On the Issue of Dispersion-Hardening Alloys

N. D. TYUTEVA and A. D. KLEMENT’EV

The phenomenon of dispersion hardening is characteristic of many engineering alloys. It is observed in alloy systems with limited solubility of the components, decreasing with decreasing temperature.

Alloys in which quenching fixes a supersaturated solid solution become strengthened during subsequent aging. The change in mechanical properties is expressed in an increase in hardness, elastic limit, and strength, and usually in a decrease in ductility, although in individual cases the ductility changes hardly at all (duralumin).

Table 1 gives examples of changes in the mechanical properties of several alloys with pronounced dispersion hardening.

Table 1

The process of dispersion hardening is also accompanied by changes in physical properties, in a number of cases to the same significant extent as the mechanical properties. Dispersion hardening is an important source for obtaining alloys with favorable combinations of properties, for example, high strength with sufficient lightness in aluminum- and magnesium-base alloys, high coercive force with high residual induction in magnetic alloys, and so on.

Of particular interest to us are iron-base alloys possessing high hardness and red hardness. It is natural to seek to use these alloys for cutting tools in connection with the general problem of cast tools. Whereas considerable experience has been accumulated with respect to light alloys, the question of cutting dispersion-hardening alloys requires its own solution.

The present work has as its aim the identification of certain features of dispersion-hardening alloys in connection with their use as a cast cutting tool.

Selection of Alloys

The theoretical treatment of the question of dispersion hardening of alloys still remains insufficient. Existing theories do not find comprehensive confirmation or a sufficiently reliable substantiation. This circumstance complicates the problem of designing alloys with the required properties.

In its simplest form, the process of dispersion hardening is observed in binary systems. In more complex systems the result of dispersion hardening will depend on the character of the interaction of the components. Any additional component may lead to a change in the mutual solubility of the components or to a change in the composition of phases of limited solubility in the solid solution, which may lead both to an increase and to a decrease in the degree of dispersion hardening of alloys.

As a rule, the appearance of stable chemical compounds promotes an intensification of dispersion hardening, and to a greater degree in multicomponent systems.

One may point to a considerable number of elements which, with iron, form systems with dispersion-hardening alloys. As examples, the following binary systems may be cited: Fe—C; Fe—O; Fe—N; Fe—Cu; Fe—Ae; Fe—Be; Fe—W; Fe—Mo; Fe—Ti, etc. Guided by the above considerations, ternary alloys Fe—W—Co and Fe—Mo—Co were selected for investigation, in which dispersion hardening manifests itself to a very strong degree, as is indicated by investigations of an earlier period.

Binary diagrams Fe—W, Co—W, Co—Fe (Figs. 1, 2, 3), and also the ternary diagram Fe—Co—W (Fig. 4), indicate the presence of the chemical compounds Fe$_3$W$_2$ and CoW, which are strengthening phases and determine—

Fig. 2.

(Phase diagram with fields labeled: liquid; liquid + ε; ε; ε + δ; δ; δ + ε; MgSi; H + δ; H; Cu?; βCu?; and temperature marks 790, 1480, 1520, 825, 1115, 403. Axes: concentration, at. %, and \(t^\circ\)C.)

Fig. 3.

(Phase diagram with fields labeled: liquid; liquid + β; β; β + γ; γ; γ + β; liquid + γ; liquid + α; α + γ; α; γ + χ; χ + γ; liquid + χ; and \(t^\circ\)C. Axes: concentration, at. %, and \(t^\circ\)C.)

undergoing dispersion hardening of the alloys of the Fe—Co—W system in the course of their precipitation. Replacing tungsten with molybdenum gives similar diagrams and analogous hardening results.

Fig. 4

For the investigation, both single-phase and two-phase alloys were taken from characteristic regions of the ternary diagram; their compositions are given in Table 2.

Table 2

For confirmation of the analogy in the behavior of tungsten and molybdenum alloys in the process of dispersion hardening, one alloy (No. 5) was taken with molybdenum.

