Milling of Stainless Steel in Cold and Heated States

(A Study of Machinability)

A. M. Rozenberg, K. A. Nassonov

In recent years, acid-resistant, stainless, and heat-resistant steels and alloys have found extremely wide application in various branches of the national economy. The need for these metals is increasing on an ever-growing scale. However, the machining by cutting of high-alloy steels is associated with great difficulties, and some of them are completely unsuitable for cutting by the usual method.

Therefore, the study of the machinability of high-alloy steels is of considerable practical and theoretical interest.

The present work gives, in condensed form, the results of a study of the machinability of stainless austenitic steel 1Kh18N9T (steel EYa1T) in face milling with high-speed-steel and carbide cutters, in both the cold and heated states [7].

Experiments on milling cold steel with high-speed-steel cutters were carried out with cooling by a five-percent solution of emulsol in water.

In this work, forged high-speed steel R18 and cast high-speed steels with additions of cobalt, sulfur, and titanium were used for manufacturing the teeth of face mills.

The best results were obtained when using forged high-speed steel R18; moreover, the optimum geometry of the cutter tooth was characterized by the following parameters: \(\gamma_N = 22—28^\circ\), \(a = a_1 = 10^\circ\), \(\varphi = 55—60^\circ\), \(\varphi_1 = 10^\circ\), \(\lambda = 30—35^\circ\), \(r = 0.5—0.7\) mm (the radius at the tool tip may be replaced by a transition chamfer with a plan angle of \(20^\circ\) and a width of 1 mm). A study of the influence of cutting speed on the durability of the high-speed-steel cutter showed that the use of cutting speeds not exceeding 15 m/min is of the greatest practical interest.

Experiments on milling steel 1Kh18N9T in the cold state with tools fitted with carbide inserts VK8, T5K10, and T15K6 established that machining of this steel is difficult. This is evidenced by the comparatively low level of cutter life. Thus, when milling at a speed of 110 m/min \((S_z = 0.126\ \text{mm/tooth};\ t = 2.5\ \text{mm})\), the life of a cutter fitted with alloy VK8 was 39 minutes, while at a speed of 170 m/min it was only 15 minutes.

Changing the cutting speed within the range of 50–160 m/min, at the same feed and depth of cut, was accompanied by a change in the life of a milling cutter tipped with T5K10 alloy from 80 to 30 min. The T15K6 hard alloy proved completely unsuitable for milling steel 1Kh18N9T in the cold state. In this case very low tool lives were obtained, not exceeding 10 min, since, owing to brittleness, the cutting edge chipped before normal wear on the flank surface had formed.

The poor machinability of steel 1Kh18N9T is associated with its physical properties, namely its high toughness, high work-hardening index, and low thermal conductivity. As a result of these properties of the steel being machined, the cutting edge of the carbide milling cutter chipped, which led to rapid tool failure, deterioration of the finish of the machined surface due to the appearance of burrs on it, and an increase in the cutting force.

The best results in milling stainless steel 1Kh18N9T in the cold state were obtained with the following optimum values of the geometric parameters of the cutter tooth:

The inclination angle of the cutting edge was taken as 7°.

For alloy T15K6, wide variation of the cutting speed, feed, and cutter-tooth geometry produced no effect and did not prevent chipping of the cutting edge.

It is known [1, 2, 6, 10, 11, etc.] that one of the methods permitting steels to be machined productively is machining them in the heated state. However, most investigations in this field are of a sporadic nature. Very often the question of the possibility of improving the machinability of steels by cutting them in the heated state has been treated one-sidedly, without covering the entire complex of the principal machinability characteristics.

In our experiments on milling heated stainless steel, the steel bars were heated in an electric furnace, after which they were fastened to the machine table by means of a special fixture.

The temperature of heating of the surface of the bars being machined was monitored by a thermocouple (an artificial copper–constantan thermocouple). The width of the milled surface was taken as 80 and 170 mm, and the depth of cut as 2.5 mm.

The influence of the preheating temperature of the steel on the life of carbide milling cutters tipped with T15K6, VK8, and T5K10 hard alloys is presented in Figs. 1, 2, and 3.

As the curves in Figs. 1, 2, and 3 show, preheating steel 1Kh18N9T makes it possible in a number of cases to increase the life of milling cutters substantially. However, the change in life of the cutting tool with increasing heating temperature of the steel does not occur monotonically, but follows curves having a maximum.

