M. M. Mikhailov and N. A. Balakin.

Hardening of Steel in a Magnetic Flux

Although the question of the hardening of steel has long since left the realm of conjectural experiments and has received a strictly scientific basis for the majority of the phenomena occurring in this process, in everyday practice the hardening of objects of great length and with a small moment of inertia of the cross-section continues to present great difficulties, gives a high percentage of rejects, and deprives one of firm confidence in the success of hardening because of severe warping in the plane of the least moment of inertia. A whole series of measures aimed at reducing the nonuniform distribution of internal stresses in the material, or subsequent straightening, which is usually practiced, for example, in the hardening of locksmith’s saws, do not give sufficiently reliable results and may be regarded only as palliatives, not as a real solution of the problem. Certain considerations concerning the electromagnetic structure of matter, the disappearance in iron of the ability to become magnetized during the transition of iron $\alpha$ into iron $\beta$, the appearance of magnetic flux under tension, etc., suggesting new paths and methods of hardening, prompted the authors of the present work to attempt to reduce warping in the hardening of thin steel plates by placing them in an artificially created magnetic flux at the very moment of hardening.

As material for the investigation, 12 specimens were taken, cut from a single piece of band-saw steel, with approximate dimensions: $50^{?}/_{?}$ length, $15^{?}/_{?}$ width, and $1^{1/2?}/_{?}$ thickness. To avoid nonuniform distribution of internal stresses in the material, the specimens were first annealed in a Heraeus furnace by heating to $900^\circ\mathrm{C}$ and then slowly cooled to room temperature. After cooling, the specimens were carefully ground and checked on a surface plate. As a result, the specimens taken for testing had the dimensions given in Table I.

Table I.

All specimens were divided equally into two groups, the first 6 pieces—Group I—being intended for hardening in pure, distilled-

water, and the latter, Nos. 7–12, group II, for hardening in distilled water and in a magnetic flux. The temperature of the water in both cases was maintained constant and equal to \(20^\circ\)C. The heating temperature of the specimens was taken as \(1000^\circ\)C. Heating was carried out in a Heraeus electric furnace; the temperature was measured with a Le Chatelier pyrometer and a corresponding Siemens and Halske galvanometer. The temperature of the water was controlled by a mercury thermometer, with an accuracy of up to \(0.1^\circ\)C. For greater assurance of the constancy of the maximum temperature at which hardening was carried out, all specimens were held in the electric furnace at the given temperature of \(1000^\circ\)C for 10 minutes each. The magnetic field was produced by means of a two-coil electromagnet fed with direct current, 110 V, 8 amp. The number of turns of the coil was 2045; the diameter of the iron core was 50 mm, the height 305 mm; the distance between the centers of the coils was 120 mm, and the cross-sectional area of the connecting iron bar was \(85 \times 20\) mm. A vessel of yellow sheet copper, 1 mm thick, with distilled water for hardening was placed directly under the electromagnet. The heated specimens were quickly removed from the furnace with special tongs and immersed in the water with the long, narrow side; then, after cooling, they were transferred to a surface plate for measuring the resulting maximum deflection arrows in the plane of the least moment of inertia.

The specimens hardened in distilled water gave the following results:

Table II.

Hence the mean deflection arrow for specimens Nos. 1 through 6 inclusive will be equal to

\[ f_{1-6} = 1.12\text{ mm} \]

Specimens that were hardened in distilled water under the action of a magnetic flux, with all other external conditions kept the same, give, analogously:

Table III.

With the mean deflection arrow

\[ f_{1-12} = 0.45\text{ mm} \]

The specimens hardened in the magnetic flux, just as in the first case during hardening in distilled water alone, were immersed in the water with the long, narrow side \(AB\), with the intention that it should coincide with the direction of the magnetic flux \(S\ N\) (Fig. 1).

