MOLYBDENUM IN CAST TUNGSTEN–MOLYBDENUM HIGH-SPEED STEELS

E. I. Marr

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

At present, in the tool industry there is a tendency to replace tungsten high-speed steels with molybdenum and tungsten–molybdenum high-speed steels. This is explained by the fact that steels containing molybdenum have a better distribution of more finely dispersed carbides and higher mechanical strength in comparison with tungsten steels. Of the total production of high-speed steels in the USA, at present about 75% consists of molybdenum and tungsten–molybdenum steels, and only 10% of tungsten steels [1]. In recent years, tungsten–molybdenum high-speed steels have also become widely used in our country. The All-Union Scientific Research Tool Institute recommends tungsten–molybdenum steels of normal productivity of the following grades for use in domestic industry [1]: R6M3F2, R9M4, and R12M3.

Table 1

In the present work, the influence of the molybdenum and carbon content in cast tungsten–molybdenum steels on the structure and certain properties of these steels was investigated. The chemical composition of the steels studied is given in Table 1.

Steel was melted under laboratory conditions in a high-frequency installation in an acid crucible with a capacity of 1 kg. The steel was cast into a metal chill mold with specimen dimensions of 25×11×60 and 11×11×60 mm. When cast in the metal mold, the specimens were quenched during cooling. The hardness of the steels in the cast (quenched) condition is given in Table 2.

Table 2

As is known, the structure of cast high-speed steel under rapid cooling may consist of the following constituents: the δ-phase, a dark constituent (δ-eutectoid), a light constituent (structureless martensite and retained austenite), and a network of carbide eutectic. Examination of the structure showed that the δ-phase is absent in all the steels studied; the steel of heat 4 has a ferritic structure. The structure of steels with a carbon content of 0.75–0.95% consists of a light phase surrounded by a network of carbide eutectic. The primary grains of the steels are divided into subgrains, along whose boundaries finely dispersed carbides are located (Fig. 1). The dark constituent (δ-eutectoid) is present only in steels with a high molybdenum content, i.e., in steels of heats 12 (Fig. 2) and 3; moreover, with an increase in the carbon content in the steel (steel of heat 3), the amount of the dark constituent decreases, which agrees with the data of works [4]. Steels with a carbon content of 1.25% have grains with a structure of coarse-needle martensite and retained austenite (Fig. 3), surrounded by a network of carbide eutectic.

Fig. 1. Microstructure of steel of heat 15 in the cast condition. ×450

The steels were subjected to special etching for carbides. For comparison of the structure of tungsten-molybdenum steels with tungsten steels, R18-type steels with 1.05 and 1.30% C were cast. The carbide network of tungsten-molybdenum cast steels (Fig. 4), as in tungsten steels (Fig. 5), is located along the boundaries of the primary grain; however, in steels with molybdenum it is finer, which is also noted in work [2]. These steels differ substantially also in the structure of the eutectic itself. Cast tung-

high-speed steels have a “skeletal” form of carbide eutectic as a result of the coalescence of eutectic carbides, while tungsten–molybdenum steels have a eutectic form (dispersed carbides of granular form), which is poorly resolved even at considerable magnification ($\times 600$). It should be noted that, as the molybdenum content increases at constant carbon content, the carbide-eutectic network becomes somewhat thicker; however, the grain size decreases. With increasing carbon content, a decrease in grain size is also observed.

Fig. 2. Microstructure of steel of heat 12 in the as-cast state. $\times 450$

Fig. 3. Microstructure of steel of heat 7 in the as-cast state. $\times 450$

In order to determine the behavior of these steels during tempering, successive tempering was carried out in the temperature range 100–700°C, for one hour at each temperature. From analysis of the microstructure, the hardness in the as-cast state, and the behavior during successive tempering, it follows that in the steel of heat 4 the $\gamma$-region was quenched in. In the steel of heat 12, because of the presence of a large amount of a dark constituent (low hardness in the as-cast state), the effect of secondary hardening is not observed during successive tempering.

From the course of the curves for successive tempering and the hardness in the as-cast state, it follows that, with increasing carbon content, the amount and stability of retained austenite increase; with increasing molybdenum content at constant carbon content, the amount of retained austenite decreases.

To select the optimum tempering temperature, these steels were subjected to multiple tempering for 1 hour at temperatures of 520–600°C at intervals of 20°C, and the red hardness of the steels was determined as a function of the temperature of triple tempering. For this purpose, steel specimens were tempered three times for 1 hour in the temperature range 520–600°C. Red hardness was determined by measuring the hardness in the cold state after a 4-hour temper at temperatures of 575–650°C at intervals of 25°C. The data for determining red hardness are given in Table 3.

Fig. 4. Microstructure of steel of melt 10 in the as-cast state; ×600 (etchant: 10 g K₃Fe(CN)₆, 10 g KOH per 100 ml of water)

Fig. 5. Microstructure of cast steel R18 with 1.3% C; ×600 (etchant: 10 g K₃(CN)₆, 10 g KOH per 100 ml of water)

In determining red hardness, it was established that the red hardness of the steels studied depends on the temperature of the preliminary tempering. With an increase in the carbon content in the steel, red hardness increases.

Table 3

As a result of studying the behavior of steels under repeated tempering and determining red hardness, it was established that for steels with a carbon content of 0.75–0.95%, tempering temperatures of 540–560°C should be considered optimal; for steels with 1.25% C—560–580°C. The maximum hardness value is reached as a result of 2–3 one-hour temperings at these temperatures. At higher tempering temperatures, the hardness obtained is somewhat lower after 1–2 temperings and drops sharply during subsequent temperings. At tempering temperatures below those recommended, a larger number of temperings is required to achieve maximum hardness, and for steels with a carbon content of 1.25%, owing to the greater stability of the retained austenite, this number is 6–7. Figure 6 gives the repeated-tempering curves for steels of heats 2 and 8.

Fig. 6

The data of the present work show that steels of heats 11 and 3 cannot be used for cutting tools because of their comparatively low hardness and red hardness. This should apparently be explained by depletion of the solid solution in carbon in the presence of a high molybdenum content in these steels. The steels of heats 2 and 8 possess the best hardness and red hardness, which is in agreement with the data of Yu. A. Geller [3]. Steels of heats 15, 1, and 7 may be

used for the manufacture of cutting tools, since at a sufficiently low alloying level they have high hardness and adequate red hardness. The increase in red hardness on going from steel heat 15 to heat 7 should be explained by the influence of carbon.

Conclusions

1. At the same degree of alloying, cast tungsten–molybdenum high-speed steels have a finer network of carbide eutectic than tungsten steels.

2. Among the present series of heats, the steels with 2.0% W and 6.5% Mo (heats 2 and 8) possess the best hardness and red hardness; moreover, as the carbon content increases, hardness and red hardness increase.

3. Steels with 2.0 W and 2.5% Mo can be recommended as low-alloy high-speed steels. These steels, at a low degree of alloying, have comparatively high hardness and red hardness.

References

1. G. A. Kossovich. New Tool Materials Improving the Durability of Cutting Tools. Moscow, 1965.
2. G. A. Kossovich, Yu. A. Geller. MiTOM No. 5, 1964.
3. Yu. A. Geller. MiTOM No. 7, 1965.
4. I. T. Tikhonov. Izvestiya TPI, vol. 68, issue 4, 1951.