DRILLING OF HEAT-RESISTANT STEELS EI316, Kh25SNZD, AND 1Kh18N9T (EYa1T) WITH HIGH-SPEED STEEL DRILLS

A. M. Rozenberg, D. V. Kozhevnikov, A. G. Dolodarenko

Heat-resistant and stainless steels are widely used in modern engineering; among them, steels based on chromium-nickel occupy a significant place.

In the present work, a study was carried out of the process of drilling cast heat-resistant steels EI316 and Kh25SNZD (through the scale and without scale) and steel 1Kh18N9T (hot-rolled).

Drilling of steels EI316 and Kh25SNZD was performed mainly with high-speed drills made of steels R18, R18F2, R9K5, and R9K10. An attempt to use drills fitted with inserts of hard alloy VK8 was not successful. The cutting edges chipped rapidly; a high temperature developed in the cutting zone, and during dry cutting cases of unsoldering of the inserts were observed.

The high-speed drills had diameters of 17–20 mm, standard geometry, and sharpening of the cutting part. In the experiments the feed remained constant and was equal to 0.235 mm/rev.

All experiments were conducted with cooling by a 5% aqueous emulsion solution. The drilling depth was 2–3 diameters without the drill emerging from the workpiece. Drill wear was periodically measured directly in the machine spindle with a specially manufactured fixture.

The pattern of characteristic drill wear is shown in Fig. 1 (magnified 4 times). The wear land is visible as a white shiny strip, increasing in width along the cutting edge from the transverse cutting edge of the drill toward the periphery. On the front surface of the drill no wear was observed at all. However, on the rear surface of the drill (especially at the peripheral corners) the lands have a large width and are clearly visible. When measuring wear, the width of the wear land was determined at the middle of the cutting edge \((h)\) and near the corners \((h_{\mathrm{yr}})\).

For the case of investigating the optimal clearance angle, the results of measurements of the maximum wear value are given in Table 1.

The results of measurements of the maximum wear value in determining the optimal point angle are given in Table 2.

It is seen from Tables 1 and 2 that the values of drill wear \((h\) and \(h_{\mathrm{yr}})\) are not sufficiently stable; therefore it is not possible to adopt any one of them as the wear criterion. Continuing drilling after final blunting and the onset of vibration during—

Table 1

Table 2

leads to breakage of drills, to rapid melting of the corners and welding of the drills in the hole of the workpiece.

In connection with the above, the construction of all dependencies was carried out on the basis of terminal tool lives, which corresponds to the rule currently accepted for testing tools made of high-speed steels up to final dulling.

At present, new grades of high-speed steels have appeared in the tool industry: R9K5, R9K10 (with increased cobalt content), and R18F2 (with increased vanadium content), which are intended mainly for machining hard-to-machine and heat-resistant materials.

There are no studies in the literature on the use of new grades of high-speed steels for drilling heat-resistant steels.

In the first series of experiments, a comparison was made of the cutting properties of drills made of various grades of high-speed steels.

Fig. 1.

It was established that steel R9K10 has the best cutting properties when machining steels EI316 and Kh25SN3D (Fig. 2). Its use for machining heat-resistant steels is especially effective under severe drilling conditions. Thus, when drilling EI316 steel through scale, the cutting speed can be increased by a factor of 1.4–1.5 in comparison with drills made of R18 steel. In other cases (EI316 without scale and Kh25SN3D through scale and without scale), the cutting speed at constant tool life is higher by a factor of 1.1–1.15. When drilling 1Kh18N9T steel, the cutting properties of drills made of R18 and R9K10 steels are approximately equivalent.

Drills made of high-speed steel R9K5 proved to be less durable than the preceding high-speed steels.

Machining of heat-resistant steels at a tool life of 20 min and \(S = 0.235\) mm/rev should be carried out at the following cutting speeds: EI316 steel at 11–13 m/min, Kh25SN3D at 15–17 m/min (the lower value in the case of drilling through scale), and 1Kh18N9T steel at a speed of 25 m/min.

The high cutting properties of R9K10 steel and its good resistance to abrasive wear are associated with the increased hardness of the drills after heat treatment.

A comparison of the cutting properties of drills made of R18F2 and R18 steels shows that, when machining heat-resistant steels EI316 and Kh25SN3D, they are equivalent, while when machining 1Kh18N9T steel, drills made of R18F2 steel have a cutting speed 5–8% higher than drills made of R18 steel.

Fig. 2

b)

Axes: \(T\), min; \(V\), m/min.

Steel Kh25SN3D, without scale
\(S = 0.235\) mm/rev
Sharpening along a conical surface

\[ \text{R9N10 } \varnothing 19.5\text{ mm}, \qquad T=\frac{34.9 \cdot 10^{8}}{V^{6.7}} \]

\[ \text{R9K5F } \varnothing 17.5\text{ mm}, \qquad T=\frac{48 \cdot 10^{6}}{V^{5.5}} \]

\[ \text{P18 } \varnothing 19.4\text{ mm} \]

Axes: \(T\), min; \(V\), m/min.

