ON DRILLING EI-316 STEEL WITH DRILLS EQUIPPED WITH CEMENTED CARBIDE INSERTS

D. V. KOZHEVNIKOV, Yu. V. SHCHEPETILNIKOV

(Presented by the joint scientific seminar of the departments of machine tools and metal cutting and of mechanical-engineering technology)

Steel EI-316 belongs to the class of heat-resistant alloys, the machining of which by cutting, owing to their poor thermal conductivity in comparison with structural steels, presents considerable difficulties.

Despite the advantage of cemented carbides over high-speed steels in wear resistance and red hardness, cemented carbides are almost never used in drilling steel. This is explained by their lower bending strength than that of high-speed steels and by their sensitivity to thermal stresses [1], which leads to chipping and spalling of the cemented carbide inserts during drilling.

A significant increase in the productivity and durability of drills equipped with cemented carbide inserts, when drilling heat-resistant and stainless steels, can be obtained by replacing flood cooling with internal pressure cooling [2] and by using new grades of cemented carbides having increased bending strength and low sensitivity to thermal stresses.

In the metal-cutting laboratory of TPI, a study was carried out of the performance of drills equipped with cemented carbide inserts when drilling cast heat-resistant steel EI-316, containing 0.18% C; 12% Ni; 26% Cr; 0.75% Si; 0.46% Mn. The experiments were conducted with standard drills of Ø12 mm and Ø20 mm equipped with VK8 cemented carbide inserts, as well as with Ø20 mm drills with rolled holes and VK8 cemented carbide inserts manufactured by the Sestroretsk Tool Plant, and Ø12 mm drills with TT10K8B alloy inserts. For comparison in productivity, experiments were carried out with drills made of high-speed steel. The drills were sharpened on two planes and had the following geometry: point angle \(2\varphi = 120^\circ\), clearance angle \(\alpha = 12^\circ\); the chisel edge of the drills was thinned to a length equal to 0.09–0.1 of the drill diameter. Drilling was performed to a depth equal to two diameters for Ø20 mm drills and three diameters for Ø12 mm drills. A 7% solution of emulsol in water was used as the cooling fluid.

As is known, feed has a major influence on the durability of drills equipped with cemented carbide inserts. The investigations carried out showed that drills equipped with TT10K8B cemented carbide inserts, unlike other alloys, operated reliably with the formation of a normal wear land of 0.5 mm on the flank face at

increasing the feed to 0.09 mm/rev. In this case, a wear land of 0.5 mm on the flank face corresponded, as a rule, to a complete loss of the drill’s cutting properties: considerable rounding of the cutting edges was observed, with small chippings along the entire length of the cutting edges. An attempt to continue drilling with such drills resulted in a significant rise in temperature in the cutting zone and in complete melting of the solder (copper). It should be noted that the small chippings and flaking of the cemented carbide sometimes observed during drilling did not subsequently lead to the formation of cracks or chipping of the inserts, which is characteristic of VK8 cemented carbide. When drilling with feeds above 0.09 mm/rev, vibrations and even breakage of the drill body were observed as a result of the increase in cutting forces.

Experiments carried out with standard drills Ø 12 mm with VK8 cemented-carbide inserts showed that increasing the feed above 0.057 mm/rev leads to the appearance of chipping and flaking on the cutting edges. Continuing drilling with drills that had even slight chipping usually led to complete destruction of the cemented-carbide inserts. For VK8 cemented carbide, a wear land on the flank face of 0.3–0.4 mm should be considered limiting wear. At larger amounts of wear, even when drilling with feeds below 0.057 mm/rev, chipping of the cemented carbide was observed.

Analysis of wear measurements showed that VK8 cemented carbide, despite its greater tendency toward brittle fracture, has greater wear resistance than TT10K8B alloy. This explains the greater durability of standard drills with VK8 inserts compared with drills with TT10K8B inserts when drilling at low feeds and cutting speeds (Fig. 1a, c). At increased cutting speeds, TT10K8B cemented carbide performs better in terms of durability, which is explained by the lower sensitivity of TT10K8B alloy to thermal stresses and its greater bending strength [1]. The comparison in productivity shown in Fig. 2a indicates that drills equipped with cemented-carbide inserts have an advantage over high-speed-steel drills only at increased cutting speeds. However, at high cutting speeds the durability of the drills is low because flood cooling is ineffective. In this case the coolant practically does not reach the cutting zone because of the high velocities of the flowing chip and the intense steam generation when it comes into contact with the heated parts of the drill. This circumstance leads to a significant increase in temperature in the cutting zone and, consequently, to intensive adhesive and diffusion wear and to an increase in thermal stresses in the cemented-carbide insert.

