Influence of Different Tempering Modes on the Tool Life of Cutters Made of R9 Steel

N. S. Kil’kov, I. T. Tikhonov, E. P. Filimonova

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

The purpose of the work was to determine the influence of accelerated high-temperature tempering on the cutting properties of R9 steel. Short-time tempering was carried out at \(600^\circ\text{C}\) in several variants: in the form of double and triple tempering followed by cooling, as usual, in air, and of some cutters in oil. The choice of such modes was determined by the fact that double tempering at \(600^\circ\text{C}\) with accelerated cooling, used in an automatic line for heat treatment of drills made of high-speed steels R18 and R9, gives a substantial increase in their service life (\(20 \div 45\%\)). In this case, the cooling rate of the tools after tempering was \(7 \div 8\) times higher than the rate of cooling in air [1].

For tool-life tests we manufactured a batch of cutters from one bar of R9 steel. The cutters were hardened by heating in factory baths to the temperature usual for R9 steel, \(1240^\circ\text{C}\). Since all the experimental specimens were heated and cooled during hardening simultaneously, this excluded the possibility that inaccuracies in the hardening mode could affect the experimental results. The duration of holding during high-temperature tempering was determined according to a previously developed procedure [2, 3]. Thus, both triple and double tempering at \(600^\circ\text{C}\) were equivalent to ordinary tempering with respect to the attainable degree of decomposition of retained austenite, which was monitored by the magnetic method. High-temperature tempering was performed in baths with a liquid aluminum alloy after a short preliminary heating of the specimens to \(550^\circ\text{C}\). The holding time for triple equivalent tempering at \(600^\circ\text{C}\) was \(8\) min, and for double tempering, \(12\) min. Immersion in oil of some cutters after their heating and holding during tempering simulated the accelerated cooling of drills during heat treatment on an automatic line. Control cutters were tempered, as usual, three times at \(560^\circ\text{C}\), with holdings of \(60\) min each and cooling in air. The average hardness values of cutters with different tempering modes varied within the range \(62.5 \div 63.0\) HRC.

Tests of cutting properties were carried out by turning annealed ShKh15 steel with a hardness of 210 HB, at a cutting depth of \(2\) mm and a feed of \(0.26\) mm/rev. During sharpening, the cutting elements of the cutters were given the following geometry: \(\gamma = 17.5^\circ\), \(\alpha = 10^\circ\), \(\varphi = 70^\circ\), and \(\varphi' = 10^\circ\). The results—

The results of life tests are presented in the plots of Fig. 1, which show that the cutting properties of the cutters, within the limits of the estimate possible on the basis of the data obtained, are the same both in the case of ordinary and accelerated equivalent tempers at 600°C. However, if the life values of cutters that underwent triple high-temperature tempering are located on the plot similarly to the points obtained in tests of cutters with normal tempering (plot “a”), then after double high-temperature tempering a somewhat greater scatter of the experimental data may be noted. Accelerated cooling in oil from the tempering temperature did not produce a noticeable positive effect either in the case of triple or in the case of double high-temperature tempering.

Fig. 1. Cutter life \(T\) as a function of cutting speed under different regimes of equivalent tempering: • — triple tempering at a temperature of 560°C; \(+\times\) — triple tempering at a temperature of 600°C (plot a); \(+\times\) — double tempering at a temperature of 600°C (plot b); \(\times\) — with cooling in oil after heating during tempering.

The work makes it possible to conclude that the life of cutters made of R9 steel after heat treatment with triple equivalent short-duration tempering at 600°C remains at the same level as after ordinary triple tempering at 560°C. Of course, the results obtained on the basis of a comparatively small number of life tests cannot claim high accuracy. This may explain the difference between our conclusions and the conclusions of work [1], where, as was indicated, an increase in the life of drills made of high-speed steel was observed after their heat treatment on an automatic line with double high-temperature tempering at 600°C. In each batch, life tests of drills were carried out, and what the possible error of the data obtained by the authors of that communication was, unfortunately, is not indicated.

LITERATURE

1. P. A. Yudkovskii, V. Ya. Bulanov, Yu. N. Zhuravlev, and A. P. Shevel. Stanki i instrument, 1963, No. 12, p. 27.

2. I. T. Tikhonov, N. S. Kilkov, E. P. Filimonova. Izvestiya vuzov. Chernaya metallurgiya, No. 10, p. 112, 1964.

3. N. S. Kilkov, I. T. Tikhonov, E. P. Filimonova. Izvestiya TPI, vol. 147, p. 206, Tomsk University Press, 1966.