For a specific machining regime (cutting speed and feed), there exists an interval (±20°C) of preheating temperatures

Fig. 1.

Tool life \(T\), min

Preheating temperature, \(T^\circ\mathrm{C}\)

Stainless steel Kh18N9T
\(S_z = 0.026\ \mathrm{mm/tooth}\)
\(\triangle\) — \(v = 307\ \mathrm{m/min}\)
\(\circ\) — \(v = 448\ \mathrm{m/min}\)

Fig. 2.

Tool life \(T\), min

Preheating temperature, \(T^\circ\mathrm{C}\)

Steel [[unclear: grade]]
T15K6
\(S_z = 0.026\ \mathrm{mm/tooth}\)
\(\square\) — \(v = 236\ \mathrm{m/min}\)
\(\triangle\) — \(v = 375\ \mathrm{m/min}\)

of stainless steel, in the region of which the life of the milling cutter reaches its greatest value in comparison with the life at other test temperatures. The temperature of heating of the steel at which the cutter life reaches its greatest value we call the “optimal temperature” of heating of the steel.

The experiments also show that, with an increase in cutting speed, the optimal temperature shifts into the region of lower preheating temperatures.

In order to study more fully the dependences obtained, we carried out special temperature investigations in which, by the natural-thermocouple method, using an MPO-2 oscillograph, the cutting temperature was measured. Some results of the investigation are presented in Fig. 4.

Fig. 3.

A joint consideration of Figs. 1, 2, and 4 shows that the optimal temperature of preliminary heating of the steel, at which the maximum life of the cutting tool fitted with alloy T15K6 is observed, corresponds to a quite definite cutting temperature, constant for all the investigated speeds and somewhat exceeding—

not exceeding 1100°C. Experiments carried out at other cutting-speed values agree satisfactorily with the results presented in Figs. 1, 2, and 4¹.

We assume that the presence of maxima in tool life under a certain temperature regime, achieved by changing the cutting speed and the heating temperature of the steel being machined, is connected above all with changes in the physicomechanical properties of the carbide cutting tool under different temperature conditions. In particular, titanium carbides, as special studies show [3, 4, 5], when heated are prone to a very considerable increase in impact toughness—by a factor of three to eight.

Apparently, the increase in impact toughness under a certain temperature regime in the titanium alloy T15K6 leads to the presence of the tool-life maxima noted in Figs. 1 and 2.

In all cases where the temperature regime corresponded to the tool-life maxima, we observed uniform, gradually increasing wear with the formation of a wear land on the flank face. When there was a departure from the indicated regime, as a rule, the process of normal wear was interrupted by chipping of the cutting edge.

Preliminary heating of steel 1Kh18N9T changes the wear process of the milling cutter not only through the temperature regime, which affects the properties of the tool material; at the same time, the properties of the work material itself also change. With heating, the hardness of the steel, its tensile strength, its conventional yield limit, and its compression polytropy index [9] decrease monotonically. A very considerable change, with heating of steel 1Kh18N9T, in its physicomechanical characteristics is naturally accompanied by a change in its “abrasive capacity” [12]. The most proper indicator of the “abrasive capacity” of the material being machined should be considered the intensity of wear of the cutting tool. Figure 5 shows—

Fig. 5.

Visible labels in the figure: Steel 1Kh18N9T. T15K6. \(S_z = 0.126\) mm/tooth. \(250^\circ\mathrm{C}\), \(350^\circ\mathrm{C}\), \(420^\circ\mathrm{C}\), \(500^\circ\mathrm{C}\). Vertical axis: relative wear \(i\), mm. Horizontal axis: cutting speed, \(V\), m/min.

on the dependence of the magnitude of relative wear (the increase in the width of the wear land on the flank surface per one hundred meters of cutter travel under the chip) on the cutting speed for four different steel-heating temperatures.

From the graph presented it is evident that, for a constant steel-heating temperature, there exists an optimum cutting speed at which the intensity of wear reaches a minimum value. At the optimum cutting speed, for all experimental steel-heating temperatures, we observe uniform wear along the flank surface of the milling-cutter tooth and sufficient stability of this wear, since precisely in this case that “favorable” temperature regime was attained under which the cutting properties of the hard alloy had the highest values (increased impact toughness). Departure from the optimum speed into the range of lower speeds was, as a rule, accompanied by chipping of the cutting edge, which accounts for the greater scatter of experimental points for speeds below 150 m/min.

