STUDY OF THE CUTTING PROCESS OF STEEL WITH ARTIFICIAL PREHEATING

V. M. Mostovoi

Introduction

It is known that the mechanical properties of metals change with changes in the heating temperature. Fig. 1 shows the dependence of the ultimate tensile strength \(\sigma_b\) and hardness \(H_b\) on the heating temperature of heat-resistant alloys. Fig. 2 presents the dependence of the logarithms of the ultimate tensile strength of alloy carbon steels on the test temperature. From Figs. 1 and 2 it is evident that, at certain preheating temperatures, \(\sigma_b\) and \(H_b\) have minimum values. Preheating of the workpieces being processed has become widely used in forging, stamping, rolling, and other types of hot working. The exception, perhaps, is one field—

Fig. 1. “Properties of metals and alloys” (Handbook), Metallurgizdat, 1949.

Fig. 2. Dependence of the logarithm of the ultimate tensile strength of alloy steels on the test temperature.

—where preheating has not yet become widespread, although in this field heating could prove no less effective than in forging, stamping, rolling, and so on. This field includes the machining of metals by cutting. At various times attempts have been made to machine me-

metals by cutting after preheating them: engineer V. I. Rukavishnikov—1936, B. M. Askinazi and G. I. Babat—1939.

The works of the above-mentioned authors are of interest to us only from the standpoint of how they posed the problem, since they do not constitute theoretical investigations and do not clarify practical possibilities. Askinazi and Babat rightly note in their work: “Attempts to carry out hot machining in practice were made more than once, but they did not yield good results because of the imperfection of the heating method.” Corresponding Member of the Academy of Sciences of the USSR, Professor-Doctor V. D. Kuznetsov, says on this matter: “The question of the influence of temperature on the cutting process as a whole has hardly been investigated; meanwhile, clarification of the dependence of cutting parameters on temperature could provide rich material for the theoretical foundations of metal cutting.” We know the fundamental works of the cutting school at the Tomsk Polytechnic Institute, headed by Professor-Doctor A. M. Rozenberg, from which it follows that regimes with identical temperatures on the rake face of the tool are accompanied by identical built-up-edge geometry, identical coefficients of friction, and identical chip shrinkage, regardless of the sources of heat generation. Cutting speed, feed, and depth of cut are regarded by the TPI school as temperature factors. If these propositions of the TPI cutting school are correct, then we should obtain quantitatively identical dependences of chip shrinkage and coefficient of friction, provided that the temperatures on the rake face of the tool in the case of preheating the blank and without preheating are the same. Clarifying the validity of these propositions is also necessary because there is no consensus on this question among the leading specialists in metal cutting. Consequently, machining of metals by cutting with preheating is of great interest from two points of view:

1. From the practical standpoint—the reduction of the power consumed and the possibility of machining difficult-to-machine alloys and steels.

2. From the theoretical standpoint—clarifying, on the one hand, the influence of temperature on all cutting parameters and, on the other hand, the nature of high-speed cutting of metals.

I. Equipment and Methodology of the Investigation

The investigations were carried out on a 1A62 engine lathe, installed at a distance of 2.5 m from the wedge clamp of an A3-46 model lamp generator.

The workpiece was heated only in the cutting zone by means of a semi-universal two-turn semi-ring inductor (a) of the author’s design (Fig. 3), mounted on the longitudinal carriage of the machine.

The forces on the cutter were measured with a two-component hydraulic dynamometer. Temperatures in the cutting zone were recorded by a combined method: with a thermoprobe, temperature-indicating paints, and, at red heat, with an optical pyrometer. The temperature on the rake face was measured with a natural thermocouple. As the cutting tool, a metal shank fitted with a T15 K6 cemented-carbide insert was used. The workpiece subjected to machining was made of machine-structural steel with a temporary tensile strength \(\sigma_b = 53\ \mathrm{kg/mm^2}\); the chemical composition of the material being machined was: \(C \simeq 0.33\%\), \(Mn \simeq 0.7\%\), \(Si = 0.15\%\).

To determine the influence only of the cutting temperature on the cutting process, the workpiece was not preheated. In this case, the temperature regime in the shear plane and on the rake face of the tool was determined by the corresponding cutting speeds.

To determine the influence on the cutting process of only the temperature of artificial preheating, the workpiece was heated in the cutting zone from 30 to 700°C, with temperature intervals of 50°C.

In this case the workpiece rotated at a very low cutting speed—\(V = 3\ \text{m/min}\), i.e., at such a cutting speed that no appreciable cutting temperature was produced. And, finally,

Fig. 3. Two-turn semi-ring inductor

to determine the combined influence on the cutting process of the temperature of preliminary artificial heating \(T^\circ\) and the cutting temperature \(T_v^\circ\), experiments were carried out with variable cutting speeds and preliminary-heating temperatures.

II. Results of the Study

Figure 4a presents the curve of the dependence of chip shrinkage along its length on the cutting speed (cutting temperature \(T_v^\circ\)), and Fig. 4b—on the preliminary-heating temperature \(T^\circ\)C. On these curves, respectively, an equality of shrinkages is found: at \(V = 20\ \text{m/min}\) and \(T = 350^\circ\text{C}\), and also at \(V = 50\ \text{m/min}\) and \(T = 600^\circ\text{C}\).

The equality of shrinkages indicates to us the equality of temperatures on the rake face of the cutter, which was confirmed by measuring the temperatures on the rake face of the cutter.

Further, with an increase in cutting speed (cutting temperature) to \(V = 23\ \text{m/min}\), Fig. 4c, the maximum value of shrinkage shifted into the zone of lower temperatures by the magnitude of the temperature increase due to the increase in cutting speed to \(V = 23\ \text{m/min}\). Finally, with a further increase in cutting speed from \(V = 23\ \text{m/min}\) to \(V = 218\ \text{m/min}\), the maximum value of the shrinkages shifted still further into the zone of microtemperatures; therefore in Figs. 4c, 4d, 4e we see only parts of descending curves.

