On Regulating the Short-Time Tempering Regime of High-Speed Steel Based on Cumulative Temperature-Time Exposure

I. T. Tikhonov

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

The long duration of ordinary tempering of high-speed steel and the difficulties arising in connection with this, especially in the automation of heat-treatment processes, have recently aroused considerable interest in accelerated tempering carried out at elevated temperatures. Works published on this question [1, 2], as well as studies carried out at TPI, have shown that when the temperature of triple tempering is raised, for example, to 620°, the holding time is reduced to three minutes; and in the case of heating to 750° for a single tempering giving maximum secondary hardness, only one second was required [2]. This indicates that, in short-time tempering, the role of thermal exposure during the period of heating the steel to the tempering temperature, as well as of temperature deviations during the holding process, increases substantially; and for sufficiently accurate execution of the regime for such tempering, the development of new control methods is necessary.

The dependence of the reaction rate on temperature, as is known, is determined by the equation

\[ V = Be^{-\frac{A}{T}} \]

which, for the time required for the reaction or process to proceed to completion, or to reach some definite stage, is transformed into the form:

\[ \tau = C \cdot e^{\frac{A}{T}}, \tag{2} \]

where \(T\) is the absolute temperature, and \(A\) and \(C\) are constants.

The purpose of tempering high-speed steel is the decomposition of retained austenite, which can be comparatively easily monitored by the magnetometric method. In the graph in Fig. 1, as an example, the course of change in the magnetic properties of specimens of hardened high-speed steel R18 is shown as a function of the duration of triple tempering at temperatures of 560, 580, and 600°C. The graph shows that the degree of decomposition of retained austenite corresponding to its decomposition after standard triple tempering at 560° and characterized by a magnetometer-pointer deflection to scale division 51.5 mv, in the case of tempering at 580° is reached as a result of holding for 22 min, and in the case of tempering at 600°—

7.3 min, and in this sense these short-time tempering regimes can be considered equivalent to the standard one. Thus, the standard tempering and the equivalent short-time temperings cause the same degree of decomposition of retained austenite, i.e., bring the tempering process to one and the same stage. In accordance with equation (2), Fig. 2 shows the dependence of the logarithm of the time of threefold equivalent temperings of high-speed steel on the reciprocal of their absolute temperature.

Fig. 1. Dependence of the magnetic properties of hardened high-speed steel on holding time during threefold tempering at different temperatures.

Fig. 2. Dependence of holding time during threefold tempering of high-speed steel on the reciprocal of its absolute temperature.

In this case, the values of the duration of equivalent temperings found by the method described above fit a straight line sufficiently well. The graph makes it possible to find the numerical values of the constants \(A\) and \(C\) and to obtain a formula for determining the duration of short-time temperings. In the case of high-speed steel R18, hardened from a temperature of \(1280^\circ\), the duration of threefold temperings at various temperatures can be calculated from the formula

\[ \tau_{\mathrm{e}} = 1.72 \cdot 10^{-18} \cdot e^{\frac{7500}{T}} \ \text{min}, \]

where \(T\) is the tempering temperature in \(^\circ\mathrm{K}\).

The existence of a general regularity linking the effect of time and temperature on the processes occurring in hardened high-speed steel during its tempering makes it possible to solve in a new way also the problem of monitoring and regulating accelerated tempering regimes. It may be assumed that, in the course of tempering, hardened steel must receive a certain temperature–time exposure \(\Theta = V \cdot \tau\), which, for tempering taking place at a constant temperature, is equal to:

\[ \Theta = B \tau \cdot e^{-\frac{A}{T}}, \tag{3} \]

where \(\tau\) is the tempering duration, \(T\) is the tempering temperature in \(^\circ\mathrm{K}\), and \(A\) and \(B\) are constants. In the case where tempering proceeds over a temperature interval and the temperature exposure during the heating period and fluctu—

temperature bath during holding, the duration of tempering will be determined by the condition

\[ B \int e^{-\frac{A}{T}}\, d\tau = \Theta, \tag{4} \]

where \(\Theta\) is a constant quantity equal to the temperature–time exposure of the standard and equivalent tempering treatments.

In practice, regulation of the tempering regime can be carried out as follows. The readings of an instrument recording the change in temperature of the tempering bath are converted by a transducer into quantities proportional to \(e^{-\frac{A}{T}}\) and are summed by an integrating device. After this sum reaches a value corresponding to completion of tempering, a signal is given indicating its end, or a device is switched on that removes the tools from the tempering bath. Since the rate of the processes occurring during tempering decreases very sharply as the temperature is lowered, the integrating device may be switched on starting from a temperature of about \(540^\circ\).

REFERENCES

1. K. A. Malinina. Metallography and Heat Treatment, No. 3, 1960.
2. G. V. Korotushenko. Izvestiya VUZov, Ferrous Metallurgy, No. 8, 1960.