Steel Kh25SN3D, after roughing
\(S = 0.235\) mm/rev
Sharpening along a two-angle plane

\[ \text{R9N10 } \varnothing 19.5, \qquad T=\frac{59.5 \cdot 10^{8}}{V^{7.12}} \]

\[ \text{R9K5 } \varnothing 17.5, \qquad T=\frac{142.6 \cdot 10^{8}}{V^{8.3}} \]

\[ \text{P18 } \varnothing 14.7, \qquad T=\frac{1134 \cdot 10^{6}}{V^{5.25}} \]

a)

Axes: \(T\), min; \(V\), m/min.

EI-316, after roughing
\(S = 0.235\) mm/rev

\[ \text{R9N10} \]

\[ \text{P18} \]

\[ \text{R9K5F} \]

Axes: \(T\), min; \(V\), m/min.

EI-316, without scale
\(S = 0.235\) mm/rev

\[ \text{P18 } \varnothing 19.4\text{ mm}, \qquad T=\frac{91.8 \cdot 10^{7}}{V^{11}} \]

\[ \text{R9K5 } \varnothing 19.5\text{ mm}, \qquad T=\frac{122.3 \cdot 10^{8}}{V^{8.15}} \]

\[ \text{R9N10 } \varnothing 19.5\text{ mm}, \qquad T=\frac{447.6 \cdot 10^{7}}{V^{6.72}} \]

Fig. 3a

Vertical axis: \(T_{\min}\)

Horizontal axis: \(V,\ \mathrm{m/min}\)

Inset text:

Double sharpening
on two planes

EI316 without scale

\[ 2\varphi \approx 118^\circ \]

\[ 2\varphi_1 \approx 70^\circ \]

\[ \psi \approx 55^\circ \]

\[ S = 0.235\ \mathrm{mm/rev} \]

Curve labels: R18; R9K10

Fig. 36

In the second series of experiments the task was to find a more productive method for sharpening drills. The following methods of sharpening the back surface of drills were investigated: helical, conical, and two-plane. It was established that sharpening drills on a helical surface on a model 3659A semi-automatic drill-sharpening machine creates excessively large clearance angles near the transverse cutting edge of the drill, as a result of which chipping of the cutting edges is observed.

A comparison of the cutting qualities of drills sharpened on a conical surface (the Washburn method) and on two planes shows that they are equivalent over a wide range of cutting speeds. In subsequent experiments studying the sharpening geometry, two-plane sharpening of the drill back surface was used. The experiments were carried out with drills made of P18 steel when machining heat-resistant steels without scale (the scale was removed by planing).

The following geometric parameters were investigated: clearance angle, point angle, double sharpening, and helix angle.

It was established that the optimum clearance angles for drills made of P18 steel when machining EI316 steel are \(\alpha = 15^\circ\), and when drilling Kh25SNZD steel, \(\alpha = 12^\circ\); the optimum point angles are, for EI316 steel, \(2\varphi = 120^\circ\), and for Kh25SNZD steel, \(2\varphi = 110^\circ\). The values of the angles \(\alpha\) and \(2\varphi\) for EI316 steel were checked at three cutting speeds: 11.83; 12.6; 13.1 meters per minute.

The influence of double sharpening of drills on their life was investigated. The results are given in Fig. 3.

For EI316 steel, drills made of P18 and P9K10 steels were tested, and for Kh25SNZD steel, only drills made of P18 steel. Sharpening was carried out with the angles: \(2\varphi = 118^\circ\) and \(2\varphi_1 = 70^\circ\); the width of the edges of the secondary cone was \(C = (0.1 \div 0.15)D\); the clearance angles had the optimum values. For EI316 steel, a twofold increase in life was achieved, and for Kh25SNZD steel, a 25-fold increase, or, at a constant life of \(T = 20\) min, the cutting speed increased by a factor of 1.4. The increase in life occurs due to strengthening of the most heavily loaded peripheral corner of the drill.

Drills with different helix angles were tested. In the study the helix angle was taken as: 26, 32, and \(44^\circ\). The first two angle values did not lead to an increase in drill life. The optimum helix angle should be considered \(\omega = 44^\circ\), at which the cutting speed increases by 10% (\(T = 20\) min).

Among new drill designs, the following were tested: drills with the cutting edges arranged in one plane, and drills of P. Ya. Ovchinnikov’s design with rolled holes for internal cooling. Drills with internal cooling were made of P9 steel and showed an increase in cutting speed when machining EI316 steel by a factor of 1.2, and Kh25SNZD steel by a factor of 1.1, in comparison with ordinary drills made of P9 steel and with emulsion supplied from above; this is explained by better delivery of the cooling liquid to the cutting zone, a reduction in temperature, and improved chip removal.

For drills with main cutting edges arranged in one plane, made of P18 steel, an increase in productivity was achieved. Thus, drills of \(\varnothing 17\) mm, when machining EI316 and 1Kh18N9T steels, make it possible to work at speeds 1.25 times higher than drills of conventional design.

Thus, the drilling experiments show that, when using the most productive grades of high-speed steels, optimum sharpening geometries, and drill designs, it is possible to obtain a 1.5-fold increase in cutting productivity when machining cast heat-resistant steels by drilling.