High-Tungsten Cast High-Speed Steel with Cobalt

V. A. Kashchuk

(Presented by Professor, Doctor A. N. Dobrovidov)

The chemical composition of high-speed steels intended for the manufacture of forged and rolled tools is determined by their good annealability and forgeability.

Between high-speed steels and hard alloys there exists a gap both in chemical composition and in cutting properties. This gap could be filled by “super-high-speed” steels, if they did not have to be forged and annealed [1].

With the existing technology for manufacturing cutting tools (forging), it is impossible to use new highly alloyed high-speed steels because of their poor annealability and forgeability. With the new manufacturing technology (casting), broad possibilities open up for the use of highly alloyed steels with more advanced characteristics [2].

Manufacture of the Steels and Their Investigation in the Cast State

In the present work, high-tungsten high-speed steel containing 24% tungsten was alloyed with cobalt over a wide range of concentrations (up to 21%) in order to identify steels with high cutting properties.

The experimental steels were melted in a high-frequency induction furnace. The crucible, with a capacity of 2 kg, had an acid lining. Plant scrap of R18 steel, ferroalloys, and specially prepared high-carbon cast iron were used as the charge. Titanium (0.22%) was added to the experimental steel, deoxidized with aluminum (0.1%), to refine the structure. Titanium was introduced in the form of ferrotitanium.

Plates with a cross section of \(8 \times 15\) mm were cast under pressure on a centrifugal machine into metal molds (chill molds).

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

The microstructure of heat No. 1 consists of dark and light constituents and carbides (Fig. 1).

Microanalysis and hardness data make it possible to consider that the light constituent is martensite and austenite, while the dark constituent consists of the products of weakly alloyed austenite decomposing above the temperatures of the martensitic transformation.

Table 1

In the as-cast state, the hardness and microstructure of the steels were investigated. The hardness values, $H_{RC}$, are given in Table 2.

Table 2

The microstructure of the steels of heats Nos. 2–5 consists of austenite, martensite, and carbides; moreover, the eutectic carbides, coalescing, form “skeleton-like” shapes (Figs. 2 and 3).

In the steel of heat No. 2, carbides of “skeleton-like” shape are poorly developed. With an increase in cobalt content, the dendritic heterogeneity of the steel increases, and the carbides of “skeleton-like” shape occupy an ever larger volume.

The amount of retained austenite increases with increasing cobalt content, while the amount of martensite decreases.

Tempering of Experimental Steels

Since, during cooling in metal molds, steels of heats Nos. 2–5 undergo hardening, the heat treatment of these steels consists only of the tempering operation.

Specimens of all heats, measuring $8 \times 15 \times 25$ mm, were subjected to triple tempering at 560°C with a hold of 1 hour. Such a tempering regime is widely used in the practice of heat treatment of forged high-speed steels. The hardness values during tempering are given in Table 3.

Additional tempers at 560°C for the steel of heat No. 1 did not raise the hardness above 55,0 $H_{RC}$. This steel does not harden during cooling in metal molds and cannot be used as a cutting tool without the full heat-treatment cycle applied to forged high-speed steel.

Table 3

Fig. 1. Microstructure of steel from melt No. 1 after casting, ×440.

Fig. 2. Microstructure of steel from melt No. 3 after casting, ×440.

Fig. 3. Microstructure of steel from melt No. 5 after casting, ×440.

After quenching from 1290°C and triple tempering for 1 hour at 560°C, specimens from melt No. 1 increased their hardness to 65.5 \(H_{RC}\).

Testing of Cutting Properties

The cutting properties were tested on lathe tools by longitudinal turning of 9KhS steel with a hardness of 240 HB, at a cutting speed of 32 m/min, cutting depth of 2.5 mm, and feed of 0.5 mm/rev, without cooling.

Plates \((8 \times 15 \times 28\ \text{mm})\) were made from the experimental steels and mechanically fastened in a specially made holder. The plates (cutters) were sharpened to the following geometry:

front angle \(\gamma = 12^\circ\),

back angle \(\alpha = 8^\circ\),

main plan angle \(\varphi = 45^\circ\),

auxiliary plan angle \(\varphi' = 15^\circ\),

nose rounding radius \(r = 0.5\text{–}1.0\ \text{mm}\).

The durability of cutters made of forged R18 steel was taken as the durability criterion.

The ratios of cutter durability obtained in the tests are given in Table 4.

Table 4

As a result of the investigation carried out, the following conclusions may be drawn.

1. When high-tungsten high-speed steel with cobalt is cast in a permanent mold, it is hardened.

2. By increasing the amount of retained austenite during quenching, cobalt intensifies the dendritic inhomogeneity of the experimental steel.

1. As the cobalt content in the steel increases, the amount of carbides of “skeletal” form increases.

2. The required heat treatment for chill-cast high-tungsten steel with cobalt is tempering at 560° C.

3. Cobalt substantially improves the cutting properties of the experimental steel.

REFERENCES

1. Dobrovidov A. N., Rozenberg A. M., and Yakovlev G. M. Machine Tools and Tooling, No. 5. Cast Cutting Tool, 1947.

2. Tyuteva N. D. Proceedings of the Tomsk Polytechnic Institute, Vol. 68, No. 1, 1951.