VERIFICATION OF ACCELERATED TEMPERING REGIMES FOR HARDENED HIGH-SPEED STEELS R18 AND R6M3 BASED ON THEIR MAGNETIC PROPERTIES

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, metal technology; equipment and technology of welding production)

The work was carried out for the Tomsk Tool Plant and had the aim of comparing the regimes of accelerated tempering of drills on the plant’s automatic line with the usual threefold tempering at \(560^\circ\)C. The comparison of these regimes was performed by monitoring changes in the magnetic properties and hardness of hardened steels during various tempering variants.

To evaluate the results of heat treatment from monitoring data on magnetic properties and hardness, specimens were made from steels R18 and R6M3. The use of specimens is explained by the fact that monitoring magnetic properties directly on drills, owing to deviations in their shape and dimensions, cannot provide high accuracy. The experiments were carried out on cylindrical specimens \(\varnothing 10\) mm, with a mass close to that of drills \(\varnothing 16 \div 27\) mm. The specimens were manufactured in batches from steels of a single delivery and from the minimum number of bars; this excluded the influence of fluctuations in the composition of steels from different deliveries on the experimental results.

To monitor the magnetic properties of the specimens, a differential-type magnetometer powered by alternating current was used. Measurement was performed at two values of the magnetic field and, correspondingly, at current strengths of 0.4 and 1.0 A. The magnetic field at a current of 1.0 A was close to the field of magnetic saturation of the specimens. Under these conditions, the change in magnetic properties will be determined primarily by the change in the values of the maximum magnetic induction of the specimens, which depends to a greater extent on the content of retained austenite in the steel. At a current strength of 0.4 A, the magnetic field is close to the region with the maximum value of magnetic permeability; consequently, under these conditions structural changes associated with the processes of strengthening or softening of the steel will have the strongest effect on the change in magnetic properties.

The readings of the millivoltmeter of the magnetometer are a relative characteristic of the change in magnetic properties depending on the conditions of heat treatment and do not correspond to any definite values, for example, the amount of retained austenite.

Hardness was measured on a TK-2 instrument on the cleaned cylindrical surface of the specimens. The hardness values are given without correction for the cylindrical shape of the specimens, which for \(\varnothing 10\) mm is \(+1\) HRC [1].

A batch of specimens of steel R18 was heat-treated according to the following variants. Half of the specimens were quenched from \(1280^\circ\) and the other half from \(1300^\circ\). The duration of the standard triple tempering at \(560^\circ\) was varied within the range \(40 \div 90\) min; that of double tempering at \(600^\circ\), within \(5 \div 20\) min. Heating for quenching was carried out in two stages: preliminary heating at \(890^\circ\), 2.5 min, and final heating at the above-indicated temperatures in a salt bath (\(100\%\) BaCl\(_2\)), 2.5 min. Cooling during quenching was carried out in molten caustic at a temperature of \(400 \div 450^\circ\). Tempering was performed in a small plant furnace with a caustic bath. The bath was loaded only with the specimens under study; therefore its temperature did not decrease after the specimens were loaded. The holding times during tempering corresponded to the time from the moment the specimens were immersed in the bath until they were removed from it.

The structure of the steel after quenching from \(1280^\circ\) was rated as grade 11, and after quenching from \(1300^\circ\), as grades \(9 \div 11\).

Tempers with different regimes, after which steel quenched from a specified temperature acquires identical magnetic properties, were termed by us mutually equivalent. The equivalence coefficient can be determined by the simple relation

\[ K = \frac{t_1}{t_2}, \]

where \(t_1\) is the heating time during tempering at one temperature and \(t_2\) at another temperature.

As a result of our previously performed work [2, 3], it was shown that, under identical quenching conditions, after standard triple tempering at \(560^\circ\) and triple tempering at \(600^\circ\) equivalent to it, high-speed steel acquires practically equivalent cutting properties and hardness. The concept of equivalence to standard tempering can also be extended to double tempers. The equivalence coefficient of two tempers can be selected by changing the time scale of one of them until the curves corresponding to the change in magnetic properties as a function of tempering duration coincide. These combined curves for the double tempering at \(600^\circ\) under study and triple tempering at \(560^\circ\) are presented in the graphs (Fig. 1). It follows from the graphs that double tempering at \(600^\circ\) with holding times of 14 min is equivalent to triple tempering at \(560^\circ\) with holding times of 60 min, i.e., the equivalence coefficient in this case is 4.3. This means that high-temperature tempering has an approximately 4.3 times more intensive effect on hardened steel than tempering at \(560^\circ\).

