EFFECT OF QUENCHING CONDITIONS ON THE MICROSTRUCTURE AND PROPERTIES OF CAST IRON–COBALT–TUNGSTEN ALLOYS

A. D. Klement’ev

(Presented by the scientific seminar of the departments of metal science, equipment and technology of heat treatment of metals; technology of metals; equipment and technology of welding production)

At the initial stage in the development of dispersion-hardening cutting alloys [1], the task was posed of studying a cast tool that undergoes quenching during the casting process and is then subjected only to tempering. This variant of the technological process is distinguished by the greatest simplicity, but it imposes special requirements on the chemical composition and on the cooling conditions of the alloys during casting, which ensure the self-hardening capacity of the alloys.

However, in the manufacture of a cast tool from dispersion-hardening alloys, another technological variant is possible, one that provides for quenching after casting and removes the indicated limitations. In this case, various kinds of intermediate treatment may be used, for example heat treatment. The use of pressure treatment for the variant of a forged tool creates additional possibilities for influencing the properties of the alloys. In connection with these considerations, a study was carried out of alloys subjected to additional quenching from temperatures of 1300°C.

Ternary iron–cobalt–tungsten alloys [2] in the as-cast state have a structure that reflects its connection with primary crystallization. The structure of the original alloy 2 (20% Co, 20% W) is characteristic (Fig. 1, a).

Alloys that undergo a gamma–alpha transformation [2], after quenching in water and oil, acquire a fine-grained structure; especially high fineness of grain was obtained in alloy 2 (25% Co, 25% W) (Fig. 1, b). Quenching in water and oil gives an entirely identical microstructure. The grain boundaries are tortuous; in alloys more highly alloyed than the initial one, isolated, very fine, rounded or point-like precipitates of the excess phase are observed (Fig. 1, b). Such a microstructure is analogous to the structure of certain forged tool steels, for example high-speed steels with a small quantity of well-separated primary carbides in the quenched state.

The formation of the microstructure of ternary alloys during quenching may be explained as follows. Upon heating for quenching, an excess phase begins to precipitate from the supersaturated solid solution, initially in a finely dispersed form, which, as the temperature rises, rapidly coarsens (a brief anneal at 1000°C gives a picture of continuous, very coarse spherical precipitates—

[[unclear: beginning of word]]ized particles of the excess phase), and upon further heating it passes into a solid solution. The alpha–gamma transformation occurs in the presence of a large number of particles of the excess phase, which promotes the formation of a large number of grains of the new phase. As the temperature is raised, grain growth is restrained by particles of intermetallic compounds located mainly along the grain boundaries. Naturally, alloys with an increased amount of excess phase should be finer-grained, which should have a favorable effect on the mechanical properties.

Fig. 1. Microstructure of alloys:
a — alloy 8 (20% Co, 20% W), quenching during casting, ×86;
b — alloy 2 (25% Co, 25% W), water quenching, ×600.

However, remnants of the excess phase along the grain boundaries adversely affect the strength of the alloys. During cooling of the alloys, the gamma–alpha transformation occurs within the boundaries of the resulting fine grains without changing their configuration.

It is of interest to compare alloys 8 and 2, which differ in their degree of alloying with cobalt and tungsten (Fig. 1). In the initial state, the bending strength of alloys 8 and 2 is, respectively, 230 and 192 kg/mm². Quenching in oil somewhat lowers the bending strength of alloy 8 (212 kg/mm²) and slightly increases the strength of alloy 2 (204 kg/mm²). Quenching in water sharply reduces the strength of both alloys, especially the more highly alloyed one (164 and 126 kg/mm²). The main reason for the decrease in strength in the latter case is the increase in internal stresses during rapid cooling in water. It is possible that cooling in oil (considering the small dimensions of the specimens used in the tests) is also excessively abrupt, which does not allow the improvement in structure to be fully realized. The strength of both alloys is practically at the same level. The greater fine-grained character of alloy 2 is offset by the presence of the excess phase, which evidently leads to a certain decrease in strength.

The change in hardness and microhardness of the alloys under consideration, quenched at different cooling rates, is indicative. The integral hardness of alloys 8 and 2 in the initial state is the same and is equal to 40 HRC; the microhardness, however, is somewhat greater in alloy 2, which can be explained by the higher degree of alloying of the solid solution in this alloy (the difference in microhardness of the alloys is not detected by the first method).

Quenching in oil lowers the hardness of alloy 8 to 39 HRC, but raises the hardness of alloy 2 to 42 HRC. The microhardness of the solid solution decreases in both alloys and remains lower in alloy 8. The increase in integral hardness in alloy 2 is explained by the appearance of particles of the excess phase, since quenching from

a temperature of 1300°C fixes a greater amount of the residual phase than occurs when the alloy is cooled from the liquid state during casting.

Quenching in water lowers the hardness of alloy 8 to 38 HRC, and that of alloy 2 to 40 HRC. Measurement of microhardness indicates a greater degree of hardening of the solid solution in both alloys than with quenching in oil, which also explains the decrease in integral hardness in alloy 2 with an excess phase.

The relative influence of cobalt and tungsten can be clarified by carrying out an analogous analysis of the hardness and microhardness of alloys 6 (15% Co, 20% W) and 7 (20% Co, 25% W). In this case, better hardenability is found for alloys with a predominant cobalt content.

Thus, the hardenability of the alloys under consideration increases with increasing cooling rate and with an increase in the relative cobalt content of the alloy.

Subsequent tempering causes dispersion strengthening, fully consistent with the degree of quenching of the supersaturated solid solution (more complete quenching corresponds to greater hardness of the tempered alloy).

Conclusions

1. The alloys studied, in the initial as-cast condition, have a microstructure reflecting its relation to primary crystallization.

Alloys with a sufficiently high degree of alloying with cobalt and tungsten (more than 20% Co, 20% W), after quenching in water and oil, acquire a very fine-grained structure.

1. Increasing the cooling rate during quenching provides better retention of the supersaturated solid solution, which corresponds to a higher degree of dispersion strengthening, but also to a corresponding decrease in bending strength.

2. The advantages arising from improvement of the structure of quenched alloys must be matched by the optimum chemical composition and cooling rate during quenching.

Literature

1. A. D. Klement’ev. Study of cast dispersion-hardening cutting alloys. Izvestiya vuzov MVO SSSR, Chernaya metallurgiya, published by the Siberian Metallurgical Institute, No. 3, 1959.

2. A. D. Klement’ev. Microstructure and properties of iron–cobalt–tungsten alloys that harden during casting. (See the present collection.)