PROCEEDINGS
OF THE TOMSK POLYTECHNIC INSTITUTE
ORDER OF THE OCTOBER REVOLUTION AND ORDER OF THE RED BANNER OF LABOR
named after S. M. Kirov

Vol. 209 1976

EFFECT OF ADDITIVES AND 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 physical metallurgy, equipment and technology of heat treatment of metals, metal technology, and equipment and technology of welding production)

The use of additives makes it possible to substantially improve the properties of cast iron–cobalt–tungsten alloys that harden in the course of casting [1]. To broaden the technological possibilities of the alloys under study, it is of interest to examine the effect on them of quenching carried out after casting [2]. A significant influence of such quenching was shown for ternary alloys without additives [2]. The present study is devoted to investigation of the quenching of alloys with additives.

The microstructure of iron–cobalt–tungsten alloys with or without additives in the as-cast state retains the features of primary dendritic crystallization [2, 3].

Alloys with additives that undergo additional quenching from 1300°C are distinguished by a great variety of structures. In some cases they are analogous to those obtained in alloys without additives [2, 3]; in others they acquire a polyhedral character (grains in the form of irregular polygons) (Fig. 1, a). The excess phase (when present) is located in the form of continuous boundary or point precipitates. The picture becomes obscured during partial decomposition of the solid solution.

The effect of additives and cooling rate on the microstructure is as follows. As analysis of the experimental material has shown, elements that expand the range of existence of the α-phase (Mo, Cr, V, Ti, etc.) [3] promote the appearance of a polyhedral structure. The occurrence of premature decomposition of the solid solution (a decrease in the hardenability of the alloy) is associated with these same elements.

Illustrative examples are alloys with additions of molybdenum, which stabilizes the α-phase and, consequently, promotes the formation of a polyhedral structure and a decrease in the hardenability of the alloy. If the initial alloy 8 is resistant to formation of a polyhedral structure during quenching in water and oil [2], then replacing 10% tungsten in this alloy with molybdenum (alloy 18), while preserving the character of the initial structure after quenching in water and oil, gives a mixed structure containing, along with polyhedra, remnants of the primary structure (Fig. 1, b). In this case the initial phase remains light, while the polyhedra prove to be more readily etched, which indicates the initial stage of decomposition of the solid solution.

The polyhedral phase proves to be harder in comparison with the residual dendritic phase. Moreover, quenching in water, in comparison with quenching in ...

quenching in oil gives a somewhat lower hardness of the corresponding phases as a result of better overall retention of the solid solution. After tempering, as should be expected, the opposite ratio of phase hardness arises. Alloys and their phases with better hardenability acquire a higher dispersion strengthening. In an alloy quenched in water, the residual phase has a higher hardness in comparison with the polyhedra and in comparison with the residual phase of an alloy quenched in oil.

Fig. 1. Microstructure of alloys, quenching in oil, ×86:
a — alloy 21 (20% Co, 20% W, 35.5% V);
b — alloy 18 (20% Co, 10% W, 10% Mo).

The lowest hardness is found in the polyhedral phase of the alloy quenched in oil. The observed regularity is of a general character.

Thus, after quenching, in alloys with a polyhedral structure a smaller degree of retention of the solid solution is achieved in comparison with alloys that preserve the initial structure.

In alloys with complete replacement of tungsten by molybdenum (alloy 17), the initial structure is masked by decomposition of the solid solution, but revealing the grain boundaries shows a resemblance to polyhedrality. Quenching in oil gives a clearly expressed polyhedral structure.

Analogously to molybdenum, but more sharply expressed, is the influence of vanadium. Addition of about 1% V to the initial alloy (alloy 8) (alloy 23) gives a result that qualitatively coincides with the picture of transformations in alloy 18 (20% Co, 10% W, 10% Mo). A vanadium addition of about 4% (alloy 21) gives results (Fig. 1, a) similar to those observed in alloy 17 (20% Co, 20% Mo).

Elements that increase the stability of the γ-phase (Co, Mn, especially Ni) [3] act in the direction opposite to that considered; they retard or completely prevent the appearance of a polyhedral structure. Excess phases also retard restructuring of the structure, especially boundary phases, if they do not dissolve at quenching temperatures. In alloys with pronounced inhomogeneity of the initial structure, for example, in the presence of clusters of point precipitates of an excess phase in the branched regions of dendrites [3], quenching leads to the production of a mixed structure (polyhedra and the initial form of the phase). After equalizing annealing, quenching fixes an entirely polyhedral structure.

