STATIC BENDING STRENGTH OF CAST HIGH-SPEED STEELS WITH COBALT

V. A. Kashchuk

(Presented by Prof. Dr. A. N. Dobrovidov)

With each passing year, cobalt high-speed steels are finding ever broader application in the tool industry of the Soviet Union.

According to a number of studies [1—3], cobalt high-speed steel should be used as a material for high-productivity cast cutting tools.

Since the literature contains no reports on the influence of cobalt on the strength of cast high-speed steel, it is of definite practical and scientific interest to carry out such a study.

The experimental steels were melted in a high-frequency induction furnace in a crucible with a capacity of 2 kg. The heats were deoxidized with aluminum, at the rate of 1 g of aluminum per 1 kg of melt. To refine the structure of the castings, titanium (0.22%) was introduced into the steel in the form of ferrotitanium. Casting was carried out on a centrifugal machine into metal molds. The specimens had the form of plates with a cross section of \(5 \times 22\) mm.

The chemical composition of the steels studied is given in Table 1.

Static bending tests of the cast steels under study were carried out on a Gagarin mechanical press under a load of 1000 kg. The distance between supports was 36 mm. The cast plates were cut with an abrasive wheel into specimens which, after grinding, had a cross section of \(4.5 \times 4.5\) mm and a length of 52 mm.

The bending strength was calculated by the formula:

\[ \sigma_{\text{bend}}=\frac{3Pl}{2b^3}, \]

where \(P\) is the load in kg;
\(l\) is the length of the specimen between supports in mm;
\(b\) is the side of the specimen cross section in mm.

Five specimens from each heat were tested. Since most of the steels, during cooling in the metal mold, underwent hardening, before testing the specimens were subjected only to threefold tempering for 1 hour at \(560^\circ\). This tempering regime was chosen by us for convenience of comparison, since it is widely used in industrial practice.

Table 1

* Content: Mn < 0.4%; Si < 0.4%; S < 0.03%; P < 0.03%.

Specimens from steels of heats 8 and 14 did not acquire full hardening during cooling in the metal mold and therefore were additionally hardened from 1290°, and then tempered. These specimens were tested immediately after tempering, and also after hardening and tempering. The test results are given in Table 2.

Table 2

Continuation of Table 2

The tests were carried out on specimens of nonstandard dimensions and had as their main purpose a comparison, in strength, of experimental steels of different chemical composition.

Conclusions

1. All the cast steels investigated have a lower strength than forged steel R18.

2. As the cobalt content increases, the strength of the steels decreases.

3. The steels of the second group possess the greatest strength; those of the third group have lower strength; those of the first group still lower; and those of the fourth group the lowest.

Literature

1. Stroeva K. M. Scientific Works of the Novocherkassk Polytechnic Institute, pp. 413–427, vol. 26, Yerevan, 1955.

2. Dobrovidov A. N., Rozenberg A. M., and Yakovlev G. M. Machines and Tooling, pp. 15–16, No. 5, 1947.

3. Toteva N. D. Proceedings of the Tomsk Polytechnic Institute, vol. 68, issue 1, pp. 29–37, 1951.