STUDY OF THE NONUNIFORMITY OF PROPERTIES IN THE CROSS SECTION OF CASTINGS MADE OF IRON–COBALT–TUNGSTEN–MOLYBDENUM ALLOYS

A. D. KLEMENT’EV

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

An important requirement imposed on a cutting tool is sufficient uniformity of cutting properties within the working part of the tool. Substantial difficulties arise in this respect in the development of cast dispersion-hardening iron–cobalt–tungsten–molybdenum cutting alloys. The present study is devoted to elucidating the factors that affect the nonuniformity of the properties of alloys across the cross section of a casting and to studying the possibility of eliminating this drawback.

The alloys were hardened during casting into a metal mold; a strengthening tempering was then carried out. With the aid of additions and vacuum melting, the structure and properties of the alloys under study are substantially improved. The structure becomes fine-grained and more homogeneous across the casting section; hardness, red hardness, and bending strength increase; and some alloys acquire high resistance in cutting [1, 2].

However, in many cases, despite sufficient homogeneity of structure (microstructure, macrostructure, fracture structure), there is considerable nonuniformity of properties across the casting section. The properties deteriorate in the direction from the periphery toward the center of the casting. Bending strength decreases noticeably, and the cutting properties deteriorate significantly.

Many alloys with a homogeneous structure, having in the initial state (in the hardened condition) practically the same hardness (HRC) throughout the entire casting section, exhibited in the central part a bending strength considerably lower than at the periphery. After tempering, a difference is also found in the hardness of the corresponding areas of the casting (up to 3 HRC). This makes it necessary, when repeating experiments or when comparing the properties of different alloys, to use symmetrical or corresponding parts of the castings.

The noted discrepancy between hardness and other properties in the hardened state is clarified by studying the microhardness of the alloys. Microscopically, the primary dendritic structure of the alloy is revealed [2]. Significant nonuniformity of properties occurred in alloys with a compact form of the central part of the dendrite grains (hereafter, grains) and with indented, finely branched edges of these grains (hereafter, intergranular regions).

Measurement of the microhardness (of hardened alloys) showed that the hardness of the grains at the periphery (at a depth of up to 3 mm) is lower than at the center.

of the casting, whereas the hardness of the intergranular regions is, conversely, higher at the periphery. Such a ratio of the hardness of grains and intergranular regions at the periphery and in the center (excluding the effect of shrinkage porosity of the casting) in a number of cases leads to leveling of the integral (HRC) hardness over the entire cross section of the casting.

The study of microhardness in different states of the alloys makes it possible to clarify the essential causes of the nonuniformity of alloy properties across the section of castings. As the investigations showed, the nonuniformity of grain hardness across the section of castings is due mainly to the degree of their hardenability. The nonuniformity of hardness of the intergranular regions depends both on the degree of their hardenability (the fine-branched part of the dendrites) and on the influence of vacuum on these regions during the formation of the casting.

Let us consider the hardenability conditions of the alloys studied. To obtain different cooling rates of the alloys during casting, a steel split mold was used, which made it possible to vary the thickness of its walls. From the inside the mold was coated with washes of various thicknesses. The mold was heated to different temperatures within the range up to 500°. The nonuniformity of alloy hardness proved to be minimal for intermediate optimum cooling rates, but rates that ensured sufficiently complete fixation of the supersaturation of the solid solution. For a casting with a section of \(8\text{–}10 \times 25\) mm the mold had walls \(10\text{–}12\) mm thick with a thin marcalite–clay coating. The mold temperature was about 200°.

The relation between the degree of hardenability of the alloy and the hardness in the quenched state and after tempering has the character usual for dispersion-hardening alloys: the more complete the quenching, the lower the hardness of the alloy and the higher the degree of dispersion strengthening.

The hardenability of alloys can be regulated within known limits by means of additions. Manganese and nickel increase hardenability, whereas chromium, silicon, vanadium, and others decrease it. This action of the elements is due to their effect on the temperature of the gamma–alpha transformation, in connection with expansion or contraction of the gamma region. A shift of the gamma–alpha transformation into the region of higher temperatures and, consequently, greater diffusion mobility of the atoms reduces the hardenability of alloys, promotes premature decomposition of the solid solution, increases the hardness of the alloys in the quenched state, and reduces the reserve of dispersion strengthening during tempering.

Alloys with a hardness level of 39–41 HRC in the initial quenched state and 65–68 HRC after tempering were investigated.

Reflecting the degree of hardenability of different regions of the alloy, the grain hardness, as would be expected, changes as follows. In the quenched state the grain hardness is lower at the periphery (by tens of HV units), and after tempering it is lower in the central part of the casting (by approximately 300 HV units). In intergranular regions, which are thin branches of dendrites, increased decomposition of the solid solution (decreased hardenability—higher hardness) should be observed in comparison with the grains. To an even greater extent this decomposition should be expressed in the central parts of the casting. The difference in hardness between the grains and the intergranular regions should also increase as a result of intracrystalline liquation (the appearance of which is caused by the high cooling rate of the alloys), leading to depletion of the peripheral regions of grains in refractory components (tungsten and molybdenum), which are responsible for dispersion strengthening of the alloys studied.

