EFFECT OF VACUUM REMELTING ON THE WEAR RESISTANCE OF R18 HIGH-SPEED STEEL

A. I. SLOSМAN, G. V. SIMONOV

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

Resistance to wear is one of the principal characteristics of steels for cutting tools, including high-speed steels. For tools operating under conditions of intensive cooling (for example, milling cutters, drills), wear resistance is a more important property than heat resistance [1].

Tool wear depends both on the conditions of its use and on the properties of the material from which it is made. One of the most radical methods of improving the properties of steels at the present time is the use of vacuum during their melting. Numerous studies [2, 3, 4] have shown that, as a result of vacuum remelting, the mechanical and physical properties of many steels and alloys change very substantially.

The aim of the present investigations was to determine the influence of the degree of vacuum during the melting of high-speed steel on its resistance to abrasive wear; casting was carried out into a stationary and a rotating chill mold.

For melting and casting, an IV-52 induction vacuum furnace was used. As a result of reconstruction of the furnace [5], it became possible to melt a small amount of metal (4–6 kg) in it and to carry out centrifugal casting without breaking the vacuum. The heats were made in a crucible manufactured from electrically fused aluminum oxide. The vacuum was produced by a VN-4 mechanical pump. The maximum vacuum attained was of the order of \(10^{-2}\) mm Hg. The leakage, determined for the cold furnace, was about 15 µ/min. The pressure in the furnace was measured with a VT-2 thermocouple vacuum gauge. Waste R18 steel from tool production was used as the charge. In order to compensate for the decrease in carbon content as a result of burn-off during melting, a certain amount of special white cast iron was introduced into the charge. This cast iron was obtained by carburizing high-speed steel and contained 4–4.5% carbon. Therefore, its introduction into the charge did not change the content of alloying elements in the steel being melted.

The process of heating and melting the charge lasted 15–20 min. After this, the liquid metal was held in vacuum for 5–10 min. Before casting, the pressure in the furnace was raised to 40–50 mm Hg by introducing argon. The temperature of the liquid metal was determined with an optical pyro-

meter OPPIR-09 and at the moment of pouring was 1500–1550°C. Pouring was carried out into chill molds made of low-carbon steel. To prevent possible welding of the casting to the chill mold, its inner surface was coated with a thin layer of a mixture of 80% zirconium dioxide and 20% refractory clay. After cooling, the castings were subjected to triple tempering at 570°, and specimens were then cut from them for wear tests.

Table 1 gives the melting and pouring conditions and the chemical composition of the heats investigated. Under each set of conditions two castings were melted, and from each casting 6 specimens were made; these were given different hardnesses within the range 60–66 RC. To reduce hardness, the specimens were additionally tempered according to the appropriate schedules.

Table 1

Tests of resistance to abrasive wear were carried out on the apparatus of G. I. Kiselev [6]. A flat grinding wheel served as the abrasive material. During the tests the wheel was periodically cleaned. Its abrasive capacity after cleaning was checked against a standard. The adopted test conditions did not lead to noticeable heating even in the surface layers of the specimens being tested. The specimens had a cross section of $4.5 \times 4.5$ mm. The load on them during testing was 3 kg. The amount of wear was determined by weighing the specimens before and after the test. Each specimen was abraded 3 times. The test results are given in graph 1 (Fig. 1).

It is evident from the graph that wear resistance depends on the melting and pouring conditions. The best wear resistance is shown by steel melted at a pressure of $10^{-1}$ mm Hg and poured by the centrifugal method; moreover, changing the rotational speed of the chill mold from 300 to 600 rev/min did not substantially change the wear resistance. However, under identical melting conditions, steel poured by the centrifugal method has greater wear resistance than steel cast in a stationary chill mold.

Fig. 1

Legend:

• ○ — pressure during melting \(10^{-1}\) mm Hg; ingot mold 300 rpm
• ◐ — pressure during melting \(10^{-1}\) mm Hg; ingot mold 600 rpm
• ◑ — pressure during melting 50 mm Hg; ingot mold 300 rpm
• ◑ — pressure during melting 760 mm Hg; ingot mold 300 rpm
• ◒ — pressure during melting \(10^{-1}\) mm Hg; ingot mold 0 rpm
• ● — pressure during melting 760 mm Hg; ingot mold 0 rpm

Axis labels:

• vertical: density, g/cm\(^3\)
• horizontal: pressure during melting, mm Hg

Plot annotation:

\(\times\) R18 steel, melted in air and cast into a stationary ingot mold

Fig. 2.

Metallographic analysis of the heats studied showed that there is no noticeable difference in their microstructure. The macrostructure of heats cast in a stationary chill mold is coarser than that of heats cast by the centrifugal method. A very noticeable difference is found in the amount of nonmetallic inclusions in heats melted in vacuum as compared with heats melted in an open furnace.

Measurement of the density of the steel by the hydrostatic weighing method showed that, with the centrifugal-vacuum pouring method, the density of the steel increases (Fig. 2).

The higher wear resistance of high-speed steel cast by the centrifugal-vacuum method is evidently explained by its greater density and by its cleanliness with respect to nonmetallic inclusions and gases. The literature contains data on the influence of these factors on wear resistance. For example, Yu. A. Geller [7] points to the dependence of wear resistance on the content of sulfur and sulfide inclusions in tool steels (Fig. 3).

Fig. 3.

Conclusions

1. Wear resistance should be regarded as one of the principal characteristics of tool steels.

2. When high-speed steel is poured by the centrifugal-vacuum method, its wear resistance increases.

3. The increased wear resistance of steel cast by the centrifugal-vacuum method is evidently explained by its greater density and by its cleanliness with respect to nonmetallic inclusions and gases.

Literature

1. V. Ertel, A. Grützner. High-Speed Steels, Metallurgizdat, 1933.

2. A. M. Samarin. Vacuum Metallurgy. Metallurgizdat, 1958.

3. L. M. Novik. The Use of Vacuum in Steelmaking Processes, Metallurgizdat, 1957.

4. Jones, Problems of Modern Metallurgy, 1 (37), 1958.

5. A. I. Slosman. Reconstruction of semi-industrial vacuum furnaces for studying centrifugal casting in vacuum, Izvestiya TPI, vol. 139, 1965.

6. G. I. Kiselev. Proceedings of SFTI, issue 26, 32, 1947.

7. Yu. A. Geller. Tool Steel, Metallurgizdat, 1961.