EFFECT OF SHORT-TERM TEMPERING ON THE PROPERTIES OF HIGH-SPEED STEEL

N. S. Kilkov, I. T. Tikhonov, E. P. Filimonova

(Presented by the scientific seminar of the departments of physical metallurgy, metal technology, and welding production)

Accelerated tempering, carried out at elevated temperatures, is already being used in automatic lines for the heat treatment of tools made of high-speed steel [1]. However, the question of its effect on the cutting properties of the steel remains insufficiently studied. According to some published data, replacement of the usual threefold tempering at 560°C by short-term double tempering at 600°C was accompanied by a 10% decrease in tool life [2, 3]; according to other data, the use of the same short-term tempering in an automatic line for the heat treatment of drills produced an increase in their life by 19–45% [1].

In our work, the cutting properties of cutters made of high-speed steel R18, quenched in the same way but tempered according to different schedules—ordinary and short-term—were compared. All cutters whose cutting properties were compared were quenched simultaneously. This eliminated the possibility that deviations in the quenching temperature would influence the experimental results; quenching was carried out under factory conditions with heating in a salt bath to 1280°C. Tempering was performed in a laboratory furnace. For accelerated tempering, a bath with liquid metal was used. To reduce the drop in the bath temperature when the cutters being tempered were loaded, they were preheated to 550°C. Short-term tempering was carried out at a temperature of 600°C with holding times determined according to the procedure described in [3]. As a result of short-term tempering, the same degree of decomposition of retained austenite in the quenched high-speed steel was attained as after its usual threefold tempering at 560°C with holding times of 60 min. Such short-term tempering will hereafter be called equivalent to ordinary tempering. For threefold equivalent tempering at 600°C, the holding time was 8.5 min; for double tempering, 11.5 min. The correctness of the short-term tempering schedules was checked by means of austenometers according to the magnetic properties of the steel.

Tests of cutting properties were carried out by turning annealed ShKh15 steel with a hardness of 207–217 HB at a depth of cut of 2 mm and a feed of 0.26 mm/rev. The cutting elements of the cutters, after sharpening, had the same geometry: $\gamma = 17.5^\circ$, $\alpha = 10^\circ$, $\varphi = 70^\circ$, $\varphi_1 = 10^\circ$. The cutters were of two types—with mechanical fastening of high-speed-steel plates measuring $8 \times 17 \times 30$ mm, and solid cylindrical cutters—

cylindrical shape, with a diameter of 16 mm and a length of 115 mm. The use of solid cutters made it possible, directly on them, to monitor with a direct-current austenometer the degree of decomposition of residual austenite and to judge more reliably whether the short-term tempering regimes had been carried out correctly. The cutters, both in the case of ordinary tempering and in that of short-term tempering, had the same hardness, which for one batch of tools was within 62–63 HRC and for the other 64–65 HRC.

The results of tool-life tests of cutters with various tempering regimes are presented in the graphs of Figs. 1 and 2. The graph in Fig. 1 corresponds to test data for inserted cutters after ordinary triple tempering at 560°C and short-term, likewise triple, equivalent tempering at 600°C. The graph in Fig. 2 was constructed from the results of testing solid cutters, for which, in addition to ordinary tempering,

Fig. 1. Dependence of the tool life of cutters $T$ on cutting speed $V$ under different regimes of triple tempering.
● — ordinary tempering at 560°;
× — short-term tempering at 600°.

Fig. 2. Dependence of the tool life of cutters $T$ on cutting speed $V$ under different tempering regimes.
● — triple tempering at 560°;
× — triple tempering at 600°;
+ — double tempering at 600°.

double and triple short-term tempering was applied. These graphs show that all three named tempering regimes ensure approximately the same tool life of the cutters. Obviously, for a more accurate evaluation of the cutting properties of tools after short-term tempering, a larger number of experiments must be carried out, followed by mathematical processing of the results. In the present work such an evaluation can be made only approximately. For this purpose, 24 tests were carried out at a constant cutting speed on cutters with ordinary heat treatment, and 20 tests on cutters after triple short-term equivalent tempering at 600°C. If the tool life of cutters with ordinary tempering, amounting to about 10 min, is taken as 100%, then the arithmetic mean value of the tool life of cutters after short-term tempering is obtained as 97%. In this case the value of the root-mean-square error reached ±30%, which

corresponds to a probable error of the arithmetic mean of ±4.5%. Thus, comparison of these data with the results shown in the graphs permits the conclusion that, in our investigations, the cutting properties of the tools can be assessed only within 10% of the value of their tool life. Within these limits, the cutting properties of tools made of R18 steel, when short-term equivalent tempering at 600°C was used, did not differ from the cutting properties obtained as a result of ordinary tempering at 560°C with holding times of 60 min.

REFERENCES

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

2. K. A. Malinina. Metallovedenie i termicheskaya obrabotka metallov, No. 3, p. 47, 1960.

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