The above-mentioned negative phenomena can be eliminated by supplying coolant through the internal channels of the drill directly into the cutting zone. In this case, drilling even at high cutting speeds and at feeds limiting for cemented carbide proceeds stably, without chipping. Thus, when experiments were conducted over a wide range of cutting speeds with a feed of 0.1 mm/rev (Fig. 3), using Ø 20 mm drills with cooling holes, equipped with VK8 cemented carbide, and with coolant supplied to the cutting zone under a pressure of 30 kg/cm², chipping and flaking of the cemented carbide on the cutting edges were absent even with a flank wear land of 0.5–0.6 mm. This is apparently explained by the absence of adhesive wear as a result of the significant reduction in temperature on the contact surfaces under internal pressure cooling. Chip removal with this method of coolant supply ...

Fig. 1. Dependence of the tool life of high-speed-steel and cemented-carbide drills on cutting speed

• a) \(T,\ \mathrm{min}\) versus \(v,\ \mathrm{m/min}\). Legend: \(\varnothing 12\ \mathrm{mm}\); \(S = 0.057\ \mathrm{mm/rev}\); ○ — TT10K8B; + — VK8.
• b) \(T,\ \mathrm{min}\) versus \(v,\ \mathrm{m/min}\). Legend: \(\varnothing 12\ \mathrm{mm}\); R18; \(S = 0.235\ \mathrm{mm/rev}\).
• c) \(T,\ \mathrm{min}\) versus \(S,\ \mathrm{mm/rev}\). Legend: \(\varnothing 12\ \mathrm{mm}\); \(v = 55.2\ \mathrm{m/min}\); ○ — TT10K8B; △ — VK8.

Fig. 2. Productivity of the drilling process with high-speed-steel and cemented-carbide drills at different tool lives

• a) \(S,\ \mathrm{mm^3/min}\). Legend: TT10K8B, \(\varnothing 12\ \mathrm{mm}\); VK8, \(\varnothing 12\ \mathrm{mm}\); R18, \(\varnothing 12\ \mathrm{mm}\). Groups: \(T = 60\ \mathrm{min}\), \(T = 30\ \mathrm{min}\), \(T = 10\ \mathrm{min}\).
• b) \(S,\ \mathrm{mm^3/min}\). Legend: R18, \(\varnothing 19.4\ \mathrm{mm}\); VK8, \(\varnothing 20\ \mathrm{mm}\), with holes. Groups: \(T = 40\ \mathrm{min}\), \(T = 20\ \mathrm{min}\), \(T = 10\ \mathrm{min}\).

Fig. 3. Dependence of the tool life of high-speed-steel and cemented-carbide drills on cutting speed

Vertical axis: \(T\), min
Horizontal axis: \(V\), m/min

Legend:

• ▲ — R9, \(\varnothing 19\) mm, flood cooling
• • — R9, \(\varnothing 19\) mm, internal cooling
• ○ — R9, \(\varnothing 19\) mm, \(P = 25\ \text{kg}/\text{cm}^2\)
• ⊕ — R18, \(\varnothing 19.4\) mm, flood cooling
• □ — VK8, \(\varnothing 20\), \(P = 30\) atm; \(S = 0.1\ \text{mm}/\text{rev}\)

For the first four curves: \(S = 0.235\ \text{mm}/\text{rev}\).

removal proceeded more freely than with cooling by pouring, and twisting and packing of chips in the drill flutes were absent. As a result, experiments with these drills showed an increase in productivity, compared with drills made of high-speed steels R18 and R9, by a factor of 2–2.5 (Fig. 2b and Fig. 3). Drilling with standard drills of diameter \(20\) mm fitted with VK8 inserts, under the same cutting conditions but with cooling by pouring, proceeds very unstably because of chipping of the cemented-carbide inserts after the very first holes.

Thus, as a result of the investigations carried out, it has been established that, for productive drilling of heat-resistant steels of the EI-316 type, harder-bending-strength cemented carbides TT10K8B and high-pressure internal cooling should be used.

The use of standard drills fitted with cemented-carbide inserts for drilling heat-resistant steels with cooling by pouring is inadvisable.

References

1. G. S. Kreimer. Strength of Cemented Carbides. Metallurgiya, 1966.
2. High duty twist drilles. “Tooling,” 15, No. 18, 1961.