Molybdenum and Tungsten in Cast High-Speed Steels

MOLYBDENUM AND TUNGSTEN IN CAST HIGH-SPEED STEELS

E. I. MARR, N. A. EROFEEV

(Presented by the Section of Metal Science and Heat Treatment of Metals of the Scientific and Technical Conference of Machine Builders of the city of Tomsk, 1967)

In this work, the influence of molybdenum, tungsten, and carbon on the alloying of phases in cast molybdenum and tungsten-molybdenum high-speed steels was investigated.

For this purpose, groups of steels with different carbon contents were cast, in which the molybdenum content was varied successively. The composition of the steels is given in Table 1.

Table 1

The steels were melted in a high-frequency installation. During casting into a metal mold, the specimens were quenched. The steels under investigation, in the cast (quenched) state, were subjected to carbide analysis using-

using an electrolyte of the following composition: 100 ml of hydrochloric acid, 10 g of sodium thiosulfate, and 75 g of potassium chloride per 1 liter of water.

On the basis of carbide analysis data, curves were constructed for the distribution of molybdenum between the phases of molybdenum (Fig. 1) and molybdenum-tungsten (Fig. 2) high-speed steels.

Fig. 1. Distribution curves of molybdenum between the phases of molybdenum high-speed steels.
— — · — steel with 1% C;
——— steel with 1.3% C.

Fig. 2. Distribution curves of molybdenum between the phases of molybdenum-tungsten high-speed steels.
— · — steel with 0.75% C;
— — — steel with 0.90% C;
——— steel with 1.25% C.

From the analysis (Fig. 1) it follows that the distribution curves of molybdenum between the solid solution and the carbides intersect at different molybdenum contents in the steel. In steels with a carbon content of 1.0%, the distribution curves intersect at 9.75% molybdenum in the steel, whereas in steels with a carbon content of 1.3% they intersect at 6.6% molybdenum in the steel.

When the molybdenum content in the steel is less than 9.75% with 1.0% carbon, and 6.60% molybdenum with 1.3% carbon, the molybdenum distribution curves in the solid solution lie above the distribution curves in the carbide phase. Such a distribution of molybdenum may be called “favorable” in the sense that, under the indicated conditions, the concentration of molybdenum in the solid solution is greater than in the carbides, since the basic properties of high-speed steel are determined by the composition of the solid solution, and not of the carbide phase.

When the molybdenum content is above the quantities indicated, a larger part of it goes into the formation of carbides, while a smaller part dissolves in the solid solution. Such a distribution of molybdenum between the phases of high-speed steel may be considered “unfavorable.”

At the indicated amount (9.75% molybdenum in the steel at 1% carbon and 6.6% molybdenum with 1.3% carbon), an equal distribution of molybdenum occurs between the phases of quenched molybdenum high-speed steel.

From the curves of molybdenum distribution between the phases of the steel it follows that the concentration of molybdenum in the solid solution, as well as the points of equal distribution of molybdenum in the solid solution and carbide phase, are inversely dependent on the carbon content in the steel.

Comparison of the data on the distribution of molybdenum between phases with the red hardness of the steels (Table 2) shows that steels with the highest red hardness lie in the region of “favorable”

Table 2

distribution. Such is steel with 6.5% molybdenum at 1.0% carbon. When the molybdenum content in the steel is above this amount, the red hardness decreases, although the alloying of the solid solution with molybdenum continues.

continues to increase. The latter should be explained by the depletion of the solid solution in carbon with a further increase in the molybdenum content in the steel.

At 10.77% molybdenum and 1.0% carbon in the steel, the $\gamma$ region is pinched off owing to a considerable depletion of the solid solution in carbon.

With a higher carbon content in the steel (1.3%), the regularities of the distribution of molybdenum between the phases remain the same as at a carbon content of 1.0%, but the heat resistance increases continuously with increasing molybdenum content in the steel. This should be explained by the fact that the heat resistance of the steel is determined not only by the content of alloying elements in the solid solution, but also by the concentration of carbon in it. In the steels considered, as the carbide phase increases there is no strong depletion of the solid solution in carbon, and therefore the heat resistance does not decrease. However, at molybdenum contents higher than those investigated, a significant depletion of the solid solution in carbon and pinching off of the $\gamma$ region are quite possible. The latter is confirmed by the increase in hardness in the quenched state with increasing molybdenum content in the steel.

Thus, the higher heat resistance of steels with a carbon content of 1.3%, despite an “unfavorable” distribution of molybdenum between the phases of the quenched steel, is ensured not only by molybdenum but also by the high saturation of the solid solution with carbon. This indicates the special role of carbon in increasing the heat resistance of high-speed steels.

Since, with increasing carbon content in the steel, the alloying of the solid solution with molybdenum decreases, as follows from Fig. 1, the simultaneous increase in the heat resistance of the steel and the increase in the amount of carbide phase should be explained by the division of the additionally introduced carbon into two parts. One part goes to the formation of primary carbides, increasing their amount and thereby depleting the solid solution in molybdenum; the other part dissolves in austenite, increasing the heat resistance of the steel. But at a high carbon content in high-speed steel, the tool will have low cutting durability because of the high brittleness caused by strong carbide heterogeneity.

It follows from this that, for the rational use of the alloying element and for obtaining a high-quality cutting tool from high-speed steel, it is necessary to select the ratio between the carbon content and the alloying element in the region of its “favorable” distribution between the phases of the quenched steel.

Figure 2 gives the curves of molybdenum distribution between the phases of quenched molybdenum–tungsten steels.

The regularities of molybdenum distribution between the phases of steels of these groups and the influence of carbon on the distribution of molybdenum remain the same as in molybdenum steels. However, the introduction of tungsten into the steel somewhat reduces the alloying of the solid solution with molybdenum and shifts the points of equal distribution of molybdenum into the region of lower molybdenum content in the steel.

From the carbide-analysis data it follows that, with a continuous increase in the molybdenum content in the steel, the alloying of the solid solution with tungsten decreases, while its content in the carbide phase increases.

The effect of increasing the molybdenum content in molybdenum–tungsten steels on red hardness and other properties remains the same as for molybdenum high-speed steels.

Conclusions

1. It has been established that the distribution of molybdenum between the phases of hardened steel depends on the carbon and tungsten content in the steel and may be “favorable,” “unfavorable,” or equal.

2. The concentration of molybdenum and tungsten in the solid solution is inversely related to the carbon content in the steel. With a continuous increase in the carbon content in the steel, heat resistance increases, but the quality of the cutting tool under such conditions may be reduced because of a decrease in strength caused by pronounced carbide heterogeneity.

3. In the region of “favorable” distribution of molybdenum between the phases of high-speed steel, there are optimum ratios between the carbon and molybdenum contents in the steel, at which the highest-quality tool is obtained with rational use of molybdenum as an alloying element.

4. With an increase in the molybdenum content in high-speed steel, redistribution of carbon and tungsten occurs between the solid solution and the carbide phase. The solid solution becomes depleted in these elements, while the carbide phase becomes enriched in them.

5. With an increase in the content of carbon, tungsten, and molybdenum in high-speed steel, the amount of the carbide phase increases continuously.