SHORT-TERM TEMPERING OF HIGH-SPEED STEEL

N. S. KILKOV, I. T. TIKHONOV, E. P. FILIMONOVA

(Presented by the scientific seminar of the departments of physical metallurgy, equipment and technology of heat treatment of metals; technology of metals; equipment and technology of welding production)

Conventional tempering of hardened high-speed steel (three heating cycles at 560°C with holding times of one hour each) considerably lengthens the overall heat-treatment cycle and substantially complicates its automation. Raising the tempering temperature makes it possible to shorten the holding times significantly. At the same time, there are concerns that accelerating the tempering process may lead to a deviation from the optimum conditions under which the steel acquires the required properties, and to a deterioration in the quality of cutting tools [1]. The question of the influence of accelerated tempering on the properties of tools made of high-speed steel has not yet been studied sufficiently fully, although the published materials provide encouraging data on the possibility of replacing conventional tempering with accelerated tempering [2, 3, 4, 5]. The present work was carried out at the suggestion of the Tomsk Tool Plant and consisted in determining the effect of short-term

Table 1

Material, heat treatment, and hardness of the tested cutters

tempers on the cutting properties of high-speed steels of grades P18, P12, P6M3F2, and P9K5.

During tempering of hardened high-speed steel, retained austenite decomposes and dispersion hardening of the martensite occurs, with the formation of secondary hardness. To verify the results of tempering high-speed steel, magnetic control methods are, as is known, widely used. It was therefore natural to assume that accelerated tempering schedules with elevated temperatures should be selected so that, after them, the steel would acquire the same magnetic properties as after ordinary tempering. We have called such short-term tempering schedules equivalent to ordinary tempering [4].

The schedules of equivalent short-term tempers were determined by the method adopted earlier [4]. As in previous investigations, in order to eliminate the influence of hardening conditions on the experimental results, all cutters whose cutting properties were compared were hardened simultaneously. Hardening was carried out under factory conditions, and tempering in laboratory baths with a molten aluminum alloy. In this case, short-term tempering of the cutters was performed with several holding times of different duration. Then, by magnetic

Fig. 1. Durability \(T\) as a function of cutting speed \(V\) for cutters made of steel P18 with different tempering schedules: ● — ordinary, x — short-term

Fig. 2. Durability \(T\) as a function of cutting speed \(V\) for cutters made of steel P12 with different tempering schedules: ● — ordinary, x — short-term

control, cutters were selected for testing of cutting properties whose magnetic properties corresponded to the properties of cutters that had undergone ordinary triple tempering. In this case, steels of grades P18, P12, and P6M3F2 were tempered at \(560^\circ\text{C}\) with holding times up to 60 min, and steel P9K5 at \(580^\circ\text{C}\). Triple short-term tempering for the first three grades was carried out at \(600^\circ\text{C}\), and for steel

R9K5—at 620°C. The specimens, which constituted the cutting part of the tool, were cylindrical, with a diameter of 16 mm and a length of 120 mm, and had a land 13 mm wide that served as the front face of the tool. The design of the tools provided for regrinding only on the back surfaces and made it possible to use one specimen for many tests. The magnetic properties of the specimens were monitored with the aid of special standards on an alternating-current magnetometer and directly on the specimens (tools) with the aid of a magnetometer supplied by direct current. In this case a galvanometer, operating in ballistic mode, was connected to the secondary winding of the instrument, and the magnetic properties of the specimens were characterized by the throw of the pointer when the current in the primary circuit was interrupted.

Fig. 3. Tool life \(T\) as a function of cutting speed \(V\) for tools made of steel R6M3F2 with different tempering regimes: ● — conventional, × — short-term

Fig. 4. Tool life \(T\) as a function of cutting speed \(V\) for tools made of steel R9K5 with different tempering regimes: ● — conventional, × — short-term

The material of the tools tested, their heat treatment, and hardness are indicated in the table.

Tool-life tests of the tools were carried out on annealed ShKh15 steel with a hardness of 207–217 HB, at a cutting depth of 2 mm and a feed of 0.26 mm/rev. During sharpening the following geometry was imparted to the cutting elements of the tools: \(\gamma = 17.5^\circ\); \(\alpha = 10^\circ\); \(\varphi = 70^\circ\); \(\varphi' = 10^\circ\). The results of the tool-life tests are presented in the graphs of Figs. 1, 2, 3, and 4, which show the dependence of the tool-life time up to complete blunting of the tool on the cutting speed.

The results of the tool-life tests show that the cutting properties of steels of grades R18, R12, and R6M3F2 in the case of their thermal

treatment with three equivalent temperings at 600°C, while steel R9K5—at 620°C with a corresponding substantial reduction in heating time—remain equivalent in cutting properties to these steels after tempering according to the generally accepted regimes for them. This confirms the conclusion previously drawn for steel grade R18 [6]. At the same time, it should be borne in mind that this assessment cannot claim high accuracy, since it was made on the basis of a comparatively small number of tool-life tests.

Conclusions

1. Accelerated tempering at elevated temperatures, in terms of its effect on hardened high-speed steel, may be considered equivalent to ordinary tempering if, after accelerated and ordinary tempering, the steel acquires identical magnetic properties.

2. The cutting properties of cutters made of high-speed steels of grades R18, R12, R6M3F2, and R9K5 after three equivalent short-time temperings remained equivalent to their cutting properties in the case of the ordinary tempering regimes generally accepted for these steels.

References

1. Yu. A. Geller. Tool Steels, Metallurgizdat, 1961.

2. G. V. Korotushenko. Proceedings of Higher Educational Institutions. Ferrous Metallurgy, No. 8, p. 128, 1960.

3. K. A. Malinina. Metallography and Heat Treatment of Metals, No. 3, p. 47, 1960.

4. I. T. Tikhonov, N. S. Kilkov, E. P. Filimonova. Proceedings of Higher Educational Institutions. Ferrous Metallurgy, No. 10, p. 112, 1964.

5. P. A. Yudkovsky, V. Ya. Bulanov, Yu. N. Zhuravlev, A. P. Shevel. Machines and Tools, No. 12, p. 27, 1963.

6. N. S. Kilkov, I. T. Tikhonov, E. P. Filimonova. Proceedings of TPI, vol. 147, p. 206, 1966.