Thermal stabilization of austenite in nickel steels

466

.\CTA

METALLURGICA,

Thermal Stabilization of Austenite in Nickel Steels* It is known that in certain steels the amount of martensite obtained on quenching specimens to a fixed reference temperature is dependent on the rate of cooling from the austenitizing temperature. Morgan and Ko [l] showed that, in an 1.1% C, 5.4y0 Ni steel, the difference in the martensite contents of specimens quenched into brine and oil was due to thermal stabilization occurring in the region of 300”-400°C. Stabilization of the austenite occurred on isothermal holding at temperatures lower than this, both above and below IIJs, bu’t it was too slow at these temperatures to reach a detectable amount even during oil quenching. The experiments have been repeated in order to obtain more data. The specimens used were cut, after a time lapse of one year, from the bars forged from the same ingot. Austenitization at 1100°C was carried out in the same neutral bath used before, and also in vacua, the results being independent of the method of austenitization. All the other experimental details were identical with those described previously [I]. The present results, shown in Figure 1, were as follows: (i) Efect of cooling rate. The amounts of martensite in the specimens were independent of the quenching medium, whether brine, mercury, or oil. In other words, within this range, the cooling rate had no effect on the progress of transformation. (ii) Age&g above MS (87°C) Holding at 178°C and 214°C for various periods up to 32 hours had no effect on the amount of martensite at temperatures between iVs and 15°C. (iiij Age&g below Ms. Stabilization was observed on ageing below Ms. Holding for 24 hours at 52°C (31 per cent martensite) caused a temperature lag (0) of 16°C and a loss of 15 per cent martensite when the transformation resumed. These results are consistent with the previous finding [I] that any difference in the amount of retained austenite in this type of steel is associated with stabilization above Ms. But it is obvious that some factors were in operation which prevented the occurrence of stabilization above MS in the present specimens. In order to ascertain the effect of any possible variation in chemical composition, specimens from. the two batches were analyzed with the following results: *Received

May 19, 1953.

VOL.

1,

Element

C Ni N2 B Pb AI, Cr, Sb, Ti,

1953

Analysis

Bi, Co, Cu, MO, Sn, Te, V, W, Zn

Morgan SCKo’s specimens

Chemical Chemical Chemical Spectrographic Spectrographic Spectrographic

1.08% 5.4% 0.005% Faint trace Trace Not detected

Present specimens 1.080/, 5.4% 0.005% Nil Trace Not detected

As hydrogen is the only element likely to have been lost during the period between the previous and the present investigations, a possible aftereffect from this element was investigated using specimens cathodically impregnated with hydrogen. About two weeks were allowed to lapse Between the hydrogen impregnation and_ austenitization, a period comparable to that between machining and austenitization in the previous experiments [l]. No stabilization effect above MS was found after isothermal holding at 183°C for periods up to 24 hours. Scrutiny of the literature shows that similar anomalous results have been reported before. Klie? and Troiano [2] found stabilization above MS in a 0.7% C, 15y0 Cr steel, while Das Gupta and Lament [3] (using specimens from the same melt) and Bogacheva and Sadovskii [4] (using a 0.7970 C, 15.2$!& Cr steel) reported that no such effect l

0 A

A A

Stab&cd at 178'~ II 2140I3 " Direct Brine Quench 01 Mercury n OiI 8’ Is

Brine quenchmg according to Morgan & Ko (I) 60

I

Temperature FIGURE

100

50

0 1. Martensite

“C

transformation

curves.

coultl

lw ohservetl.

