Automatic Surfacing of Cutters with High-Speed Steel under a Layer of Flux

N. A. Balakin and N. D. Tyuteva

There are several methods for manufacturing cutting tools. Methods of automatic surfacing of cutting edges are of great importance. Methods are known for gas surfacing, electric-arc surfacing, and surfacing with carbon and metallic electrodes.

It is possible to surface with a metallic electrode with a protective coating or under a layer of flux. The latter method must be automated, and it is especially suitable for manufacturing the blades of cutters. Automatic welding by this method is carried out under a layer of flux, which eliminates the possibility of burnout of the principal elements. As the flux one may use inexpensive blast-furnace slag (from blast furnaces operating on charcoal), under a layer of which we surfaced the blades of turning cutters.

Experimental Part

Surfacing is performed on a holder made of tool steel with a carbon content of 0.4–0.5%. Automatic electrode feed is carried out on a welding machine with a constant wire-feed speed. The holder is made of the usual type, with a milled recess for the deposited metal. The recess for surfacing must not have sharp transitions. On average, the surfacing depth is 5–7 mm. Before surfacing, the cutter (holder) is molded in graphite or carbon plates. Graphite plates are better, since they burn out much less and last longer. Only the place on the holder where the cutting blade is deposited is molded. The molding plates enclose the welding place on three sides and are held by a clamp or by a simple device made for this purpose.

Technique for Surfacing Cutters with an Automatic Machine

The electrode is steel RF-1. Surfacing is carried out under a layer of granular flux EMK-31. If, during automatic surfacing, the electrode is set normal to the surface of the cutter on which surfacing is being done, poor fusion is obtained (weak fusion of the deposited steel with the holder). The reason for lack of fusion is insufficient heating of the surface of the part, since the arc burns predominantly on the liquid deposited metal, as a result of which unfused places are obtained. To eliminate this phenomenon, the electrode was given an inclination at an angle \(\alpha = 70^\circ\) in the direction opposite to the movement of the electrode. An electrode set at an angle \(\alpha = 70^\circ\), while ensuring good fusion with the holder, melted its surface to a slight depth, which contributed to the complete elimination of alloying of the deposited surfacing by the molten...

metal of the holder. This is confirmed by metallographic investigations.

In the deposited metal there are no pores or lack of fusion, and it is completely homogeneous. As already indicated, with this welding method the holder is not subjected to particularly intense heating, while surfacing is carried out with a current of \(I = 400\) A with an electrode diameter of 4 mm. The surfacing speed is many times greater than in surfacing by the Benardos method (with a carbon electrode); therefore the deposited metal cools very rapidly owing to better heat removal into the cold holder. The accelerated cooling led to the formation of a fine-grained structure in the deposit. This latter circumstance significantly improved the cutting qualities of cutters surfaced by the proposed method.

Hardness tests showed that the surfaced cutter does not require hardening; treatment is limited only to triple tempering to temperatures (successively) of 560, 570, and 580°C. In surfacing under a layer of flux, as is known, oxidation, carburization, nitriding, or burn-out of alloying elements is completely eliminated; consequently, the chemical composition of the deposited steel corresponds to the chemical composition of the electrode made of steel RF-1. Tempering of the deposits gives hardness values very close to the hardness after tempering of forged high-speed steel.

In automatic welding under a layer of flux it is possible to alloy the deposits with such elements as carbon, cobalt, molybdenum, boron, nitrogen, etc. The deposits obtained are convexities, the edges of which protrude slightly beyond the limits of the holder. The excess of the deposited portion is removed with abrasive wheels, and the blade is given the usual geometry of a turning cutter.

Heat Treatment of Surfaced Cutters

After surfacing, the cutters are cooled in air. Air cooling constitutes hardening for the cutting blade made of high-speed steel. Thus, the heat treatment of cutting surfaced blades is reduced only to double and triple tempering, with holding at the tempering temperature for one hour (Table 1). In this case the hardness \(R_c = 55\) after surfacing increases to \(R_c = 65\).

Table 1

Change in hardness with tempering temperature of cutters surfaced automatically

Investigation of the Macro- and Microstructure of Surfaced Blades

Investigation of the macrostructure indicates a sufficiently good bond between the base metal and the deposited metal, the absence of lack of fusion, and the formation of a dense, bubble-free deposit.

The microstructure of the deposited blade is a network of ledeburitic carbides; moreover, the network of primary carbides may be

of a sufficiently massive and coarse, or rather fine, structure, depending on the conditions of crystallization (cooling). In the spaces between the network of eutectic carbides there are martensite and austenite.

Fig. 1

Fig. 2

In Figs. 1, 2, and 3 the microstructure of high-speed steel is shown immediately after surfacing. Etching with nitric acid.

Fig. 3

Fig. 4

In Fig. 4 is the microstructure of the cutter after double or triple tempering at \(t = 570^\circ\mathrm{C}\). Etching with sodium picrate. A considerably larger quantity of secondary carbides is visible.

Cutting Test

The automatically surfaced cutters were tested on ShKh-15 steel with hardness \(H_a = 200\) (Brinell). The depth of cut was 3 mm; the feed was 0.274 mm/revolution.

The geometry of the cutting elements was as follows: rake angle 18°; clearance angles 10°; principal cutting-edge angle in plan 75°; auxiliary cutting-edge angle in plan 15°; nose radius 1 mm.

The durability of the surfaced cutter was compared with the durability of a cutter made of forged high-speed steel, heat treated according to the following regime: quenching from 1315°C, tempering at 560°C (twice, with a one-hour holding time each).

The tests showed that a cutter produced by automatic electrode-arc surfacing is not inferior in durability to an ordinary forged cutter.

Conclusions

The method of automatic electrode-arc surfacing of cutters under a thick layer of flux has a number of advantages in comparison with other known methods of autogenous surfacing, namely:

1. A thick layer of slag (flux) prevents changes in the chemical composition of the deposited metal.

2. The cutter does not require heat treatment, except for tempering, since quenching is carried out during cooling under the same conditions.

3. In cutting properties, the cutter is not inferior to ordinary (forged) cutters made of high-speed steel, produced by forging followed by quenching and tempering.