Manufacture of Twist Drills from Cast High-Speed Steel

I. O. Khazanov, Yu. K. Korzunin, G. G. Zakharova

(Presented by the scientific seminar of the cluster of departments: metallurgy; equipment and technology of heat treatment of metals; equipment and technology of welding production; and metal technology)

Existing methods for manufacturing drills provide for the use of hot-rolled bar stock as the starting blank for a drill.

The bars are cut to the length of the blank for the working part of the drill; then a shank made of steel 45 is welded to them. The welded blanks are annealed, turned, and ground, after which they are either rolled on a sector-rolling mill with subsequent twisting, or a milled drill is made from the turned blank [1].

The manufacture of drills, as well as other types of tools, from high-speed steels is invariably associated with the formation of a considerable quantity of waste of scarce and expensive high-speed steel. This waste is obtained in the form of chips, small excess pieces of the working part of the drill, and simple piece waste that can no longer be used in tool production because of its substandard small dimensions. In the best case, this waste is remelted into cutters and cast drills, but in most cases it is turned in for remelting. The plant loses the metal it needs and suffers losses because of the difference in the cost of the waste as compared with standard metal.

By remelting waste into other types of tools at tool plants, it is possible to increase tool output significantly, which will give the plants additional profit through metal savings.

However, a cast tool, along with good qualities, has a substantial drawback: it has considerable brittleness, and the principal form of its wear is chipping of the cutting edges [2]. This drawback apparently restrains the increase in output of such types of cast tools as drills, milling cutters, counterbores, etc.

The most promising is the production of such a mass-consumption tool as drills. Our tool industry produces hundreds of millions of drills annually, and the demand for them increases every year.

The authors of the present work developed, at the Tomsk Cutting Tool Plant, a new technology for manufacturing drills of 20 mm and larger from cast blanks.

The proposed technology provides for remelting high-speed-steel waste and chips as the starting material for drill blanks; rods for the required drill sizes are cast from this material.

The cast rods are annealed and cut into short blanks corresponding to the working part of the drill; the entire subsequent technology for manufacturing the drill coincides with the factory technology for manufacturing drills by the sector-rolling method [1].

It is known that steel R18 has low plasticity and is the most difficult to deform in comparison with steels R12 and R6M3 [3, 4].

Therefore, at present, in most plants producing drills by the sector-rolling method, steels R12 and R6M3 are used mainly. In the cast state, high-speed steel has very limited plasticity [5]; therefore, at metallurgical plants, ingots of small weight (from 200 to 500 kg) with a strictly limited carbon content are used for direct rolling, since this carbon content determines the plasticity of cast high-speed steel. The strongly developed structure of the carbide eutectic in large ingots (weighing 300 kg and more) [5] forces manufacturers first to forge them, after which the forged blanks are rolled.

In our case, small-volume and low-weight castings are used as blanks for rolling drills; in these castings, owing to the high cooling rate during crystallization, the grain is strongly refined and coarse eutectic carbides are not formed, which increases the plasticity of cast high-speed steel.

Remelting of steel scrap was carried out in an induction furnace with an MGP-102a machine generator, in an acid crucible with a capacity of 150 kg. The fine charge was loaded into the crucible and covered from above with a layer of broken glass. After the first portion of charge had melted, briquetted chips were introduced into the crucible up to 30% of the melt weight. Compensation for carbon, molybdenum, tungsten, chromium, and vanadium losses was carried out using high-carbon high-speed cast iron of the following composition: carbon—5–5.5%; chromium—4%; tungsten—6–12%; vanadium—2.3%; molybdenum—3.5%. The composition of the high-carbon cast iron was selected depending on the composition of the charge being remelted. When smelting steel from an R6M3 charge, the high-carbon cast iron was melted on the basis of R6M3 steel; for R12 steel, cast iron was smelted from R12 steel. This facilitates make-up of the metal being melted, since in high-speed cast iron, with additional introduction of ferroalloys, any content of burning-out alloying elements can be obtained.

The remelted steel R6M3 or R12, brought to the standard composition, was poured into metal molds, Fig. 1, which are provided with a feeder part. The feeder part of the mold is made by the method used for preparing shell molds. The chill molds, on the ends and working surfaces, are coated with a nonstick dressing of chromium oxide or aluminum oxide in mineral or vegetable oil.