In a

a compo~i t ion similar

1.I $6 (‘, 5.3(i(,t,ij Xi steel,

to that

used in the present

[5j found no stabitizaion above room temperature (25°C). These authors also reported that their steel showed no difference in the amounts of retained austenite at 25°C‘ after cooling at different rates, although in a previous paper from the same laboratory, Howard and (‘ohen [G], using apparently the same steel, recorded a difference of 12 to 14 per cent in retained austenite between specimens quenched in oil and 10 per cent NaOH to room temperature (unspecified, probably 25’C according to’ the given data!. Cohen and his co-workers [7] also found that the austenite content of a 1.07, C, 3.82$& Ni steel oil quenched to room temperature was dependent on the specimen size. Jt must be concluded that there are some unknown factors which determine whether or not stabilization will take place above ~14s. No suggestion can be made at present as to the nature of these. The effect of boron on stabilization is now being studied. Thanks are due to Mr. H. Moriogh of The British fast Iron Research Association for the spectrogr~lphic analyses and Dr. E. Marks of Messrs. Richards Thomas Baldwins, Ltd., for the nitrogen analyses. T. Ko and B. EIIMONDSON invest~~~~~io~,

Harris

and

Cohen

Department of Metallurgy University of Birmingham Birmingham, England References 1. MORGAN, E. R. ajld Ko, T. Acta Met., 1 (1953) 36. 2. KLIER, E. P. and TROIANO. A. R. Trans. A.I.M.E., 162 (1945) 175. 3. DAS GuPT.~, S. C. and LAMENT, B. S. Trans. A.I.M.E., 191 (1951) 727. 4. BOG.~C~IW& G. ?\‘. and SADOVSKII, V. D. Dok. _qkad. Nauk, SSSR., 83 (1952) 569. 5. HARRIS, u’. J., JR. and COHEN, M. Trans. A.I.M.E., 180 (1949) 447. 6. HOWARD, R. T., JR. and COHEN, M. Trans. A.I.M.E., 176 (19%) 384. 7. .-~NTIA,D. P., FLETCHER, S. G. and COHEN, M. Trans. Amer. Sot. Metals, 32 (1944) 290.

Spin-Orbit

Coupling Effects in Ferromagnetic Metals*

The remarks made in a recent note by Argyres and Kittel [l) on the ferromagnetic ground state are relevant to some theoretical work on ferromag*Received

May 22, 1953.

The criticism

of 2.22

effective

in Ixtragnf&

magnetic

(.-\)

of the assumption

electr-011s per atom

for

appears at first sight to 1~ valid. Such values have been deduced from saturation magnetization data, assuming the effects to be due entire]!, to electron spin and unaffected l)~the orbital motion of the electrons. Owing to spin-orbit coupling, this assumption is not entircl!. correct, coupling effects being demonstrated b\~the deviations from two of the values of the gyromagnetic ratio g’ and the spectroscopic splitting factor g. The only treatment giving a clear physical picture of spin-orbit coupling effects, on the basis of the collective electron theory, is that due to Brooks [2]. a1 quantitative investigation of this treatment has recently been carried out bl- the author for nickel, using the results of the d bancl calculations for that metal by Fletcher and Q~ohlfarth [3] and Fletcher [4]. Brooks’ treatment has now been extended to include the effects of all five atomic d functions, but the additional computational complexities introduced thereby have necessitated the consideration of only three for quantitative purposes, as in Brooks’ original paper and in [3]. On thiy basis theoretical values of 2.22 and 1.82 have been obtained for g and g’ respectively. The calculations have also been extended to include second order terms. The vafue 2.22 for the g factor of nickel gives 0.54 for the number of effective magnetic electrons per atom, in agreement with Argyres and Kittel’s estimate based, presumably, on an experimental value of g. This change would have the effect of increasing the value of the low temperature electronic heat coefficient y calculated by Fletcher [4] to about 1.9 X 10e3 cal moP degU2, Ilnn' slightly larger than the experimental value for nickel. The calculated degeneracy temperature would be reduced correspondingly. 1\ direct comparison of the calculated g factor with values deduced from ferromagnetic resonance Bxperiments is not at present possible, since the experimental values derived in different cases vary over a considerable range, e.g. 2.19-2.42 for nickel. The calculated value of g’ for nickel is in disagreement with observation; it is not clear, incidentall~~, wh; the correction to the effective number of electrons should not rather be made using g’ in place of g. It may be pointed out, in commenting on the more fundamental aspects of the theor\-, that the iron and 0.6 for nickel