Cast Tungsten–Molybdenum–Cobalt High-Speed Steels

E. I. Marr

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

Recently a large number of works have appeared [1, 2, 3] in which the opinion is expressed that partial or complete replacement of tungsten by molybdenum in high-speed steels is possible. A marked improvement in the cutting properties of these steels is achieved by introducing cobalt. Steels of this type have higher cutting properties in comparison with tungsten steels.

In the opinion of Yu. A. Geller [2], one of the directions for increasing the productivity of high-speed steels is to increase the wear resistance of these steels. A significant increase in the wear resistance of high-speed steels can be achieved by increasing their hardness. Steels [2, 3] with hardness up to HRC 69–70 are recommended. In work [3] a steel of the following composition is given: 1.5% W, 9.5% Mo, and 8% Co. This steel, at a hardness of HRC 70, has high mechanical and cutting properties. Studies have shown [4] that in the cast state steels with molybdenum and tungsten have an identical structure; however, the steel with molybdenum has a finer eutectic network.

The purpose of the present work was to study the possibility of partially replacing tungsten by molybdenum in high-speed steels for the manufacture of cast tools, with simultaneous alloying of these steels with cobalt. The tungsten content was varied over wide limits, from 1 to 7%; correspondingly, the molybdenum content was also varied from 8.0 to 5.0%. As is known, molybdenum, in its action in high-speed steels, is analogous to tungsten, and 1% (by weight) molybdenum replaces approximately 2% (by weight) tungsten. The amount of cobalt introduced into the steel was 5 and 10%. The chemical composition of the heats is given in Table 1.

The steel was melted in a high-frequency installation in an acid crucible with a capacity of 2.0 kg. Casting was carried out into a metal mold on a centrifugal machine, with specimen dimensions of \(12 \times 12 \times 120\) mm. The specimens were quenched during the process of cooling in the mold.

In order to study the behavior of these steels as a function of tempering temperature, the specimens were subjected to successive tempering for 1 hour at various temperatures (Fig. 1). The graph shows the hardness curves of steels from heats of the first and second series with maximum and minimum tungsten content as a function of tempering temperature. Steels with intermediate tungsten compositions fall well between these curves in the order of decreasing content.

tungsten. Judging from the hardness in the cast (quenched) state and the course of the curves for subsequent tempering, it should be assumed that, as the tungsten content in these steels increases, the amount of retained austenite increases. The higher secondary hardness of steels with 10% cobalt compared with steels with 5% cobalt should apparently be explained by the precipitation of an intermetallic $\vartheta$-phase [5]. In Fe—W—Co and Fe—Mo—Co alloys an intermetallic compound is formed which corresponds to the for-

Table 1

Fig. 1. Change in HRC hardness as a function of the tempering temperature of steel (1 hour).

mulas $\mathrm{Co}_7\mathrm{W}_6$ and $\mathrm{Co}_7\mathrm{Mo}_6$. In [5], the presence of an intermetallic phase in cast cobalt steel was established, and the opinion is expressed that a $\vartheta$-phase may form at 10% Co in high-speed steels with 18 and 15% tungsten.

Table 2 gives the hardness data for the steels after triple tempering for 1 hour at various temperatures. These data are in good agreement with the statements of some authors [6, 7] that, during tempering, cobalt considerably accelerates the transformation of retained austenite and shifts this transformation into the range of lower temperatures. With an increase in the cobalt content to 10%, the maximum hardness is achieved as a result of triple tempering at a temperature of 520°C, whereas with 5% cobalt the maximum hardness is achieved at temperatures of 540–560°C.

Table 2

Fig. 2. Effect of the temperature of triple tempering for 1 hour on the red hardness of tungsten–molybdenum steels with 5% Co.

Figure 2 and Figure 3 present graphs of the red hardness of these steels as a function of the temperature of triple tempering for 1 hour. Red hardness was determined by measuring Rockwell hardness after 4-hour heating at temperatures of 600–675°C at intervals of 25°C. The red hardness of steels with 5% cobalt is somewhat lower than

red hardness by 10%. In accordance with the decrease in tungsten content, red hardness decreases successively both at 5 and at 10% cobalt.

From the data on the determination of red hardness it follows that steels with 10% cobalt, upon reaching maximum hardness after triple tempering at a temperature of 520°C, have lower red hardness than at higher tempering temperatures, which contradicts the data of work [7] for high-cobalt tungsten-molybdenum steels.

Fig. 3. Effect of the temperature of triple tempering for 1 hour on the red hardness of tungsten-molybdenum steels with 10% Co

On the basis of the data on red hardness and maximum hardness, the optimum tempering temperatures should be considered to be: for steels with 5% cobalt, 540–560°C, and with 10%, 540°C. However, the final resolution of this question is connected with endurance tests of tools made from these steels.

Conclusions

1. On the basis of the investigations carried out, it has been established that reducing the tungsten content and correspondingly increasing the molybdenum content in cast steels containing 5 and 10% cobalt does not change the value of the maximum hardness in the tempered condition—67–69 HRC.

2. With decreasing tungsten content, the red hardness of the steels decreases somewhat in the order of decreasing tungsten; however, the red hardness of these steels remains higher than that of P18 steel in the forged condition.

With an increase in cobalt content from 5 to 10%, red hardness increases.

1. Cobalt in cast high-speed steel shifts the transformation of retained austenite into the region of lower temperatures.

References

1. J. Hinnüber, E. Hengler. Stahl und Eisen, 1964, 84, No. 26.
2. Yu. A. Geller. MiTOM, No. 7, 1965.
3. James R., Handyside et al. Metal Progress, 1963, vol. 83, No. 6.
4. G. A. Kossovich, Yu. A. Geller. MiTOM, No. 5, 1964.
5. V. F. Moiseev, Yu. A. Geller. MiTOM, No. 4, 1965.
6. I. K. Kupalova. MiTOM, No. 9, 1965.
7. N. F. Vyaznikov, A. N. Popandopulo. Proceedings of LPI, No. 234, 1964.