Fig. 1. Metal mold for casting blanks for drills

Before the first pouring, the chill molds are heated to 400–500°C. The chill molds are quick-detachable, and their number was determined by the volume of one ladle of steel. In our case, 10 chill molds were used, with the weight of one casting 2.8–3.0 kg, which amounts to 30 kg of steel in one ladle. After the first ladle had been poured, before the second pouring the foundrymen had time to disassemble and prepare the chill molds for the next pouring. With good application of anti-burn-on paint to the chill mold, 7–8 pourings can be made without additional painting, only with touching up the upper part of the chill mold at the point where the stream of liquid metal strikes. Touching up was performed immediately after disassembling the hot chill mold, using a rag swab. The paint dries very quickly, and the chill mold is again ready for pouring. The solidification rate of the casting in chill molds for blanks 30 mm in diameter was 14 deg/sec. While the last chill molds were being poured, the first ones were already being disassembled, the cast blanks were removed from them, the chill molds were lightly touched up and reassembled for pouring. The operation of reassembly and removal of castings took 4–6 minutes. Since the shape of the castings is simple, the durability of the chill molds is high. The chill molds were made of steel 10, St. 20. Free shrinkage of the castings is provided in the chill molds; otherwise hot cracks may appear in them. The chill molds were installed in a row on a bottom plate. When metal was poured into chill molds heated to 400–550°C, the castings obtained were free of pores and had a clean surface. The weight of the riser part of the casting was 0.7–0.8 of the weight of the usable bar, although this requirement is fulfilled differently in each individual case, depending on the size of the casting. The principal factor for controlling the quality of the casting should be the absence of a shrinkage cavity in the sub-riser part of the castings.

In designing the chill molds, no thermal calculation was made; in our case the weight of the chill molds was 3–4 times the weight of the casting. The chill molds have a good appearance after more than 100 pourings have been made in them, and there was not a single case of their failure due to burning. If burning appears, the working cavity can be bored out for a larger casting size, which will increase the service life of the chill molds.

The loss by oxidation of the elements in our case was: carbon—10%, chromium—3–4%, molybdenum—5%, vanadium—6–7%; tungsten practically did not burn out. A 150 kg heat lasted 1.5 hours. A heat on an LPZ-60 high-frequency unit weighing 50 kg lasted about one hour. The first experimental heats were made on an LPZ-60 unit with a weight of 20 kg; the melting time was 20–25 minutes.

The castings obtained in the chill mold have a very fine grain and a thin network of carbide eutectic, Figs. 2, 3. The hardness of the blanks cast in the chill mold is HRC 56; therefore they must be annealed, since they are to undergo machining. Annealing was carried out in shaft furnaces with sand packing. To remove scale and the decarburized layer, a technological allowance for turning was provided in order to obtain a high-quality blank. The hardness of the castings after annealing is HB—260 kg/mm². The fine-grained structure of the castings made it possible to obtain drills from them by the method of direct rolling.

The cast blanks obtained were rolled on a sector-rolling mill according to the plant technology. After rolling and twisting, the drills were annealed together with the plant batch of drills according to the standard regime [6]. The presence of a decarburized layer was checked by Sadovskii’s method and was not detected. After annealing and subsequent machining, the drills underwent heat treatment, hardening and tempering according to the standard regimes for steels R6M3 and R12.

Drills in finished form with a hardness of HRC—63–64.5 were tested for torsion and bending in comparison with drills made from rolled blanks. The torsional work for 26 mm drills made from cast blanks was 78 kg·m; drills made from rolled blanks showed a torsional work of 77 kg·m. The bending force for drills made from rolled blanks proved to be higher by 350–400 kg. The drills passed laboratory cutting tests

Fig. 2. Microstructure of R6M3 steel cast in a metal mold, ×640, normal annealing

Fig. 3. Microstructure of R12 steel cast in a metal mold, ×640, normal annealing

according to GOST 2034—64 and had no traces of wear or chipping. In external appearance, drills made from cast blanks do not differ from factory-made ones.

From the results of tests of the mechanical and cutting properties it follows that drills made from cast blanks meet the requirements of GOST 10902—64.

The manufacture of drills by the proposed technology from high-speed-steel waste will give tool plants significant savings.

REFERENCES

1. A. M. Brandelis. New Technology for the Production of Twist Drills. Mashgiz, Moscow, 1953.

2. N. S. Degtyarenko. Cast Cutting Tool and Its Field of Application. VNII, Moscow, 1959.

3. N. M. Fadoshina, E. I. Malinkina. Stanki i instrument, No. 6, Moscow, 1966, p. 30.

4. G. A. Kossovich, Yu. A. Geller. MITOM, No. 5, Moscow, 1964, pp. 3–9.

5. Yu. M. Chizhikov. Processes of Pressure Working of Alloy Steels and Alloys. Metallurgiya, Moscow, 1965, p. 248.

6. Yu. A. Geller. Tool Steels. Metallurgizdat, Moscow, 1968, p. 396.