QUALITY CONTROL OF HIGH-SPEED STEEL DRILLS TEMPERED UNDER VARIOUS CONDITIONS

A. E. Kameristov, N. S. Kilkov, E. S. Sergeeva, N. P. Sidorova,
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 check the quality of high-speed steel drills after their heat treatment under various conditions used at the Tomsk Tool Plant.

For steels of grades R18 and R6M3, the cutting properties of drills after their heat treatment with standard triple tempering at \(560^\circ\) with holding times of 60 min were compared with the cutting properties of drills heat-treated on unit No. 4 with double tempering at \(580^\circ\) and cycles of 8 min each, and on the automatic line TKhA-15 with double tempering at \(590^\circ\) and holding times of 15 and 18 min, depending on the drill size. To increase accuracy in monitoring the temperature–time effects of tempering by changes in magnetic properties, cylindrical specimens \(\varnothing 10 \pm 0.02\) mm and \(l = 130\) mm were used, made from the steel grades under study and heat-treated simultaneously with the drills. The correct shape of the specimens and the constancy of their dimensions made it possible to reduce significantly the deviations in the readings of the austenometer in comparison with those observed when monitoring the magnetic properties of drills.

The cutting-property tests were carried out in the TIZa laboratory on steel 45. Drills \(\varnothing 17.5\) mm were tested at a cutting speed \(V = 29\) m/min, feed \(S = 0.43\) mm/rev, and drilling depth of 50 mm; drills \(\varnothing 9.5\) mm were tested at \(V = 30\) m/min, \(S = 0.2\) mm/rev, and drilling depth of 30 mm. Drilling was performed using a cooling liquid (5% emulsol in water). The drill tests were continued until complete blunting. The heat-treatment conditions, hardness of the drills on their working part, and the average values of the results obtained in endurance tests of five drills are given in Table 1. Also indicated there are the values of the root-mean-square deviations, determined by ordinary methods of statistical processing, of the mean endurance values of the drills, characterized by the number of holes drilled before blunting.

The microstructure of the drills consisted of hidden- and fine-crystalline martensite with carbides and corresponded, in the case of drills \(\varnothing 17.5\) mm, to grades 10–10.5, and in the case of drills \(\varnothing 9.5\) mm, to grade 10.

In previously performed work it was shown that the magnetic properties of high-speed steel, monitored with an austenometer, can serve as a characteristic for evaluating the intensity of the temperature–time effect of tempering on hardened steel, which determines its

Table 1

cutting properties. Moreover, if hardened high-speed steel, as a result of two temperings of the same multiplicity but with different temperatures and durations, acquires identical magnetic properties, then it will also have the same hardness and, within the accuracy of the investigations carried out (±5% of the tool-life values), the same cutting properties [1]. Therefore, comparison of the cutting properties of drills with different tempering schedules was made in accordance with the readings of an austenometer, which characterize their magnetic properties.

Figure 1 shows the change in drill life as a function of the austenometer readings, which determined the intensity of the effect of tempering with different schedules and were obtained when monitoring the magnetic properties of cylindrical specimens of R6M3 steel that had undergone heat treatment together with the drills. Despite the considerable magnitude of the standard deviations of the tool-life values, it may be concluded that a decrease in the temperature-time effect during tempering on the TXA-15 and unit No. 4, compared with standard tempering, was accompanied by some deterioration of the cutting properties. In this case, R6M3 steel proved more sensitive to changes in the tempering schedule than R18 steel.

Fig. 1. Drill life as a function of austenometer readings under different tempering schedules: I—double tempering at 590° for 18 min; II—double tempering at 580°C with cycles of 8 min each; and III—triple tempering at 560°C for 60 min; ●—● R18 steel, drills Ø 17.5 mm; ○—○ R18 steel, Ø 9.5 mm; ×--× R6M3 steel, Ø 17.5 mm.

The second cycle of investigations was carried out with R6M5 steel, the production of drills from which had been begun by the plant. All drills to be tested, Ø 19.5 mm, and cylindrical specimens Ø 10 mm were made from steel of one delivery and were quenched under identical conditions on the automatic TXA-15 line from a final heating temperature of 1240°C. The grain size number corresponded to 10–11. After tempering, the structure consisted of finely acicular martensite and carbides, except for the drills after double tempering at 560°C with holding times of 10 min each, when retained austenite was also present in the structure. Table 2 gives the tempering schedules, hardness, austenometer readings, and mean values of the results of tool-life tests of five drills, as well as the magnitudes of the standard deviations of the tool-life values, characterized by the number of holes drilled until the drill became dull. The tool-life tests were carried out at a cutting speed \(V = 36\ \text{m/min}\), feed \(S = 0.43\ \text{mm/rev}\), and drilling depth of 60 mm.