Figure 1 gives the curves corresponding to the change in hardness as a function of tempering duration. These curves show that, within the accuracy of its measurement, the hardness retains an almost constant value of \(64 \div 65\) HRC for all the tempering regimes investigated.

The heat treatment of steel R6M3 was carried out under the same conditions and on the same specimens as for steel R18. Quenching of specimens of steel R6M3 was carried out from temperatures of \(1220^\circ\) and \(1240^\circ\)C. After quenching from \(1220^\circ\), the grain size of the steel was rated at 9.5, and after quenching from \(1240^\circ\), at 8.5. Tempering was performed at two temperatures—triple tempering at \(560^\circ\) and double tempering at \(600^\circ\). The change in the magnetic properties and hardness of the specimens as a function of the duration of heating during tempering is shown in the graphs (Fig. 2). The experimental values of the magnetometer readings after double tempering at \(600^\circ\) and triple tempering at \(560^\circ\) fall quite well on a single curve at an equivalence coefficient of these tempers equal to 3.85.

Thus, in this case a double tempering at 600° with heating periods of 15.6 min each may be regarded as equivalent to the standard triple tempering at a temperature of 560°.

Comparing the change in properties during tempering of specimens of steels R18 and R6M3, the following features of the behavior of the latter may be noted.

Steel R6M3, both in the hardened and in the tempered state, is distinguished by a higher magnetic induction in comparison with steel R18.

Fig. 1. Magnetic properties and hardness of steel R18 as a function of the duration of heating during tempering at temperatures of 560 and 600°C

Fig. 2. Magnetic properties and hardness of steel R6M3 as a function of the duration of heating during tempering at temperatures of 560 and 600°C

On the other hand, the magnetic properties of steel R6M3 change relatively less with changes in tempering regimes. Thus, in the case of triple tempering at a temperature of 560°, when the duration of heating is increased from 40 to 90 min, the change in magnetometer readings for steel R18 amounted to 12.3%, whereas for steel R6M3 it was only 5%. Naturally, the method of magnetic control of tempering regimes will be considerably less sensitive for steel R6M3 than for steel R18. Further, when the magnetometer operates with sufficiently strong magnetic fields \((I = 1.0\ \text{A})\), in the case of steel R6M3 there was almost no difference in the magnetic properties of specimens hardened from 1220 and 1240°, whereas for steel R18 a change in the hardening temperature by 20° affected the magnetic properties quite distinctly (Fig. 1). Differences in hardening regimes, as the graphs in Fig. 2 show, can be detected from the magnetic properties of steel R6M3 only when the magnetometer operates with fields in the region of maximum magnetic permeability \((I = 0.4\ \text{A})\), although even in this case the difference in the magnetic properties of the specimens when the hardening temperatures are changed is less significant than for steel R18. The hardness of steel R6M3 after double tempering at 600°C shows a tendency, though a very slight one \((\sim 0.5\ \text{HRC})\), toward some decrease in comparison with the hardness after equivalent triple tempers at 560°C.

After the above data had been obtained, some of the specimens were subjected to heat treatment together with drills on the plant’s automatic line. Control of their magnetic properties and hardness showed that the double-tempering regimes at 600°C used on the automatic line may, in their effect on hardened high-speed steels R18 and R6M3, be regarded as equivalent to ordinary tempering.

Conclusions

1. Magnetic methods for monitoring tempering regimes in the case of steel R6M3 are approximately 2.5 times less sensitive than for steel R18, which is associated with significantly smaller changes in the magnetic properties of steel R6M3 under deviations in tempering regimes.

2. The hardness of hardened steel R18 after double equivalent tempering does not differ from the hardness obtained as a result of the usual triple tempering at 560°C. After accelerated tempering of steel R6M3, however, a tendency toward a certain decrease in its hardness can be observed.

References

1. I. S. Kamenichnyi, A Concise Handbook for the Heat-Treatment Technologist. Mashgiz, 1963.

2. I. T. Tikhonov, N. S. Kilkov, E. P. Filimonova. Short-time tempering of high-speed steel R18. Izvestiya vuzov, “Ferrous Metallurgy,” No. 10, p. 112, 1964.

3. N. S. Kilkov, I. T. Tikhonov, E. P. Filimonova. The influence of short-time tempering on the properties of high-speed steel. Izv. TPI, vol. 147, p. 206, 1966.