STUDY OF R18 STEEL CAST BY THE CENTRIFUGAL VACUUM METHOD

A. N. Dobrovidov, A. I. Slosman

(Presented by the scientific seminar of the Departments of Metallurgy, Metal Technology, and Welding Production)

The method of centrifugal casting in vacuum makes it possible to combine the advantages of vacuum melting (obtaining a metal of strictly specified composition, clean with respect to gases and nonmetallic inclusions, with high mechanical and physical properties) with the advantages of the centrifugal casting method (obtaining a dense casting with a good structure and a clean and accurate surface) [1]. Therefore this method makes it possible to produce castings of higher quality in comparison with castings obtained by other methods. In particular, it can be used to substantially improve the quality of cast tools.

To determine the advisability of using centrifugal casting in vacuum in the manufacture of cast cutting tools, a study was carried out on high-speed steel of type R18, melted and poured under various conditions.

Melting was carried out in an induction vacuum furnace of the IV-52 type. The furnace was adapted for centrifugal-vacuum casting: an inductor designed for melting a charge weighing 4–6 kg was made, and a mechanism for centrifugal casting into a chill mold was mounted in the furnace [2]. A diagram of the reconstructed furnace is given in Fig. 1.

Electrofused magnesite, chromomagnesite, aluminum oxide, and zirconium dioxide were used as crucible materials. When magnesite, chromomagnesite, and aluminum oxide were used, the crucible was made by sintering a mixture of the appropriate composition around a graphite template, heated by high-frequency current to a temperature above 2000°; in this process the crucible was sintered with melting of its inner surface. Zirconium crucibles, made at a refractory-products plant, were inserted directly into the inductor. Crucibles made of magnesite and chromomagnesite had lower durability than crucibles made of aluminum oxide and zirconium dioxide. Under our melting conditions, contamination of the molten metal by lining materials was not observed.

The vacuum in the furnace was produced by a VN-4 mechanical pump. The maximum attainable vacuum was \(10^{-2}\) mm Hg. The leak rate, determined for the cold furnace, was about 15 µ/min.

The current source was a PV-50/2500 type converter. The installation operated under the following conditions:

generator excitation current 6–7 A,

inductor voltage  400–500 V,
inductor current  90–120 A,
\(\cos \varphi\)  0.7–0.9.

The process of heating and melting the charge lasted 10–15 min. The liquid metal was then held in vacuum for 3–10 min. During the holding period, gases were released from the metal, and nonmetallic inclusions floated up and decomposed.

Fig. 1. Installation for melting and centrifugal casting in vacuum:
1—casing, 2—mold-rotation mechanism, 3—mold, 4—funnel, 5—crucible, 6—casing cover, 7—burner cover, 8—mechanism for rotating the installation

In order to prevent “boiling” of the liquid metal during its cooling, the pressure in the furnace before casting was raised to 40–50 mm Hg by introducing argon into it. After the argon was introduced, the inductor power was switched off, the motor of the centrifugal machine was switched on, and casting was carried out. The casting was cooled in vacuum down to 300–400°.

Figure 2 shows the mold used in the work. The mold material was low-carbon steel. The heats were cast under various conditions (Table 1). When casting in an open furnace, the quality of the surface of the castings was worse: films and subskin bubbles were present.

Pieces of R18 steel from tool-production scrap served as the charge for the heats. Chemical analysis showed that during vacuum remelting there is no loss of chromium, tungsten, or vanadium. This is explained by the fact that at the temperatures and pressures prevailing in the furnace during holding, the deoxidizing capacity of carbon exceeds the deoxidizing capacity of the other alloying elements [3]. Therefore, if a sufficient amount of carbon is present in the melt (in particular, when melting high-speed steel), oxidation of the alloying elements does not occur. Owing to evaporation of the alloying elements, the chemical composition of the steel also cannot change noticeably, since the vapor pressure of these elements under the holding conditions

close to (or below) the vapor pressure of iron [4]. A decrease in the content of metallic alloying elements due to interaction with the walls of the crucible is not observed. Under the melting conditions used, carbon likewise practically does not interact with the lining. For substantial development of the reaction of interaction between carbon dissolved in the liquid metal and the crucible, lower pressures and longer holding times are required [5]. Carbon burnoff depends on the holding time of the liquid metal in vacuum and proceeds mainly through reaction with oxygen dissolved in the metal. To compensate for the decrease in carbon content as a result of burnoff, a certain amount of specially prepared white cast iron, obtained by carburizing high-speed steel, was introduced into the charge. The content of tungsten and other alloying elements in this cast iron was the same as in R18 steel, while the carbon content varied from 4 to 4.5%.

Fig. 2.

After cooling, the castings were subjected to triple tempering at a temperature of 570°. Specimens were then prepared from them for studying structure, heat resistance, nonmetallic inclusions, density, wear resistance, and mechanical and physical properties.

For studying the macrostructure, the specimens were etched with a hot 50% solution of hydrochloric acid in water. All castings had, for the most part, a dense fine-grained structure. In the central part of the castings—

Table 1

Conditions of melting and pouring of the investigated heats

Notes. 1. The pressure in the furnace was measured with a VT-2 thermocouple vacuum gauge. 2. The pouring temperature was determined with an OPPIR-09 optical pyrometer.

castings exhibited slight porosity and a somewhat coarser structure.