An increase in cutting speed above the optimum also leads to an increase in relative wear, owing to the usual influence of speed on the wear process of the cutting tool. From the same figure it follows that, with an increase in the temperature of preliminary heating of the steel, there is a regular shift of the optimum cutting speeds toward the range of lower speeds, since it is natural that the “favorable” temperature regime for a higher steel-heating temperature can be reached at a lower cutting speed.

At the same time it should be emphasized that, as the temperature of preliminary heating of the steel is raised, the minimum of relative wear not only shifts toward lower cutting speeds, but also acquires ever smaller absolute values. This is illustrated by Table 1, which gives the values of the minimum relative wear corresponding to the optimum cutting speeds at different steel-heating temperatures. Table 1 was compiled from the same experimental data as Fig. 5.

Table 1

In the reduction of the magnitude of relative wear with increasing heating temperature there is manifested, in our opinion, the influence of the physicomechanical properties of the steel, changing with heating, and above all of its “abrading ability.”

Tool-life tests carried out with the use of hard alloys VK8 and T5K10 agree qualitatively quite satisfactorily with the results obtained for the hard alloy T15K6 [7].

Table 2 gives the results of several experiments in which the cutting properties of the hard alloys investigated were manifested to the best advantage. Table 2 indicates the grade of hard alloy, the machining conditions, the temperature of preliminary preheating, the tool life of the milling cutter obtained at this temperature, its comparison with the tool life in cold machining, and the productivity of the corresponding conditions per unit time (calculated per one tooth of the milling cutter), which characterizes—

Table 2

...with the minute feed and, finally, the productivity of the tool-life period, characterized by the volume of metal removed by the cutter tooth before it became dull when milling heated steel. The table presented also indicates the ratio of the width of the milled surface \((B)\) to the cutter diameter \((D)\) that occurred in the experiments.

The results of the investigations given in Table 2 show that, when milling heated stainless steel with carbide cutters, it is possible to increase substantially the life of the cutting tool in comparison with cold machining. According to the table, this increase in life ranges from 1.3 to 32 times.

The best results among the three grades of hard alloys were obtained when alloy T5K10 was used. In this case the highest productivity was achieved at optimal heating temperatures of the stainless steel, at comparatively low cutting speeds and large feeds per tooth.

Tool life is a decisive, but not the only, index of machinability. Let us consider the second characteristic of machinability—namely, the nature of the dynamic relationships observed in the process of cutting heated stainless steel.

Figure 6 presents the dependence of the mean circumferential force in face milling of steel 1Kh18N9T on its heating temperature, and Fig. 7 presents a similar dependence of the principal component—

Fig. 6.

—of the cutting force when turning stainless steel. The experiments were carried out at different cutting speeds at which no built-up edge was present.

It follows from the graphs shown in Figs. 6 and 7 that, for different cutting speeds, the cutting force may increase, decrease, or remain approximately constant within a certain interval of steel heating temperatures.

Let us examine the factors on which the magnitude of the principal component of the cutting force depends. In general form, for the principal component of the cutting force one may write the following functional dependence:

\[ P=\Phi(\sigma,\varepsilon,f), \]

where: \(\sigma\)—the strength characteristic of the steel;

\(\varepsilon\)—the degree of plastic deformation of the layer being cut;

\(f\)—the area of the metal layer being cut.

Passing to the specific cutting force, or keeping the cross-section of the cut constant, it may be noted that the magnitude of the cutting force can simultaneously be affected by two factors that change independently with heating of the steel. These are, first, the weakening of the steel as a result of the strength characteristics that decrease with preheating and, second, the influence of the degree of plastic deformation of the layer being cut, which changes with heating of the steel.

Table 3 presents the values of the physicomechanical characteristics of the steel at different heating temperatures. Here are given the values of the conventional yield strength \((\sigma_0)\), the exponent of the stress polytrope \((n)\) in compression, and the Brinell hardness \((H_B)\), obtained by us in special experiments; the values of the ultimate strength \((\sigma_{\mathrm{в}})\) at different heating temperatures for steel 1Kh18N9T are also given, according to data of A. A. Shmykov [13].

Fig. 7.

Table 3

Analysis of the data in Table 3 shows that, as the heating temperature of the steel increases, all the listed physicomechanical characteristics decrease monotonically.

We shall trace the change in the degree of plastic deformation of the layer being cut by the change in chip shrinkage (Fig. 8) at different cutting speeds and steel heating temperatures. From the figure presented it is clear that, for a cutting speed of 33 m/min, a sharply expressed maximum of chip shrinkage is observed in the region of steel heating temperatures of 280–300°.