Melting technology

Melting was carried out in a fireclay crucible with induction heating by high-frequency currents obtained from a tube generator of type AZ-46, with a power of 60 kW at 250,000 cycles per second. The melting process,

to avoid the influence of the atmosphere, was carried out under slag (molten glass). Deoxidation was by aluminum. Casting was performed into a massive metal mold, which ensured a sufficiently high cooling rate. Casting of milling-cutter blades (weighing about 100 g) was carried out in a chill mold. In most cases the fracture structure revealed a coarse-grained dendritic structure of the cast specimens.

Heat Treatment of the Alloys

All the alloys were quenched during casting into a metal mold and were tempered at various temperatures in order to determine the optimum tempering temperatures giving the greatest hardness. The tempering time was 1 hour.

Fig. 5

The graph of the change in hardness at various tempering temperatures of alloys quenched during casting is presented in Figs. 5 and 6.

Fig. 6

In order to determine the possibility of quenching during cooling in metallic molds, and also to exclude possible accidental results, all alloys were annealed and quenched.

Quenching was carried out in a barium chloride salt bath from a temperature of 1300° after a 4-minute hold, with cooling in oil. The corresponding results for a number of alloys are given in the graph in Fig. 7.

Fig. 7

A comparison of the graphs in Figs. 5, 6, and 7 leads to the conclusion that quenching during the casting process and special quenching give very similar results, which accounts for the identical hardening results in both cases.

Table 3 gives the limits of hardness change in the process of dispersion hardening.

Table 3

The greatest increase in hardness was obtained by alloys Nos. 10, 5, 1, 2, 3—from 17 to 39 units $R_c$. The composition of these alloys lies within the range from 15 to 30% tungsten and cobalt. The greatest hardness was shown by alloy No. 10, of composition 30% W and 30% Co.

The greatest degree of dispersion hardening was given by alloys Nos. 10, 5, 9, 2, 1, i.e., the same as in the preceding case, not counting alloy No. 9, whose composition is W—50%, Co—20%.

Table 4 gives the tempering temperatures corresponding to the greatest degree of dispersion hardening of the alloys. This temperature varies within the range 600–700°.

Table 4

Table 5 gives hardness data after tempering for 6 hours at a temperature of 950°.

Table 5

The retention of hardness at a high level under such high and prolonged tempering, as well as the high temperature of optimal hardening, indicate the high red-hardness of alloys of the Fe—Co—W and Fe—Co—Mo systems.

Cutting Test

These tests pursue the aim of revealing the characteristic features of the behavior of the alloys in the cutting process, without claiming a comprehensive evaluation of the cutting properties of these alloys. For this purpose, face milling was carried out, which imposes sufficiently varied and severe requirements on the alloys tested. In addition, the use of the available experience in testing cast cutting tools makes it possible to draw a broader conclusion about the cutting properties of these alloys. For the test, knives for face mills were cast in a chill mold from alloys Nos. 2 and 10.

Milling was carried out on a horizontal milling machine with a single-tooth cutter (the inserted test blade) on the end face of a round blank made of ShKh-15, \(d=130\) mm, \(H_B=180\). Tool life was determined by the number of passes until 1.5 mm of wear on the rear face of the blade. The standard was a forged blade made of RF-1, for which, under the cutting conditions \(n=48\) rpm, \(S=14\) mm/min, \(t=3\) mm, the tool life amounted to 7 passes.

Table 6 gives the results of cutting tests of blades Nos. 2 and 10 under two variants of heat treatment.

Table 6

The results of the cutting test indicate the increased brittleness of the alloys, which prevents the utilization of the wear-resistant properties of cast blades. Very slightly increasing wear over the course of three passes in testing blade No. 2, at its hardness \(R_c=58\), indicates good wear resistance. This latter circumstance makes it possible to conclude that it is advisable to study these alloys when they are used for lighter types of work or with a different geometry of the cutting edge, which will make it possible to realize the high red hardness and wear resistance of these alloys.

Conclusions

1. The greatest effect of dispersion hardening is given by alloys Nos. 10, 5, 1, 2, 3, containing \(W=15\div30\%\), \(Co=15\div30\%\). The hardness reaches \(R_c=69\).

2. Most of the alloys obtained had sharply pronounced brittleness. The fracture was coarse-grained.

3. Testing by face milling did not yield positive results because of the brittleness of the alloys.

4. Alloys with a high degree of dispersion hardening possess high wear resistance and red hardness; the possibility of using them requires special study.