The microstructure of all heats is typical of cast high-speed steel. It consists of dark and light constituents and eutectic. The degree of vacuum and the number of revolutions of the mold (within the range from 200 to 800 rpm) did not have a noticeable effect on the dimensions of the structural constituents. Evidently, the main factors influencing structure formation are the cooling rate and the chemical composition of the castings.

Table 2

Chemical composition of the heats studied

In Table 3 are given the results of measuring the microhardness of individual constituents of the heats made in vacuum and in an open furnace.

When Rockwell hardness was measured, all castings, irrespective of the melting and casting conditions, showed approximately the same hardness. After casting they had a hardness of 61–62 RC, and after triple tempering, 64–65 RC.

Heat resistance was determined by measuring the hardness of specimens in the cold state. Specimens subjected after casting to normal tempering were heated to 600, 625, and 650° and held at these temperatures for 4 hours. After cooling, the hardness was measured. The measurement results show that the heat resistance of the vacuum-treated steel is 5–8 degrees higher than that of the non-vacuum-treated steel.

Table 3

Contamination by nonmetallic inclusions in the heats studied (according to Gerty)

Quantitative metallographic analysis of nonmetallic inclusions, carried out by the linear-intercept counting method [6], showed a very substantial reduction in steel contamination as a result of vacuum remelting. In this method of studying inclusions, a glass plate with lines intersecting at right angles is inserted into the eyepiece; the unetched polished section is moved parallel to one of them, and the number of inclusions intersected by this line is counted. Contamination is expressed as the number of nonmetallic inclusions intersected by the line when the polished section is moved by 1 cm. In the work, 10 cm were examined on each polished section at a magnification of 300, and 3 polished sections were taken from each heat. The results of the analysis are given in Table 4.

In addition, for heats 14 and 31 the content of nonmetallic inclusions in weight percent was determined by the method proposed by M. S. Aronovich and I. M. Lyubarskii [7]. Counting was carried out at a magnification of 300. From each heat, 3 polished sections were taken and 60 fields were examined on each of them. The following results were obtained:

heat 14 \((10^{-2}\ \text{mm Hg})\)—0.0108% inclusions,

heat 31 (open furnace)—0.0660% inclusions.

Qualitative analysis of nonmetallic inclusions was carried out by examining them in ordinary light in bright and dark fields and in polarized light at various magnifications. The analysis showed that in vacuum-treated steel the nonmetallic inclusions are distributed more uniformly. The bulk of the inclusions consists of aluminosilicate glasses and aluminum oxide. In steel melted in an open furnace, in addition, oxides of other metals were found (most likely iron, manganese, chromium, vanadium), as well as sulfides (apparently complex sulfides FeS—MnS).

Fig. 3

The study of the effect of melting and pouring conditions on the density of the steel showed that the density increases with increasing degree of vacuum during melting. The density was determined by the hydrostatic weighing method. The specimens were weighed in air and in distilled water. The increase in density observed as a result of vacuum remelting is evidently explained by a decrease in porosity and in nonmetallic inclusions.

The influence of the degree of vacuum in the furnace during melting on the specific electrical resistance of the steel was also investigated. Measurements were made on cast tempered and annealed specimens. Data on the electrical resistance of specimens in the as-cast tempered state are given in Fig. 3. It is evident that the increase in the electrical conductivity of the steel with increasing degree of vacuum during its melting is associated with a decrease in pores, nonmetallic inclusions, gases, and harmful impurities in the steel.

Conclusions

1. In casting by the centrifugal-vacuum method, the quality of the surface of the castings is improved.

2. When high-speed steel is melted in vacuum, no loss of alloying elements occurs. This makes it possible to reduce the variation limits of the chemical composition of the steel being melted and also gives a saving of alloying elements.

3. The hardness of high-speed steel after remelting in vacuum does not change; the heat resistance of the steel increases by 5–8 degrees.

4. As a result of vacuum remelting, the amount of nonmetallic inclusions is reduced severalfold, and the remaining inclusions are distributed more uniformly.

5. Centrifugal-vacuum pouring improves the physical properties of the steel (density, electrical conductivity).

6. Casting in vacuum presents no technical difficulties.

LITERATURE

1. D. G. Lyubetskii. Proizvodstvo stali i otlivok v vakuume [Production of Steel and Castings in Vacuum]. Mashgiz, 1959.

2. A. I. Slosman. Izv. TPI, vol. 139, 1965.

3. F. Handers, H. Knüppel, K. Brotzmann. Stahl und Eisen, 76, No. 26, 1956.

4. E. S. Kalinnikov. Vakuum v proizvodstve stali [Vacuum in Steel Production]. Metallurgizdat, 1962.

5. H. A. Wriedght, J. Chipman. Metallurgia, IX, 1195, 1956.

6. G. V. Hardy and R. T. Allsop. Journal of the Iron and Steel Inst., vol. 195, part 3, 1960.

7. M. S. Aronovich, I. M. Lyubarskii. Metallurg, No. 9, 1936.