Hardness of Nickel

J. B. Dumas observed that the hardness of nickel is such that it is scratched by glass of hardness 2.6 on Mohs' scale; S. Bottone gave for the cutting hardness 1410 when that of cobalt is 1450, iron, 1375, and copper, 1360; H. Copaux found a hardness of 3.5 on Mohs' scale; and T. Turner reported the hardness of nickel to be 1410 when that of cobalt is 1450; that of copper, 1360; that of the diamond, 3010; and that of iron is 1375. L. Thompson stated that the purified metal which he prepared was as soft as copper. F. Robin found the penetrative hardness of nickel to be 105 to 190, and, when annealed, 130 to 150. The value for copper is 52 to 54. M. Waehlert gave 56 for the Brinell's hardness of nickel; C. A. Edwards, 144.0; and A. Krupkowsky, 59.1. W. B. Price and P. Davidson obtained for BrinelFs hardness of bars which had been cold-rolled from 0.5 in. to 0.134 in.:

	As rolled	Annealed at
		250°	350°	450°	550°
Hardness	235	262	255	248	228

L. Jordan and W. H. Swanger gave for the scleroscope hardness of 99.94 per cent, nickel, 5.0; for the Rockwell hardness, 42 to 44; and for Brinell's hardness, 68 to 78. They also found for cold-worked, and nickel annealed 30 min. at different temp.:

	Cold worked	Annealed for 30 min
		650°	750°	850°	950°
Rockwell	63-68	52-59	51-59	35-52	15-18
Scleroscope	12-19	8-13	8-13	7-11	4-6

Brinell's Hardness of Nickel annealed at Different Temperatures.

The results of F. Sauerwald and co-workers on the impact hardness of nickel are summarized in Fig. Measurements were also made by H. Pecheux, T. Kawai, W. A. Mudge and L. W. Luff. D. J. MacNaughtan and A. W. Hothersall found the Brinell's hardness of electrodeposited nickel is increased from 162 to 250 by additions of sodium sulphate, and the substitution of potassium chloride by sodium fluoride in the bath of nickel sulphate, boric acid, and potassium chloride. M. Guichard and co-workers found the hardness of electrodeposits to vary from 155 to 420; D. J. MacNaughtan studied the porosity of the deposits. F. Sauerwald observed that the drop hardness decreased slowly between 20° and 300°, then more rapidly between 300° and 400°, increased slightly at 450°, decreased rapidly between 600° and 700°, increased slightly at 800°, and then decreased on to 914°. The decrease between 300° and 400° is attributed to the magnetic transformation; and the maximum at 450° is ascribed to blue-brittleness similar to that of iron. L. Guillet, and J. Cournot and S. Silva measured the hardness and fragility of nickel at temp, ranging from 20° to - 190°. T. K. Rose found that the metal hardened by cold-work is softened by annealing between 530° and 700°. Measurements were also made by A. Kurth. According to K. Ito, the Brinell's hardness, H, of nickel is:

	-43°	-18°	0°	35°	51°	65°	82°	109°	125°	148°
H	91.5	89.4	89.0	87.4	86.6	87.0	86.6	86.0	84.8	84.0

and the relation can be represented by logH₂ - H₁ = a(2 - 1), where the temp, coeff. A=0.00015. The temp, coeff. of the hardness a and the m.p. T_m on the absolute scale of a number of metals are related by T_m(a+0.00145) = 2.5 - vide iron. F. Sauerwald and E. Janichen studied the adhesion of the compressed powder. O. Schwarz found for the percentage reduction by rolling, the Brinell's hardness:

Reduction	0	6.0	9.1	20.8	39.5	60.2	80.0	88.0 per cent.
Hardness	42.0	102.8	116.0	156.1/td>	192.8	216.0	231.2	236.0

Observations were also made by G. Tammann, B. Bogitch, F. Sauerwald and K. Knehans, F. Krau, H. Schottky and H. Jungbluth, H. O'Neill, G. Wazau, H. J. Tapsell and J. Bradley, F. P. Romanoff, M. Guichard and co-workers, Kroll, and W. Kroll. D. H. Browne and J. F. Thompson found that the effects of varying physical and chemical conditions on the Brinnell's and Shore's hardness of nickel are shown by the following tests for nickel bars hot-rolled, and afterwards cold-wiled to the reduction stated:

		0.06 % C		0.08 % C		0.09 % C		0.26 % C
		Brinell	Shore	Brinell	Shore	Brinell	Shore	Brinell	Shore
Hot-rolled		99	13	103	15	112	13	137	14
Reduction by cold rolling	25%	170	28	170	28	187	28	241	34
	50%	196	28	212	29	217	33	293	39
	66%	217	33	217	33	235	35	302	44
	75%	228	35	223	35	235	35	332	40
Annealed at 900°		89	9	92	9	92	9	99	11

D. H. Browne and J. F. Thompson found for the Brinell's hardness of nickel with different proportions of carbon:

Carbon	0.06	0.08	0.09	0.26 per cent.
Annealed at 900°	89	92	92	99
Hot-rolled - 75 per cent, reduction.	99	103	112	137
Cold-rolled - 75 per cent, reduction.	228	223	235	332

The Acoustic Properties of Nickel.

A. Mallock found the velocity of sound in nickel to be 49734 metres per second. G. W. Pierce studied the subject. J. Kleiber found that the velocity of sound in metals is proportional to the sq. root of the product of the sp. ht. and the linear coeff. of expansion. W. F. Barrett found that the conductivity of nickel for sound is 14.9 when that of air is unity. F. Robin studied the acoustic properties, and found that purified nickel exhibits little resonance. T. Gnesotto and L. A. Alberti observed breaks in the viscosity and rigidity curves of nickel at the Curie point. The curve, Fig., for forgeable annealed nickel shows that the duration of sound diminishes as the temp, rises to about 60°, after which the curve remains horizontal or falls very slowly up to about 150°. The resonance falls again up to 300°, after which, under the influence of the allotropic change, β-nickel, it rises up to 340°, subsequently falling again once for all, until, on reaching a red-heat, the sound is extinguished. The curves for stressed and annealed nickel are also shown in Fig. 3. Ordinary and somewhat impure nickel possesses fairly considerable resonance. As the result of interstrain the peculiarity exhibited in the curve at about 150° disappears, and the sinuosity of the curve at the peculiar point just noted is greatly diminished. Annealing restores to the metal its original properties, and the curve of the duration of the sound becomes parallel with that of pure annealed nickel.