COBALT ALLOYING OF CAST HIGH-SPEED STEEL WITH HIGH TUNGSTEN AND VANADIUM CONTENT

V. A. KASHCHUK

(Presented by Prof. Dr. A. N. Dobrovidov)

In recent years, production of cast cutting tools has been established at a number of plants in the Union. In manufacturing tools by casting:

a) the amount of machining is significantly reduced;
b) the production cycle is shortened;
c) consumption of high-speed steel is reduced;
d) an additional possibility is opened for improving the cutting properties of the tool by alloying the high-speed steel during the casting process with a number of components whose use had been limited by the need to process ingots by pressure.

Among the various methods for producing cast cutting tools of simple geometrical form and small dimensions, the greatest technological advantage is offered by the method of centrifugal casting into permanent metal molds (chill molds). With this manufacturing method it is possible to obtain a tool in an almost finished form, requiring no machining except grinding and sharpening.

Owing to the high cooling rate in the chill mold, the tool in most cases is hardened during the cooling of the casting, and its heat treatment consists of tempering. Since hardening takes place during the cooling of the casting, that is, from the maximum possible temperature, a larger quantity of alloying elements passes into the solid solution (austenite) than in the hardening of a forged tool, as a result of which the red hardness and cutting properties of high-speed steels are increased [1—5].

High-speed steels with a high vanadium content surpass ordinary high-speed steels of the R18 type in cutting properties; moreover, it should be taken into account that vanadium has an effective action only with a simultaneous increase in the carbon content in the steel [6—8].

Cobalt high-speed steels are distinguished by still higher cutting properties [9—11]. Cobalt dissolves in the metallic matrix, slows diffusion processes, and promotes an additional increase in red hardness. Cobalt and vanadium intensify the dispersion hardening of hardened high-speed steel.

In the present work, cast high-vanadium steels with a tungsten content of 20% were alloyed with cobalt over a wide range of concentrations (up to 27.6%) in order to identify steels with high cutting properties.

Melting and Casting Technology

The experimental steels were melted in a high-frequency furnace with an AZ-46 tube generator. The crucible, with a capacity of 2 kg, was made from a mixture of 80% quartz sand and 20% refractory clay. Factory scrap of R18 steel, ferroalloys, and “high-speed cast iron”—a specially carburized high-speed steel with 4.5% C—were used as the charge.

Melting was carried out under an acid slag—melted broken glass. Before pouring, the steels were deoxidized with aluminum (0.1%). To refine the structure, titanium (0.22%) was introduced into the steel in the form of ferrotitanium. From the experimental steels, plates with a cross section of \(15 \times 8\) mm were cast on a centrifugal machine with a vertical axis of rotation and an adjustable number of revolutions into permanent metallic molds.

The chemical composition of the experimental steels is given in Table 1.

Table 1

Investigation of the Structure in the Cast State

The cutting properties of a cast tool are determined by its structure, which forms during crystallization of the casting and depends on the physicochemical state of the steel, the quality of the casting, etc.

Steel of heat 8, poured into a chill mold, did not harden during cooling of the casting. Apparently, the cooling rate in the chill mold was below the critical hardening rate of this steel, and during cooling the austenite decomposed above the martensitic transformation point.

The microstructure of steel of heat 8 (Fig. 1) consists of products of austenite decomposition above the martensitic transformation temperature, carbide eutectic, and retained austenite. The hardness of the steel after casting is \(53.0\,H_{\mathrm{Rc}}\).

As a result of alloying with cobalt, cast high-vanadium high-speed steel with 20% W acquires the ability to harden during cooling in the chill mold. Thus, already after the addition of 5.65% Co the steel after casting has a hardness of \(63.5\,H_{\mathrm{Rc}}\), and after the addition of 12.48% Co—\(64.0\,H_{\mathrm{Rc}}\). The microstructure of steels of heats 9 and 10 consists of cryptocrystalline martensite, retained austenite, and carbide eutectic (Fig. 2).

* The content in all heats was: \( \mathrm{Mn} < 0.4\% \); \( \mathrm{Si} < 0.4\% \); \( \mathrm{S} < 0.003\% \); \( \mathrm{P} < 0.003\% \).

Fig. 1. Microstructure of steel, melt 8, after casting, ×440.

Fig. 2. Microstructure of steel, melt 10, after casting, ×440.

The microstructure of the steels of heats 11–13 (Fig. 3) consists of the same constituents as the structure of the steels of heats 9–10. However, it should be noted that, as the cobalt content in the steel increases, the amount of retained austenite after casting increases, while the amount of martensite decreases, which is confirmed by the decrease in the hardness of the steel. Thus, the steel of heat 11 already has a hardness of 60.0 \(H_{Rc}\), the steel of heat 12—52.0 \(H_{Rc}\), and the steel of heat 13—48.0 \(H_{Rc}\).

In studying the macrostructure, the steels of heats 12 and 13 show a definite grain orientation characteristic of dendritic structure. In the steels of the other heats, no dendritic structure is visible.