In Fig. 4, at certain temperatures, a general tendency is revealed toward a decrease in shrinkage with an increase in the temperature of preliminary heating—\(T^\circ\)—and the cutting temperature—\(T_v^\circ\); this tendency is shown most clearly in the curves of Fig. 5.

Figure 6 presents the curves of shrinkage through the thickness of the cut. Here the character of the change in the curves is the same as for the curves of chip shrinkage along the length.

The chip shrinkage across the width of the cut, Fig. 7, with increasing temperature may practically be considered unchanged.

Fig. 4. Dependence of chip shrinkage along the length on the preheating temperature

Fig. 5. Dependence of chip shrinkage along the length on the cutting speed

The coefficient of friction between the contact surfaces of the chip and the cutter changes according to the same law as the chip shrinkage along the length and thickness of the cut, Fig. 8. The coefficient of friction, when the temperatures on the rake face of the cutter are equal in the case of preliminary heating of the blank in the cutting zone (Fig. 8b) and in the case of heating of the metal in the cutting zone due to the cutting speed (Fig. 8a), has equal values.

Fig. 6. Dependence of chip shrinkage in thickness on the temperature of preliminary heating

Fig. 7. Dependence of chip shrinkage in width on the temperature of preliminary heating

With a further increase in cutting speed from \(V = 23\ \text{m/min}\) and higher, and also in the temperature of preliminary heating, the coefficient of friction decreases.

In contrast to the curves of Figs. 4, 6, 8, when the temperatures on the rake face of the cutter are equal, the force \(P_z\) does not remain equal (Fig. 9). Thus, for example, at \(V \simeq 20\ \text{m/min}\) and the temperature \(T_v = 350^\circ\text{C}\) corresponding to this speed, the minimum value of the force is \(P_z = 130\ \text{kg}\) (Fig. 9a), whereas at \(T = 350^\circ\text{C}\) and \(V = 3\ \text{m/min}\), the force is \(P_z = 115\ \text{kg}\) (Fig. 9b). The same can be said of the maximum values of the force \(P_z\): in the first case the force is \(P_z = 172\ \text{kg}\), and in the second case \(P_z = 152\ \text{kg}\). The same phenomenon is also observed with the force \(P_x\) (Fig. 10) and the stress in the shear plane (Fig. 11). As is seen from Figs. 9, 10, 11, the cutting forces \(P_z\) and \(P_x\), as well as the stresses in the shear plane \(\sigma_s\), have lower values in the case when a specified temperature is established by preliminary artificial heating of the blank ...

chips in the cutting zone, and not by means of the corresponding cutting speed. This means that in the first case the cutter removes a chip from metal whose properties have been weakened by preheating, whereas in the second case—from metal that is heated in the shear plane as a result of the cutting work. From the standpoint of the law of conservation of energy it cannot be otherwise: additional energy is expended on the plastic deformation of cold metal and, consequently, on its heating.

Fig. 8. Dependence of the coefficient of friction on the temperature of preheating

Fig. 9. Dependence of the main cutting force on the temperature of preheating

With a further increase in cutting speed from \(V = 23\ \text{m/min}\) and higher, and also with an increase in the temperature of preheating, the force \(P_z\), Fig. 9 c, d, e and Fig. 12 a, b, the force \(P_x\), Fig. 10 c, d, e, and the stress in the shear plane \(\sigma_s\), Fig. 11 c, d, e, decrease steadily. Consequently, raising the cutting temperature \(T_v^\circ\) by means of the cutting speed \(V\), or the temperature of artificial preheating \(T^\circ\), inevitably leads to an increase in the productivity of the cutting process.

Fig. 10. Dependence of the feed force on the preheating temperature

Fig. 11. Dependence of the stress in the shear plane on the preheating temperature.

Fig. 12. Dependence of the main cutting force on the cutting speed

Conclusion

The investigations carried out make it possible to draw the following conclusions:

1. At identical temperatures on the rake face of the tool, regardless of whether these temperatures were obtained as a result of the effect of cutting speed, or as a result of preliminary heating of the workpiece in the cutting zone, or as a result of the combined action of both factors, the coefficient of friction between the contact surfaces of the chip and the tool, and the chip shrinkage, prove to be identical.

This confirms the fundamental point of view of the cutting school at the Tomsk Polytechnic Institute, headed by Professor and Doctor A. M. Rozenberg, and definitively establishes the principal role in the cutting process of the temperature on the rake face of the tool and its prevailing influence on all phenomena accompanying the cutting process.

1. The cutting forces and stresses in the shear plane, at equal temperatures on the rake face of the tool, have lower values in the case of cutting metal preheated in the cutting zone.

2. With an increase in the temperature of preliminary heating of the metal in the cutting zone, beginning from a certain temperature, the cutting forces, stresses in the shear plane, chip shrinkage, and coefficients of friction invariably decrease, which makes it possible, when cutting metals with preliminary heating, to increase significantly the productivity of the cutting process. Thus, for example, under our experimental conditions, the value of the force \(P_z\) decreased by a factor of 3.65.

3. The use of preliminary heating makes it possible to machine, comparatively easily, difficult-to-machine alloys and steels with the use of metal-ceramic and thermocorundum plates in roughing operations.

4. The investigations carried out made it possible to reveal more fully the physical essence of the phenomena accompanying the cutting process and, on this basis, to explain the nature of high-speed cutting of metals¹.

1. Izvestiya TPI, vol. 75, V. M. Mostovoi, “On the essence of the processes in high-speed cutting of steel.” ↩