Thus, the hardness of the intergranular regions should increase toward the center of the casting; in reality, however, this hardness

decreases (in the hardened alloy by approximately 100 HV units). One of the reasons for such a decrease in the hardness of the interdendritic regions is the appearance of looseness caused by shrinkage microporosity, which arises in the thin-branched dendrite tips, where isolated volumes of liquid can easily appear during crystallization. Vacuum intensifies porosity, especially during melting with incomplete gas-formation processes.

Pouring without vacuum or under pressure should reduce this defect. And indeed, pouring almost at atmospheric pressure (pouring began when air was slowly admitted into the furnace and its pressure reached approximately 200 mm Hg) led to a decrease in the looseness of the interdendritic regions in the middle of the casting. The difference in the hardness of the interdendritic regions at the periphery and in the center of the casting when poured under vacuum is about 100 HV units, whereas with nonvacuum pouring it is, respectively, several tens of HV units.

It should be noted that the hardness of the grains and of the interdendritic regions in the center of the casting is approximately the same (at the periphery, the hardness of the corresponding regions differs by tens of HV units). However, this uniformity of microhardness in the center of the casting in the hardened state of the alloy is due to premature decomposition of the solid solution and to some remaining looseness, which have opposite effects on the hardness of the interdendritic regions. Subsequent tempering reveals a significant nonuniformity of microhardness in the central part of the casting (up to 100 HV units), but still roughly half as much as when poured under vacuum. Removing the vacuum during pouring has no effect on the nonuniformity of microhardness at the periphery of the casting.

Thus, nonvacuum pouring reduces the nonuniformity of microhardness in the center of the casting, bringing it to the level of nonuniformity at the periphery. The main reason for this remaining nonuniformity is the premature decomposition of the solid solution in the interdendritic regions. This is well confirmed by experiments involving additional hardening, which ensures through, uniform hardenability over the entire cross section of the casting. As a result, a small scatter of microhardness is observed over the whole cross section of the casting both in the hardened and in the tempered states.

A certain increase in the cooling rate during pouring in air has no substantial effect on the leveling of microhardness.

In alloys poured under vacuum, despite additional hardening, considerable nonuniformity of microhardness is observed in the center of the casting and nonuniformity of various properties over its cross section, which is due to looseness in the interdendritic regions. The microhardness near the casting surface is fairly uniform both in the hardened and in the tempered states. The hardness of the grains differs little within the entire cross section of the casting, both after hardening and after tempering, whereas the interdendritic regions in the center have a significantly lower hardness (by approximately 200 HV units) than at the casting surface.

The nonuniformity of integral hardness over the casting cross section in the hardened state amounts to as much as 0.5–1 HRC, and in the tempered state to as much as 1–1.5 HRC, which is substantially less than in the absence of additional hardening.

Conclusions

Investigation of microhardness makes it possible to clarify the causes of the nonuniformity of properties across the cross section of castings of the alloys studied. The responsibility

responsible for the reduction of such alloy properties as bending strength and cutting properties are the regions of fine-branched dendrites (intergranular regions), in which premature decomposition of the solid solution occurs, as well as the friability that appears in these regions.

In the outer regions of the casting, owing to better hardenability, the nonuniformity of microhardness is relatively small. In the central parts of the casting the degree of hardenability is lower, especially in the branches of the dendrites, where, in addition, friability appears, caused by shrinkage microporosity and intensified under the influence of casting under vacuum.

Nonvacuum casting reduces friability in the intergranular regions and improves the properties of the central part of the casting. The use of additional hardening, which ensures through, uniform hardenability of the casting, leads to an equalization of microhardness both in the hardened and in the tempered condition; this should also be promoted by a reduction of intracrystalline segregation during heating for hardening. The simultaneous use of nonvacuum casting and additional hardening makes it possible to obtain sufficient uniformity of microhardness (within several tens of HV units) throughout the entire cross section of the casting.

To increase the density of the casting, maximum degassing of the metal during vacuum melting is necessary. During casting it is advisable to use inert-gas pressure, as well as pressure casting or centrifugal casting.

The elimination of nonuniformity of properties over the casting cross section is substantially simplified in alloys with a fine-grained, compact primary crystallite structure without fine-branched formations.

REFERENCES

1. A. D. Klement’ev. Investigation of cast dispersion-hardening cutting alloys. Izv. vuzov, Ferrous Metallurgy, No. 3, 77–83, 1959.

2. A. D. Klement’ev. Cast dispersion-hardening cutting alloys. Modern design of cutting tools. Proceedings of Toolmakers of Western Siberia, coll. 1, TsINTIAM Publishing House, Moscow, 242–252, 1962.