The influence of the intensity of the temperature-time effect of the various temperings on the properties of the steel is shown by the graph presented in Fig. 2. This graph combines the results of experiments 5–8 and 11–14 (Table 2), corresponding to double temperings at 560 and 580°C with heating times within 10–30 min in a Sadovskii furnace. As a characteristic of the magnetic properties, the readings of a laboratory austenometer obtained on cylindrical specimens were used. In semilogarithmic coordinates of the graph, three combined curves are plotted for two tempering temperatures (560 and 580°), corresponding to the change in the magnetic properties, hardness, and life of R6M5 steel drills as a function of the intensity of the temperature-time effect—

Table 2

of the effect of different tempering regimes. The graph makes it possible to note an increase in drill life with increasing tempering intensity, despite a certain decrease in the hardness of the hardened steel. The only exception to the general pattern is experiment No. 11, corresponding to tempering \(580 \times 2 \times 10\). In the graph of Fig. 3, the life values of the drills from the remaining experiments are additionally plotted; their data were not used in the graph of Fig. 2. As a result, to the outlier point of experiment 11 there is added the outlier point of experiment 1 with tempering \(560 \times 2 \times 21\), and still greater uncertainty arises in drawing the envelope line. Two variants are possible: a curve with a shallow maximum in the region of temperature-time treatments close to standard tempering, with a decrease in life beyond the limits of the graph (solid line), and a curve with a maximum in the region of increased scatter of the data from life tests (dashed curve). In this case the first variant appears more probable, since the large scatter of the results of life tests after tempering in the temperature-time treatment region under consideration may be associated with the interaction of two factors oppositely affecting drill life—

Fig. 2. Magnetic properties, life, and hardness of R6M5 steel drills as functions of the duration of double tempering at 560 and 580°C.

Fig. 3. Life of R6M5 steel drills as a function of the duration of double tempering at 560 and 580°C.

factors—the high hardness and the retention in the structure of the hardened steel of some amount of residual austenite. On the other hand, since tempering treatments with such intensity of action are recommended for R6M5 steel—tempering \(550 \times 3 \times 60\) and the tempering \(580 \times 2 \times 10\) that replaces it [2]—a more detailed study of these tempering treatments with a sufficiently large number of life tests is highly desirable. At the same time, the graph in Fig. 3 makes it possible to confirm the conclusion of the work cited above [1] that, in the case of tempering treatments with an intensity close to that of the standard threefold tempering at \(560^\circ\), identical magnetic properties of the hardened and tempered high-speed steel correspond to identical cutting properties as well (experiments 3, 9, 10, 12, 13, 14).

To assess the quality and the possibilities of controlling tempering regimes by the magnetic properties of the steel using factory austenometers, the graph in Fig. 4 compares three curves showing the change

of the magnetic properties of hardened R6M5 steel after double tempering at 560 and 580°C for various durations. In this case the curves correspond to the readings of three austenometers—a laboratory instrument, one of the design of the Chelyabinsk plant, and one of TIZ. On the laboratory instrument, the magnetic properties were monitored with the aid of cylindrical specimens, and the deviations in the pointer readings were within only ±0.2 scale division. On the plant austenometers, however, the magnetic properties were monitored directly on the drills. The course of the curves shows that all the austenometers have approximately the same sensitivity. However, owing to deviations in the shape and dimensions of the drills, the readings of the plant austenometers are characterized by a large scatter of values. As a result, for a group of drills with the same tempering, the austenometer readings broaden into a band corresponding to a considerable range of variation in the tempering conditions. Therefore the plant austenometers cannot serve as a means of reliable control of the quality of drills and of the correctness of their tempering conditions. As the limits of deviation of the austenometer readings marked in the graph of Fig. 4 show, in this respect the austenometers of the Chelyabinsk plant design and of TIZ are approximately equivalent to one another.

Fig. 4. Magnetic properties of hardened R6M5 steel as a function of the duration of double tempering at 560 and 580°C according to the readings of various austenometers: ○—○ laboratory; ▲--▲ Chelyabinsk plant design; ×---× TIZ.

REFERENCES

1. N. S. Kilkov, I. T. Tikhonov, E. P. Filimonova. Collection of papers of the scientific-technical conference “Technical Progress in Mechanical Engineering,” dedicated to the 70th anniversary of the Mechanical Faculty of TPI. Part IV. Tomsk University Press, 1970, pp. 166–172.

2. P. Andjus. Scientific-technical abstract collection of NIIMASH, “Metal-Cutting and Measuring-Control Instruments,” issue 10, 1970, pp. 11–13.