EFFECT OF THE MAIN COMPONENTS MOLYBDENUM AND CARBON ON THE HARDNESS OF IRON–COBALT–TUNGSTEN ALLOYS DURING TEMPERING

A. D. KLEMENT'EV

(Presented by the scientific seminar of the departments of physical metallurgy, metal technology, and welding production)

Iron–cobalt–tungsten (—molybdenum) alloys are known for their magnetic properties, as well as for a high degree of dispersion strengthening [1]. The high hardness and heat resistance of these alloys indicate the possibility of developing, on their basis, materials for high-productivity tools [2, 3].

This article is devoted to an experimental study of the effect of the main components and molybdenum on the dispersion hardening of cobalt–tungsten alloys on an iron base. In their initial state the alloys are carbon-free; however, the study of the effect of carbon on hardness and heat resistance is also of great interest. The study of alloys with carbon is important also because the melting of carbon-free alloys is associated with certain technological difficulties.

The alloys were melted in a vacuum induction high-frequency furnace in a corundum crucible [4]. The use of vacuum was intended to improve the mechanical properties and to achieve better control of the chemical composition of the alloys. The degree of vacuum corresponded to the usual conditions of industrial melting \((10^{-1}\ \text{mm Hg})\).

The castings had the form of bars with a cross section of \(10\text{–}15 \times 25\) mm. The alloys were quenched during casting in a metal mold and subsequently subjected only to tempering (the simplest variant for manufacturing a cast tool). Special quenching in water or oil was carried out in order to clarify the degree of hardening of the alloys under the basic technological variant. The conditions of quenching in the metal mold were assumed to correspond to those that can be obtained under production conditions. At the same time, with respect to the degree of hardening, the alloys prove to be unequal, since for different alloys the cooling rate attained in the metal mold will fix the supersaturated solid solution to different extents.

The composition of the alloys studied and their initial hardness (after casting) are given in Table 1.

In order to determine the general regularities of the process of dispersion hardening of cobalt–tungsten alloys, preliminary studies of a number of alloys were carried out under various tempering regimes, over wide intervals of temperatures and durations. This made it possible to outline the conditions for comparative tempering, carried out at one temperature—670° [5].

The change in hardness during tempering at 670° makes it possible to judge the nature of dispersion hardening of the alloys under conditions of a severe thermal regime, which imposes rather high requirements on the heat resistance of the alloys. Preliminary investigations showed that the change in hardness during tempering of the alloys studied is determined only by the holding time at the given temperature and is practically independent of interruptions in tempering (for example, for carrying out intermediate hardness measurements) and of the heating and cooling rates used in this process. This makes it possible to clarify the features of the change in hardness during tempering on the same alloy specimens and increases the reliability and stability of the hardness-measurement results.

Table 1

Chemical composition and initial hardness of the alloys studied

An indicator of the heat resistance of alloys is the degree of decrease in maximum hardness in comparison with the hardness at the end of the comparative tempering (stability of hardness during tempering), as well as the magnitude itself of the final hardness.

Tempering of the alloys was carried out in a lead bath. This ensured rapid heating of the specimens, which is especially important for short tempering times.

The effect of the main components on the process of dispersion hardening was studied on alloys with different cobalt and tungsten contents (Table 1). The comparative-tempering data are presented in the graph (Fig. 1), which gives a clear idea of the features of the influence of cobalt and tungsten on the dispersion-hardening process, as well as of the dependence of hardness and heat resistance on the ratio of these components.

Alloys beginning with 15% Co at a content of 25% W (alloy 9), or 15% W at a content of 20% Co (alloy 1), have a hardness of about 60 $HRC$, acceptable for tool materials, at the end of comparative tempering (670°, 4 h).

The relative influence of cobalt and tungsten on the level of maximum hardness and heat resistance (hardness after 4 h of tempering) is clearly represented by comparing the following alloys. An increase, in comparison with alloy 8 (20% Co, 20% W), of cobalt by 5%

(alloy 14) raises the level of maximum hardness somewhat more and substantially increases heat resistance more than the same increase in tungsten (alloy 7). At a higher concentration of cobalt and tungsten in the alloys, cobalt, as in the preceding alloys, contributes to a greater extent to an increase in heat resistance, whereas increasing the tungsten content is more intensive in raising the level of maximum hardness. Thus, an increase, compared with alloy

Fig. 1. Change in hardness of alloys with different contents of cobalt and tungsten (Table 1) as a function of tempering duration

14 (25% Co, 20% W), in cobalt concentration by 5% (alloy 11) or tungsten by 5% (alloy 2) in the first case gives higher heat resistance than in the second. However, tungsten is more effective in raising the maximum hardness. Even an increase in the cobalt content by 10% (alloy 5) does not make it possible to attain the same level of hardness.

The graph (Fig. 1) gives a clear picture of the peculiar nature of the influence of cobalt and tungsten on the kinetics of the process of dispersion hardening of the alloys. When the tungsten content in the alloys predominates in comparison with cobalt (alloys 6, 9, 7), the process of dispersion hardening is sharply accelerated, the curves rise steeply upward, and thereby

to a greater extent, the higher the tungsten content, whereas alloys with the opposite component ratio (alloys 10, 14, 11, and 5) are characterized by a smoother increase in hardness and a more stable retention of it during tempering. An increase in the cobalt concentration in the alloys shifts the hardness maximum toward higher tempering temperatures and longer tempering times. The predominance of tungsten in the alloys, as compared with cobalt, leads to an acceleration of the hardening process, while the predominance of cobalt slows overaging and gives a more effective increase in heat resistance.

