Modification of Iron-Cobalt-Tungsten Alloys

MODIFICATION OF IRON–COBALT–TUNGSTEN ALLOYS

A. D. Klement’ev

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

Dispersion-hardening iron–cobalt–tungsten alloys possess properties very important for tool materials, in particular high hardness and heat resistance. The main drawback and obstacle to their use as cutting alloys is brittleness, caused by the coarse-grained structure and by unfavorable intercrystalline phenomena arising during the process of dispersion hardening.

One of the important means of eliminating brittleness and improving the properties of these alloys is modification. Studies [1, 2] have shown that, under the influence of a whole series of small additions, the structure of the alloys is substantially refined, and the hardness, red hardness, strength, and cutting properties of the alloys under study are increased. However, questions concerning the relationship between the structural features of modified alloys and their properties and the technology of producing the alloys remain unclear. The present investigation is devoted to the study of these questions.

Melting and casting of the alloys were carried out in a vacuum induction high-frequency furnace [3]. The use of vacuum was intended to reduce the gas saturation of the metal and to eliminate unfavorable intercrystalline precipitates of a nonmetallic phase. The castings had the form of bars measuring \(10—15 \times 25 \times 100\) mm. The alloys were quenched during casting in a metal mold and were subsequently tempered. The amount of additions was selected, depending on their influence on the structure and properties of the alloys, within the range from fractions of a percent to one percent and, in a number of cases, up to several percent. At increased contents of additions, the modifying effect (refinement of the structure) was combined with the alloying action of the elements. The influence of additions was investigated on an alloy with \(20\%\) tungsten and \(20\%\) cobalt, and also on alloys with partial replacement of tungsten by molybdenum. The elements nickel, manganese, chromium, silicon, vanadium, titanium, niobium, and carbon, introduced separately and in combination, were studied as additions. Nickel, chromium, and carbon were introduced together with the charge, while the remaining elements were introduced before casting.

The additions exert a substantial influence on the microstructure, macrostructure, and fracture structure of the alloys. The fracture (Fig. 1), instead of being coarse-grained and dendritic, becomes fine-grained, homogeneous, and sometimes porcelain-like. This usually has a favorable effect on the strength and cutting properties of the alloys. However, in some cases, despite

No refinement of the grain in the fracture or improvement of the indicated properties is observed, or it is insignificant. This is not difficult to explain when defects of the fracture structure appear, such as, for example, naphthalene-like structure, but in other cases the cause should be sought in changes of the finer structure, in particular in changes of the microstructure.

The initial microstructure (Fig. 2) is characterized by tortuosity of the crystal boundaries, reflecting its connection with the primary dendritic structure.

Analysis of the microstructure of the modified alloys makes it possible, in accordance with the classification [4], to distinguish two principal types of structural change:

1. Formation of a fine-grained structure with compact grains (Fig. 2б).
2. Formation of a finely branched dendritic structure (Fig. 2в). A combination of these structural changes also occurs.

Changes caused by the alloying effect of additions may be superimposed on these principal changes in the structure. In a number of cases the interdendritic spaces are filled with decomposition products of the solid solution. In some cases a polyhedral structure appears, partially superimposed on the dendritic one or displacing it completely, which is due to the effect of the element on the conditions of recrystallization of the alloy during cooling.

Fig. 1. Types of fractures of alloys with 20% Co, 20% W and additions:
а—initial alloy (without additions); б—2% Si; в—0.35% Ti; 1% Ni, 1.5% Mn, 0.3% Si, 0.15% C, ×2.5.

The effect of the investigated additions on the structure is as follows: titanium and niobium in concentrations up to 0.2% lead to a sharp decrease in the size of the primary crystals. With an increase in their concentration, serration of the boundary regions and excessive point precipitates arise. Vanadium, in contrast to titanium and niobium, gives less refinement of the crystals and greater branching of the structure. Silicon leads to a decrease in the crystals and to increased decomposition of the solid solution, especially in the peripheral regions of the grains. Manganese, in an amount of 0.5%, leads to refinement of the crystals; at a concentration of 1% and higher, it sharply refines the dendrite arms. The effect of nickel is analogous to that of manganese. Chromium at a concentration of 1% leads to the appearance of a polyhedral structure with traces of solid-solution decomposition over the grain field.