As the cutting speed increases, the maximum of the curves shifts into the region of higher steel heating temperatures, while its absolute value decreases. It may be assumed that the character of the force relationships is governed by the combined influence of the degree of plastic deformation of the chip, which changes with heating of the steel, and the weakening of the steel with heating.

Thus, for a cutting speed of 33 m/min, when the steel was heated to 300°, its strength characteristics decreased, but the cutting force nevertheless increased because of the predominant influence of the greatly increasing degree of plastic deformation (the chip reduction increased approximately 2.5-fold), while the decrease in strength characteristics was reflected only in a slower growth of the cutting force. At a cutting speed of 190 m/min, although the chip reduction increased with increasing steel-heating temperature, because of the predominant influence of the strength characteristics decreasing in this case, a reduction in cutting force was observed.

Figures 9 and 10 respectively show the dependences of the coefficient of friction and the cutting temperature on the steel-heating temperature

Fig. 8.

at various cutting speeds. These dependences were obtained in the same experiments as the results presented above in Figs. 7 and 8.

Analysis of the dependences shown in Figs. 8, 9, and 10 makes it possible to conclude that the rule known in cutting theory [8], according to which, for a given work material and cutting-angle modes, equal temperatures correspond to equal values of the coefficient of friction and chip reduction, is valid only when cutting low-alloy steels. This rule does not extend to the cutting process of high-alloy steels, in particular steel 1Kh18N9T, in the heated state. This is evident, for example, from the joint consideration of Figs. 8, 9, and 10, from which it follows that, as the cutting temperature rises due to the steel-heating temperature, the chip reduction changes along a hump-shaped curve. However, as the cutting temperature rises due to an increase in cutting speed (vertical sections from top to bottom, Fig. 8), the reduction decreases at all experimental steel-heating temperatures.

At the same time, it should be emphasized that, in cutting heated stainless steel, over a sufficiently wide range of variation of cutting conditions, we observed an unambiguous relationship between chip reduction and the coefficient of friction. A change in chip reduction always qualitatively followed the corresponding change in the coefficient of friction between the chip and the rake face of the tool, which, in particular, can be observed when considering Figs. 8 and 9 [9].

Special experiments on the study of microroughnesses of the machined surface established that heating stainless steel

has little effect on changes in the surface finish during milling. As the heating temperature of steel 1Kh18N9T is increased, a certain decrease is observed in the magnitude of the microroughnesses of the machined surface. For cutting speeds at which the formation of a built-up edge is excluded, this decrease is more noticeable at the lower cutting speed; however, the surface finish with heating of the steel from 20 to 650°C remains within the limits of one finish class.

The present work is the result of a comprehensive study of the possibility of improving the machinability of stainless steel 1Kh18N9T by machining it in a heated state.

Fig. 9.

Fig. 10.

In this connection, three principal machinability indicators were studied: the tool life of the cutting tool, the magnitude of the forces and power expended in cutting, and the finish of the machined surface.

As a result of the investigations carried out, the possibility was established of improving the machinability of steel 1Kh18N9T by machining it in a heated state by the method of face milling with a carbide tool. The work indicates machining conditions under which it is possible to increase significantly the life of milling cutters by machining steel 1Kh18N9T in a heated state; dependences are given of the circumferential fo—

... during milling and of the principal component of the cutting force during turning of stainless steel.

REFERENCES

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5. Brushtein B. E. Cutting properties of sergonite as a function of its structure. Oborongiz, 1940.

6. Kuznetsov V. D. Physics of the Solid State. Vol. III, Tomsk, 1944.

7. Nassonov K. A. High-speed milling of EI1T steel in the heated state. (A study of machinability.) Dissertation. Tomsk, 1954.

8. Rozenberg A. M., Eremin A. N. Toward a theory of the metal-cutting process. Machine Tools and Instruments, No. 10, 1949.

9. Rozenberg A. M., Nassonov K. A. Factors influencing the process of deformation during cutting. Proceedings of TPI, Vol. 85, 1957.

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1. In the range of cutting speeds below 150 m/min, for alloy T15K6 we found a second maximum of \(m\) tool life, which appears at cutting temperatures not exceeding 950°C; moreover, the magnitude of this temperature depends on the cutting speed. The scope of the present article does not permit the presentation of additional experimental material. ↩