THE EFFECT OF BORON ON THE HARDENABILITY OF BALL-BEARING STEEL ShKh-15

N. D. TYUTEVA

(Presented by Prof. A. N. Dobrovidov, Doctor of Technical Sciences)

To determine the change in the hardenability of ShKh-15 steel under the influence of boron, the author cast several heats of this steel.

The steel was melted in a high-frequency furnace. The weight of a heat was 3.5 kg. A heat was carried out in one crucible as follows: it was deoxidized with aluminum (0.1%) and half of the heat was poured, without boron, into a mold. The remaining part of the heat was modified with boron in the form of ferroboron. Thousandths of a percent of boron were added. The heat with the ferroboron addition was held for 1 minute before pouring. Since it was intended to study the effect of boron on forged ShKh-15 steel, the steel ingots were forged. During forging the cross-section of the ingot was reduced 16-fold. After forging, the steel bars were annealed according to the schedule for cast ShKh-15 steel.

The heat treatment of a specimen before testing for hardenability by the end-quench method consisted of the following: heating to a temperature of 1100°, holding for 2 hours, then the box with the specimens was quickly transferred into a furnace heated to 850°. During heating the specimens were protected by being packed with cast-iron chips in a metal box. The holding time at a temperature of 850° was 45 minutes. During this time the specimens reached a temperature of 850°. Then end quenching followed. As is known, during heating to high temperatures the grain grows. Grain growth may increase the hardenability of the steel independently of the influence of boron. To eliminate the influence of this factor, heating to 1100° was introduced for specimens both with boron and without boron.

Experiments show that, when heated to 1100°, the austenite grain of steel with boron and without boron grows identically [4]. This is seen in Fig. 1.

In the apparatus for end quenching, the distance from the nozzle to the end of the specimen is 25 mm. The height of the jet is 60 mm. The addition of thousandths of a percent of boron increases the hardenability of ShKh-15 steel.

Fig. 2 shows how the hardness (Vickers) changed on standard specimens for end quenching. The results of hardness measurements show the nature of the change in hardness when quenching steel with boron and without boron.

The study of the influence of boron on the hardenability of structural steels has been the subject of a number of works [1, 2, 3].

Fig. 1

Fig. 2

In some works [2] there are indications that boron increases the hardenability of steel with an elevated carbon content, for example, 1.2%, less strongly. When the carbon content is increased from 0.80 to 1.2%, the effect of boron on hardenability decreases (Fig. 3).

Fig. 3

In the work published by Z. L. Regirer [1], boron was introduced in the form of the complex alloy “grainal.” Data are given on the amount of alloying elements that can replace 0.0025–0.003% boron.

Content of the alloying element in steel replacing 0.025–0.0030% boron

For example, in steel of composition: 0.18–0.23% C; 0.7–0.9% Mn; 0.20–0.35% Si; 0.4–0.7% Ni; 0.15–0.25% Mo and 0.40–0.60% Cr, 0.0025–0.0030% boron replaces 0.27% Ni; 0.16% Cr and 0.1% Mo.

The author arrives at the logical conclusion that the amount of boron introduced and of the alloying elements replaced by it varies depending on the nature and degree of alloying of the steel. Some authors [3] come to the conclusion that, in terms of its effect on hardenability, boron can replace chromium and manganese in low-alloy structural steels. The authors state that, thanks to the addition of boron, the hardenability of steel

shifts toward the upper boundary of the region defined for the steel under consideration. The properties of the steel as a whole remain unchanged. When comparing heats of different compositions, it may be concluded that the effect of boron additions (\(\sim 0.001\%\)) corresponds to a manganese content of approximately \(0.25\text{--}0.30\%\). From consideration of the hardenability curve of steel containing \(0.41\%\) C; \(1.11\%\) Si; \(1.47\%\) Mn and \(0.48\%\) Cr, it follows that \(0.48\%\) Cr has a more significant effect than the boron introduced into the steel (\(0.001\%\)) (Fig. 4).

Fig. 4

The influence of the latter corresponds to an addition of \(0.25\%\) chromium. Kreiner and Kroneis [3], by the results of their work, confirm the positive influence of small additions of boron.

The action of boron additions consists in the fact that boron delays the beginning of phase transformations in steel, without noticeably affecting the points of the final transformations. The influence of a boron addition on the properties of thermally improved structural steels is similar to the action of chromium or manganese in an amount of approximately \(0.3\%\) [3].

Conclusion

Boron increases the hardenability of steel with a carbon content above \(0.8\%\) (steel ShKh 15). It is natural to consider replacing part of the chromium in ShKh 15 steel with boron.

References

1. Regirer Z. L. Influence of boron-containing additions on the properties of steel. Proceedings of the Central Scientific Research Institute of NKTP and the Tank Industry, No. 1 (21), 1945.

2. Wray P. R., Boron Steels for Constructional Parts and for Carburizing. Metal Progress, July, pp. 52--55, 1951.

3. Kreiner H. Kroneis M., Stahl und Eisen, 72 (26), 1652--1657, 1952.

4. Potaszkin R. et Jaspart. M., Revue de Metallurgie, May, No. 5, p. 406, 1952.