, 20, and J. A. Ewing, Phil. Trans., 1889, 180, 239. 5 Many of the figures which, through an error, were inaccurately Stated in the first paper are corrected in the second. upon the magnetism of iron, while the presence of only 2-25 % was comparatively unimportant. When nickel was added to the iron in increasing quantities the coercive force increased until the proportion of nickel reached 2o%; then it diminished, and when the proportion of nickel was 32 % the coercive force had fallen to the exceedingly low value of 0-5. In'the case of iron containing 7-5 %Of tungsten B B /ti Coercive

Element' Per Cent' for H =45. residual. for H = 8. Force. C O-03 168O0 9770 1625 1-66

Cu 2-5 14300 10410 5-4

Mn 2-25 14720 10460 IOSO 6-0

Mn 15-2 O . . .

Ni 3-82 16190 9320 1375 2-76

Ni 19-64 7770 4770 90 20-0

Ni 31-4 4460 1720 357 O-5

W 7-5 15230 13280 500 9-02

- Al 2'25 16900 10500 1700 1-0

Cr 3-25 . 12-25

Si 2- 5 16420 4080 168O 0-9

Si 5-5 15980 3430 1630 O-85

(W), the residual induction had a remarkably high value; the coercive force, however, was not very great. The addition of silicon in small quantities considerably diminished permeability and increased coercive force; but when the proportion amounted to 2-5 % the maximum permeability (u =5IO0 for H =2) was greater than that of the nearly pure iron used for comparison, while the coercive force was only 0'9.6 A small percentage of aluminium produced still higher permeability (;.¢=6000 for H =2), the induction in fields up to 60 being greater than in any other known substance, and the hysteresis-loss for moderate limits of B far less than in the purest commercial iron. Certain non-magnetizable alloys of nickel, chromium-nickel and chromium-manganese were rendered magnetizable by annealing. Later papers7 give the results of a more minute examination of those specimens which were remarkable for very low and very high permeabilities, and were therefore likely to be of commercial importance. The following table gives the exact composition of some alloys which were found to be non-magnetizable, or nearly so, in a field Of 320.

An. = Annealed. Un. = Unannealed.

State. Percentage Composition. I, for H =320. Un. Fe, 85-77; C, 1-23; Mn, 13.

An. Fe, 84'64; C, 0-15; Mn, 15-2

An. Fe, 80-16; C,0~8; Mn, 5-04; Ni, 14-55. Un. Ditto

Un. Fe, 75-36; C, 0-6; Mn, 5-04; Ni, 19.

An. Fe, 86-61; C, I-o8;Mn, 10-2;W, 2-11.

A very small difference in the constitution often produces a remarkable effect upon the magnetic quality, and it unfortunately happens that those alloys which are hardest magnetically are generally also hardest mechanically and extremely difficult to work; they might however be used rolled or as castings. The specimens distinguished by unusually high permeability were constituted as follows:- Silicon-iron.-Fe, 97-3; C, 0-2; Si, 2-5.

Aluminium-iron.-Fe, 97-33; C, 0-18; Al, 2-25. The silicon-iron had, in fields up to about IO, a greater permeability than a sample of the best Swedish charcoal-iron, and its hysteresis loss for max. B =9000, at a frequency of 100 per second, was only 0-254 watt per pound, as compared with o-382 for the Swedish iron. The aluminium-iron attained its greatest permeability in a field of O-5, about that of the earth's force, when its value was 9000, this being more than twice the maximum permeability of the Swedish iron. Its hysteresis-loss for B =9000 was 0-236 per pound. It was, however, found that the behaviour of this alloy was in part due to a layer of pure iron (“ ferrite ”) averaging 0-I mm. in thickness, which occurred on the outside of the specimen, and the exceptional magnetic quality which has been claimed for aluminium-iron cannot yet be regarded as established.

A number of iron alloys have been examined by Mme. Curie (Bull. Soc. d'Encouragement, 1898, pp. 36-76), chiefly with the object of determining their suitability for the construction of permanent magnets. Her tests appear to show that molybdenum is even more effective than tungsten in augmenting the coercive force, the highest values observed being 70 to 74 for tungsten steel, and 80 to 85 for steel containing 3- 5 to 4% of molybdenum. For additional information regarding the composition and qualities of permanent magnet steels reference may be made ° The marked effect of silicon in increasing the permeability of cast6iron has also been noticed by F. C. Caldwell, Elect. World, 1898, 2, 1 .

3 7 Trims. Roy. Dub. Soc., 1902-4, 8, 1 and 123.