ON METHODS FOR DETERMINING SHORT-TERM TEMPERING CONDITIONS FOR HARDENED HIGH-SPEED STEEL

I. T. Tikhonov

(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)

In our investigations, the conditions for short-term tempering were selected in such a way that, in hardened high-speed steel, the same degree of decomposition of retained austenite—monitored by magnetic properties—was achieved as after the usual triple tempering at \(560^\circ\mathrm{C}\) with holding times of 60 min. In this case such tempers were called equivalent to the usual one [1]. It was further shown that the holding time of an equivalent temper is related to its temperature by an exponential dependence:

\[ \tau = C \cdot e^{\frac{A}{T}}, \tag{1} \]

where \(T\) is the tempering temperature in \({}^\circ\mathrm{K}\); \(A\) and \(C\) are constants.

For the case of triple equivalent tempers of hardened R18 steel, formula (1) took the form:

\[ \tau_{\mathrm{e}} = 1.72 \cdot 10^{-18} \cdot e^{\frac{37500}{T}} \ \text{min.}, \tag{2} \]

where \(\tau_{\mathrm{e}}\) is the holding time during tempering at temperature \(T\), measured in \({}^\circ\mathrm{K}\) [2].

Another researcher, P. A. Andzheus, for the same purpose recommends a formula of a somewhat different form:

\[ 20 \lg 3\tau_T + 0.47T - 308.30545 = 0, \tag{3} \]

where \(\tau_T\) is the time of a single holding during triple tempering, in min. (identical with \(\tau_{\mathrm{e}}\) in the preceding formula), and \(T\) is the tempering temperature in \({}^\circ\mathrm{C}\) [3]. This dependence, like the preceding one, was obtained experimentally, but control of the processes occurring during tempering was carried out not by the magnetic method, but by the change in hardness of the hardened steel. It is therefore of interest to compare the results obtained by these different methods.

Experiments carried out by us according to an improved method made it possible to reveal a certain dependence of the constants \(C\) and \(A\) in equation (1) on the temperature of heating the steel for hardening, and to refine their values. Thus, for R18 steel hardened from \(1230^\circ\mathrm{C}\), \(A = 38000\) and \(C = 9.34 \cdot 10^{-19}\), while in the case of hardening from \(1300^\circ\), \(A = 42200\) and \(C = 6.0 \cdot 10^{-21}\). Graphically this is represented in Fig. 1, where, in semilogarithmic coordinates, on the basis of experimental data, two straight lines are plotted:

1—for hardening from 1230°C and 2—1300°C. On the same graph a dashed line is plotted, corresponding to equation (3) and passing, in the interval of the tempering temperatures investigated, between straight lines 1 and 2, which indicates very good agreement of the results obtained by both methods. To avoid ambiguity it should be stipulated that both formula (1) and formula (3) determine only the holding time during tempering. The heating time, which depends on the dimensions of the specimens and on the heating device, must, for practical purposes, be taken into account separately.

Figure 2 shows the dependence of the magnetic properties (a) and hardness (b) of hardened R18 steel on the duration of tempering, expressed in conventional units. As the conventional unit there was adopted a sixty-minute holding during triple tempering at 560°C, as well as holdings of equivalent temperings determined by formula (1). Comparing these graphs, it is easy to note that the values of the instrument readings characterizing the magnetic properties of the steel after equivalent temperings at different temperatures fall on one curve considerably better than do the corresponding hardness values. The points on the lower graph, representing the mean values of four to five hardness measurements, are located within the error range of 0.5 HRC usual for Rockwell-type instruments. In [3] the hardness was determined on a Vickers instrument, and, to solve the problem posed, it was evidently necessary to carry out a labor-consuming experiment.

Fig. 1. Dependence of the duration of holdings of equivalent temperings on their temperature for R18 steel: 1—hardened from 1230°C; 2—1300°C; 3—according to data of P. A. Andrews (3)

Fig. 2. Dependence of magnetic properties (a) and hardness (b) of hardened R18 steel on the duration of tempering in conventional units at different temperatures: ●—560°C; ▲—580°C; ▼—600°C; ■—620°C

Thus, it may be considered once again confirmed that after equivalent short-term temperings, the regimes of which are determined by the magnetic method, hardened high-speed steel acquires the same hardness as after standard tempering. Both methods of determining short-term tempering regimes—by the magnetic properties and by the hardness of the steel—give results that agree well. However, the magnetic method appears to us somewhat simpler. Our experiments were ...

were carried out with a differential-type magnetometer. The instrument had two primary coils connected in series and supplied with alternating current. The rectified current of the secondary coils, connected in opposition to one another, was recorded with a millivoltmeter. A standard—a specimen that had undergone only hardening—was placed in one coil, and specimens after tempering were placed in the other. Specimens 6 mm in diameter and 60 mm long were tempered in a bath of molten aluminum alloy. To reduce the heating time, the specimens were preheated in a lead bath at a temperature of 560°C for 30 sec. The additional preheating time was not taken into account.

REFERENCES

1. I. T. Tikhonov, N. S. Kilkov, E. P. Filimonova. Izvestiya vuzov. Ferrous Metallurgy, No. 10, p. 112, 1964.

2. I. T. Tikhonov. Izvestiya TPI, vol. 147, p. 203, Tomsk University Press, 1966.

3. P. A. Andzhyus. Metallovedenie i termicheskaya obrabotka metallov, No. 8, p. 66, 1966.