The process of formation of the structure of quenched alloys may be represented as follows. Upon heating of the alloy, owing to the α–γ transformation, which causes phase work hardening, conditions are created for recrystallization of austenite [4]. Under the action of additions, the temperature of the polymorphic transformation may be shifted, which should be refle...

... on the degree of phase hardening, the conditions under which recrystallization phenomena proceed, and grain growth.

In the region of lower temperatures, when diffusion processes are relatively weakly developed, there is a large number of coarse spheroidized particles of the excess phase, which hinder grain growth. Even in the vicinity of such particles, the effect of strain hardening that stimulates recrystallization ceases. Quenching fixes a more or less fine-grained structure. When, however, the polymorphic transformation occurs in the region of high temperatures, the diffusional mobility of atoms increases greatly; the particles of the excess phase, dissolving, rapidly decrease as the temperature rises and lose their influence on grain growth. Under these conditions, with a sufficiently homogeneous alloy matrix, the process of recrystallization takes place under the influence of phase hardening, forming a polyhedral structure. The sizes of the polyhedra depend on the final temperature and the holding time during heating, but also on the degree of phase hardening. Obviously, hardening will be less at a higher temperature of polymorphic transformation, owing to the increase in the plastic properties of the alloy, approaching the conditions of critical deformation, thereby promoting coarsening of the polyhedra.

Additions that raise the temperature of the polymorphic transformation at the same time promote premature decomposition of the solid solution; this explains the appearance, in a number of cases, simultaneously with polyhedrality in the initial structure of the alloy, also of products of solid-solution decomposition. Mixed structures containing, along with polyhedra, regions of an unrearranged phase (Fig. 1,b) are formed under intermediate conditions in comparison with those considered above. The presence in the polyhedral regions of signs of solid-solution decomposition reflects the elastoplastic processes occurring in them, whereas the unrearranged regions experienced only the external action in the alloy. A change in the cooling rate is reflected in both types of structural regions in essentially the same way (with an increase in the cooling rate, better fixation of the solid solution occurs); however, the difference in the degree of supersaturation of the solid solution in the regions under consideration is, for the reason indicated, not the same.

Difficulties in the formation of a polyhedral structure in alloys with pronounced structural inhomogeneity, especially in the presence of an excess phase, are explained by mechanical obstacles to structural rearrangement. Hence follows the stimulating effect, on the appearance of a polyhedral structure, of hot working by pressure or of any heat treatment that promotes an increase in the uniformity and homogenization of the alloy.

In our investigations, the usual polyhedral structure was distinguished by coarse grain size and caused low mechanical properties of the alloys. Alloys which, owing to additions in the initial state, acquired a considerable bending strength after quenching had an increased level of dispersion strengthening as a result of better fixation of the supersaturated solid solution, but they substantially reduced strength and resistance during cutting, losing their advantage.

Alloys intended for tools subjected to additional quenching after casting should not contain elements that expand the range of existence of the $\alpha$-phase in amounts capable of shifting the polymorphic transformation into the region of excessively high temperatures; or this tendency must be suppressed by means of elements that increase the stability of the $\gamma$-phase.

Conclusions

1. The alloys studied, after quenching at different cooling rates, possess a great variety of structures.

The work establishes and explains the dependence of the character of the microstructure on the chemical composition and cooling rate of the alloys.

1. Alloys with additions that are quenched during the casting process and acquire high mechanical and cutting properties, after the application of additional quenching (they usually have a polyhedral structure) lose their advantages.

2. The principles of alloying alloys that are quenched after casting have been outlined.

References

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

2. A. D. Klement’ev. The influence of quenching conditions on the microstructure and properties of cast iron-cobalt-tungsten alloys (see the present collection).

3. A. D. Klement’ev. Microstructure and properties of iron-cobalt-tungsten alloys quenched during casting (see the present collection).

4. V. D. Sadovskii, K. A. Malyshev, B. G. Sazonov. On methods of grain refinement in alloyed cast steel by heat treatment. Tr. IFM, issue 17, Ural Branch, USSR Academy of Sciences, publ. by the USSR Academy of Sciences, Moscow, 1956.