A whole series of external technical difficulties (for example, the impossibility of creating, with the means available, a more powerful magnetic flux, the great expense and laboriousness of preparing test specimens) did not allow the authors to develop their experiments to the desired extent, and compelled them to confine themselves only to the conclusions from the observations made, thereby merely marking out the first milestones along which, undoubtedly in the very near future, the study of steel hardening must move, both from the point of view of a correct understanding of the very nature of the internal transformations, and from considerations of a purely practical character—the development of a method for hardening long and thin plates and, in general, profiles subject to severe warping. Leaving the final word to future investigators of this question, when the influence of the strength of the magnetic flux will be precisely taken into account quantitatively, in the sense of determining the most favorable conditions for its paralyzing action on warping, at the present time the authors consider it possible to accept the following propositions:

Proposition I. Infinitely small elementary particles (electrons), during the hardening of steel, are in a state that permits the action upon them of an external magnetic field.

Proposition II. An external magnetic field may be used in the hardening of steel as a means that diminishes (and may even entirely eliminate) the warping of parts with sharp variations in the moments of inertia of their sections with respect to different axes.

For greater assurance that the results obtained do not depend on the structure of the steel, two microsections were prepared: one, hardened in distilled water, with the greatest deflection of warping, taken from specimen No. 4; the other, on the contrary, with the smallest deflection of warping, hardened in distilled water and in the magnetic flux, from specimen No. 7. Under a magnification of 1000 times and upon careful examination of the microsection, it was not possible to detect any features or differences in the structure of the steel, which was to be expected, taking into account that one or another orientation of infinitesimally small particles—molecules or even electrons—lies far beyond the limits attainable by modern microscopy.

In parallel with determining the influence of the magnetic field on the warping of steel during hardening, the temporary resistance limit and the hardness were determined.

The tensile test was carried out on an Amsler Laffon machine and gave, for:

a) specimens hardened in distilled water,

\[ k_{z_1}=69.2\ \mathrm{kg/mm^2} \]

b) and hardened in water and in the magnetic flux,

\[ k_{z_2}=71.1\ \mathrm{kg/mm^2} \]

The material used consisted of specially prepared specimens (3 pieces each), hardened under the same conditions as specimens Nos. 1–12.

The hardness test was performed on the same specimens that had been taken for determining warping; moreover, for the sake of the greatest uniformity, a special standard \(K\) was prepared (see drawing 2).

Hardness was determined with a Shore scleroscope; the blows were delivered by a striker with a diamond tip.

On each side of the plate, 18 observations were made. The hardness-determination locations, as can be seen from the sketch of standard \(K\), were the intersection points of a 5-millimeter grid.

In view of the insignificant transverse dimensions of the plates, their fastening during the hardness test was carried out by embedding the broad side (flatwise) in sealing wax. The mean values of all 36 determinations for each specimen are given in Table IV.

Table IV.

Thus the average hardness of specimens Nos. 1–6, hardened in water, was equal to \(\tau_{1-6}=46.2\) on Shore’s scale when tested with a diamond tip, and \(\tau_{7-12}=47.0\) for the specimens hardened in water and a magnetic flux.

The results obtained in determining the ultimate tensile strength and hardness, although they do not show particularly sharp differences in mechanical properties when hardening in a magnetic flux and without it, nevertheless do not contradict propositions I and II put forward by the authors. It is quite possible to suppose that the difference between \(K_{z_1}\) and \(K_{z_2}\), on the one hand, and \(\tau_{1-6}\) and \(\tau_{7-12}\), on the other, will be more noticeable with an increase in the power of the magnetic flux, and will make it possible to say something new in this direction.

To the article by M. M. Mikhailov and N. A. Balenin, “Hardening of Steel in a Magnetic Flux”

Fig. 1.

[Diagram labels:]

• steel
• \(t = 1000^\circ\mathrm{C}\)
• \(\mathrm{H_2O}\)
• \(t = 180^\circ\)
• \(A\)
• \(B\)
• \(S\)

Fig. 2.

(template K)

[Diagram labels and dimensions:]

• \(60\ \mathrm{mm}\)
• \(5\)
• \(5\)
• \(3\)
• \(3\)
• positions \(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18\)