The influence of carbon is of an alloying character. When the carbon content exceeds 0.2%, a eutectic appears. At first it has no effect on the size and shape of the dendrites. However, with increasing carbon concentration the dendrites change their shape and decrease in size owing to an increase in the amount of eutectic distributed in the interdendritic spaces.

A tendency toward branching of the structure is often observed with complex introduction of elements (Fig. 2в).

The modifying action of additions can be explained [4] by an increase in the number of crystallization centers and the appearance of a modified structure of the first type, or by the hindering effect of impurities on the growth of primary crystals and the appearance of a modified structure of the second type.

Fig. 2. Microstructure of alloys with 20% Co, 20% W and additions:
a—initial alloy (without additions); b—0.2% Ti; c—0.2% Ti, 1% Mn, 1% Ni, 0.5% Si, 0.15% C. 3×135.

As studies have shown, in some cases the same additions, depending on their concentration, can lead to both types of modification. Thus, titanium at a concentration of 0.2% leads to the appearance of small compact grains, whereas at a higher concentration, in combination with other elements, it causes the appearance of a branched structure.

A comparison of the properties of the alloys shows that the most favorable modification is that which leads to the appearance of a refined structure with a compact form of the primary crystals. The presence of indented crystalline formations, as well as the presence in them of fine precipitates of excess phases, leads to a deterioration of the properties of the alloys.

This deterioration of properties can be explained as follows. The indented nature of individual regions of the structure leads to a sharp nonuniformity of the properties of the alloys. The crystallization process itself of solid solutions (which dispersion-hardening alloys are), especially under rapid cooling (in accordance with the technology adopted in the study), should lead to significant intra-

crystalline segregation. This, in turn, causes nonuniformity of the decomposition process within individual grains of the solid solution during tempering. In addition, the thermodynamic conditions of solidification are not the same in the branched regions of the alloy, with a highly ramified crystal surface, and in the central, unbranched part of these crystals. Finally, in heavily dissected regions of the structure, accumulations of an excess phase are observed first of all, which further increases the inhomogeneity of the properties of the alloys.

Another important circumstance is the danger of microscopic porosity appearing in regions of the dissected structure of the alloy. In the regions of branched dendrites, isolated volumes of liquid appear, the crystallization of which is accompanied by the formation of small shrinkage pores. When reactions accompanied by gas evolution proceed in the metal during crystallization, the porosity increases. During crystallization under vacuum, the pore sizes will increase still more.

A number of alloys having satisfactory strength and a high degree of dispersion hardening exhibited low resistance during cutting. In the structure of these alloys, a sharply expressed branching of crystals was observed. Wear of the cutting edge of the tool proceeded nonuniformly. As metallographic studies of the cutting part of the tool showed, small spalls of the cutting edge occur at the places where the structure branches.

Measurement of hardness and microhardness reveals looseness in places of a strongly dissected structure in alloys cast under vacuum. The looseness is considerably less when casting with reduced and with removed vacuum.

Conclusions

1. The types of modified structures and their relationship to the properties and production technology of the alloys studied have been established.

2. Modification leading to refinement of primary grains having a compact shape promotes a sharp increase in the mechanical and cutting properties of the alloys.

3. The formation of finely branched dendrites during modification adversely affects the properties of the alloys.

Literature

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

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

3. A. D. Klement’ev. Vacuum induction high-frequency furnace for experimental melts. Information bulletin No. 16, Publishing House of the Bureau of Technical Information of the Tomsk Sovnarkhoz, 1–4, 1960.

4. M. V. Mal’tsev. Modification of the structure of metals and alloys. Metallurgiya Publishing House, Moscow, 27–36, 1964.