Cobalt–tungsten and cobalt–molybdenum alloys are very similar [1]; nevertheless, it is of interest to determine the specific features of molybdenum alloys under the conditions studied. The corresponding data are given in Table 1 and in Fig. 2.

As can be seen from the graph, alloys with molybdenum (alloys 17, 15) are distinguished by a lower level of dispersion hardening in comparison with alloys having the same tungsten content (alloys 8, 2). Ternary

Fig. 2. Change in hardness of alloys with molybdenum (Table 1) as a function of tempering duration

tungsten and molybdenum alloys (alloys 8 and 17) differ by a lower level of strengthening in comparison with the quaternary iron–cobalt–tungsten alloy (alloy 18) with the same total content of tungsten and molybdenum.

The high hardness of the molybdenum alloys in the initial state (Table 1) is noteworthy; this is explained by insufficient hardenability, i.e., partial decomposition of the solid solution during cooling of the alloys when cast into a metal mold. These considerations are confirmed metallographically and also by the results of additional hardening of the molybdenum alloys. The hardness after such hardening decreases substantially, while the level of dispersion hardening increases. Repeated hardening of the mixed alloy (alloy 18), however, has almost no effect on its hardness.

Thus, cobalt–molybdenum alloys do not acquire sufficient hardenability during casting. The use of molybdenum

along with tungsten has a positive effect on the dispersion strengthening of the alloys. It is advisable to replace up to 50% of the tungsten with molybdenum.

The study of the effect of carbon on cobalt–tungsten alloys was carried out on alloy 8 (20% Co, 20% W), taken as the starting alloy. This alloy has a relatively high hardness—up to 62 HRC (Fig. 1)—and at the same time a sufficient reserve for increasing it in the event of a favorable effect of additions. In addition, at the quenching temperature (1300°) the alloy lies at the boundary between the alpha and gamma regions and at the saturation limit of the complex solid solution with tungsten [3]; consequently, the introduction of even very small foreign additions should cause readily detectable changes in the structure of the alloys. Alloys with a carbon content of up to 0.52% were studied (Table 1, Fig. 3).

Increasing the carbon content to 0.3% (alloy 55) raises the level of dispersion hardening of the alloys, both in hardness and in its stability during tempering. The higher hardness at the end of tempering

Fig. 3. Change in hardness of cobalt–tungsten alloys with different carbon contents (Table 1) as a function of tempering duration

compared with the starting alloy is an indication of the increased heat resistance of the carbon-containing alloys. The increase in hardness is especially significant at the beginning of tempering. The maximum hardness of alloy 55 (0.28% C) is 67 HRC; the hardness at the end of a 4-hour tempering is 63.5 HRC.

When the carbon content is above the optimum concentration, a decrease in strengthening is observed, especially at the end of tempering (loss of heat resistance). Alloy 57 (0.52% C) at the end of tempering has a lower hardness than carbon-free alloy 8. The initial hardness of the alloys increases with increasing carbon concentration, which is quite natural for carbon-containing alloys. Microscopically, the effect of carbon is manifested in the appearance of a eutectic when the carbon content exceeds 0.2% [5].

The introduction of small additions of carbon, in addition to increasing dispersion hardening, substantially simplifies the technology of alloy production (the selection of charge materials and melting methods).

Conclusions

1. Under conditions of self-hardening of cobalt–tungsten alloys when cast into a metallic mold, sufficiently high hardness (above 60 HRC) with an intensive tempering heat regime (670°) can be obtained in alloys at a certain minimum content of cobalt and tungsten (with 15% Co—at least 25% W, and with 15% W—at least 20% Co).

The predominance of tungsten content in comparison with cobalt leads to an acceleration of the hardening process and makes it possible to obtain higher alloy hardness. With the opposite ratio of the components in the alloy, a higher level of heat resistance is achieved.

1. Cobalt–molybdenum alloys are distinguished by weaker self-hardening in comparison with cobalt–tungsten alloys and do not acquire sufficient strengthening upon tempering. Alloys containing both tungsten and molybdenum have the highest level of dispersion hardening. In cobalt–tungsten alloys, partial (up to 50%) replacement of tungsten by molybdenum is advisable.

2. Cobalt–tungsten alloys used as a base for tool materials should not be carbon-free, but low-carbon. Small additions of carbon (0.2–0.3%) lead to significant strengthening of the alloys. The admissibility of carbon in the alloys substantially simplifies the technology of obtaining them and the selection of charge materials.

LITERATURE

1. W. Köster. Archiv für das Eisenhüttenwesen, No. 1, 17–23, 1932.
2. Yu. A. Geller. Metallovedenie i obrabotka metallov, No. 3, 1–16, 1955.
3. A. D. Klement’ev. Izvestiya vuzov MVO SSSR. Chernaya metallurgiya, publ. Siberian Metallurgical Institute, No. 3, 77–83, 1959.
4. A. D. Klement’ev. “Informatsionnyi listok,” No. 16. Bureau of Technical Information of the Tomsk Sovnarkhoz, Tomsk, 1960.
5. A. D. Klement’ev. Modern Designs of Cutting Tools. Proceedings of the Conference of Toolmakers of Western Siberia, coll. 1, Moscow, TsINTIAM, 242–252, 1962.