Fig. 3. Microstructure of the steel of the heat after casting, ×440.

Behavior of Castings During Tempering and Red Hardness

When the castings were cooled in chill molds, as a result of the high cooling rate, all the experimental steels containing cobalt were hardened. The effect of cobalt on the stability of retained austenite can be most clearly revealed by studying the hardness of the steel during repeated tempering.

Specimens of all steels (3 from each heat), measuring \(8 \times 15 \times 20\) mm, after casting were subjected to sevenfold tempering at \(560^\circ\text{C}\), with a holding time of 1 hour. The results of hardness measurements after tempering are given in Table 2.

It is evident from Table 2 that the steel of heat 8, during sevenfold tempering at \(560^\circ\text{C}\), did not increase its hardness above 55.0 \(H_{Rc}\). Additional tempering at various temperatures also did not produce an increase in hardness. As indicated above, this steel is not hardened when cooled in chill molds. To harden the steel of heat 8, more intensive cooling is required than can be provided by steel molds. After quenching in oil from \(1290^\circ\) and triple tempering for 1 hour each at \(560^\circ\text{C}\), the hardness of this steel increased to 65.0 \(H_{Rc}\).

It is also evident from Table 2 that cobalt increases the stability of retained austenite during tempering. If in the steel of heat 9 the maximum hardness was observed after one tempering, in the steel of heat 11—after triple tempering, then in the steel of heat 13, after sevenfold tempering at \(560^\circ\), it is not possible to raise the hardness value above 51.5 \(H_{Rc}\).

Table 2

After carrying out additional temperings at various temperatures for the steel of heat 13, the following tempering schedule was selected (Table 3).

Table 3

In determining the red hardness of the steel, the method developed by Yu. A. Geller was used. According to this method, red hardness is characterized by the maximum temperature of fourfold heating with holding for 1 hour, up to which the steel retains a hardness not lower than 60.0 $H_{RC}$. In determining red hardness, separate specimens were used for each heating temperature. The red-hardness values are given in Table 4.

Table 4

Cutting tests

Cutting tests were carried out on lathe tools by the method of longitudinal turning of 9KhS steel with a hardness of 240 HB, at a cutting depth of 2.5 mm and a feed of 0.5 mm/rev, without cooling.

From plates cast from the experimental steels, inserts (8×15×25 mm) were cut off with an abrasive wheel; these were then mechanically fastened in a specially made holder and served as the cutting part of straight turning tools.

The tools were ground to the following geometry:

front rake angle . . . . . . . . . . $\gamma = 12^\circ$
back clearance angle . . . . . . . . $\alpha = 8^\circ$
auxiliary back clearance angle . . . $\alpha' = 8^\circ$
principal angle in plan . . . . . . . $\varphi = 45^\circ$
auxiliary angle in plan . . . . . . . $\varphi' = 15^\circ$
nose rounding radius . . . . . . . . $r = 1.0\ \text{mm}$

Cutting tests were carried out at a cutting speed of 32 m/min.*)
For comparison, inserts made of forged steel R18 were also tested.
The criterion of blunting was taken to be complete seating of the tool.

The results of the cutting tests are given in Table 5.

Table 5

Annealing of steels

To determine the annealability of the experimental steels, isothermal annealing was carried out according to the following schedule:

a) heating to a temperature of 870°C;
b) holding at a temperature of 870°C for 3 hours;
c) cooling to a temperature of 740°C at a rate of 30–50°C per hour;
d) holding at a temperature of 740°C for 5 hours;
e) cooling to a temperature of 550°C at a rate of 30–50°C per hour;
f) further cooling in air.

*) This cutting regime was chosen specifically to reveal the behavior of the experimental steels under severe operating conditions.

This annealing regime is widely used in the practice of heat treatment of high-speed steel and is called softening annealing. The hardness after annealing is given in Table 6.

Table 6

Conclusions

1. The steel under investigation, without cobalt, is not completely hardened during cooling in the chill mold.

2. By lowering the temperature of the martensitic transformation and reducing the critical quenching rate, cobalt gives the steel the ability to harden during cooling in the chill mold. When the cobalt content is increased to 12.48%, the transformation products of austenite that decomposed above the martensitic-transformation temperature are replaced in the steel microstructure by martensite and retained austenite. With a further increase in the cobalt content in the steel, the amount of retained austenite increases.

3. When more than 16% cobalt is introduced into the steel, signs of a dendritic structure are observed in its structure.

4. Cobalt introduced into the steel in amounts above 10% increases the stability of retained austenite during tempering.

5. While impairing the annealability of the steel, cobalt significantly increases its red hardness and cutting properties.

6. It is advisable to manufacture cutting tools of simple geometric shape and small dimensions by casting in metal molds from steel containing 1.5–1.6% C; 19.8–20.5% W; 5.5–5.6% Cr; 5.0–5.2% V; and 12.5–20.4% Co.

References

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