Features of Hardening of Cast Iron–Cobalt–Tungsten–Molybdenum Alloys during Two-Stage Tempering

A. D. Klement’ev

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

Studies of cast iron–cobalt–tungsten–molybdenum dispersion-hardening alloys, being developed as tool materials, have shown that the properties of the alloys are highly sensitive to tempering conditions. In particular, for alloys that harden during casting, the set of properties important for cutting tools is improved when stepped tempering regimes are used [1, 2]. In a number of cases, an increase in the maximum hardness of the alloy during complex tempering, as compared with simple tempering, was observed.

In connection with this, a study was carried out of the hardening conditions under two-stage tempering, which made it possible to obtain a clear substantiation of some advantages and features of such tempering [3]. However, the minimum tempering time used in that study—two minutes—was insufficiently short and did not make it possible to establish the regularities of the initial stage of dispersion hardening.

The purpose of the present work is to study aging, beginning from the earliest possible phase of development of this process (from a 4-second hold), and to obtain a more complete picture of dispersion strengthening during stepped tempering.

For the study, an alloy was selected that is characterized by a moderate hardening rate and a sufficiently high level of strengthening ($66.5\ HRC$). The chemical composition of the alloy was: 20% Co, 10% W, 10% Mo, 2% Mn, 0.1% Ti (the remainder iron).

The experimental procedure ensured a high and controllable rate of heating of the specimens, which is important for short tempering holds. Heating was carried out in a lead bath. For the shortest holds, specimens 1 mm thick were used along with the main specimens—2.5 mm. Allowance for the time required to heat the specimen through, and reduction of the time for bringing the specimen to the required tempering temperature under conditions in which the aging process undergoes significant development, were carried out as follows. Taking into account that heating of the alloy to 400–450°C over several minutes proceeds without a noticeable increase in hardness (Fig. 1), the specimens were given preliminary through-heating in a lead bath at this temperature. The heating time in the tempering bath was then determined. During darkening of the sp—

the samples were transferred to this bath and held there until they acquired the heating color of the bath. One fourth of the time of such heating, determined for each specimen size, was included in the nominal tempering time.

Tempering at a single temperature was carried out on the same specimens, since interruptions in tempering, necessary for intermediate examination of the specimens, do not affect strengthening.

The possibility of differentiating hardness within very narrow ranges of its values, in addition to using the same specimens at all stages of tempering at one temperature, was achieved by determining hardness in parallel by several methods. Hardness was determined by Rockwell \(HRC\) (except for thin specimens), \(HRA\), by Vickers, and in some cases refinements were made by microhardness.

For continuity of the results of this work in the general research plan, the results of hardness measurements are given on the \(HRC\) scale.

Figure 1 shows a plot of the change in alloy hardness as a function of tempering temperature and duration. The selected intervals of time and temperature make it possible to obtain a clear picture of the dispersion hardening of the alloy. The hardnesses of the alloy vary within very wide limits for different combinations of tempering temperatures and durations. The highest value of hardness at

Fig. 1. Change in alloy hardness (20% Co; 10% W; 10% Mo; 2% Mn; 0.1% Ti; balance iron) as a function of tempering temperature and duration.

in single-stage tempering can be obtained at temperatures of 600–650°C.

Of special interest is the possibility of obtaining maximum hardness at higher temperatures (700–750°C) and very short holding times, which can be achieved under practical conditions by means of high-frequency heating.

Relatively low temperatures require considerable tempering durations, and their use may be justified in the event of obtaining a more favorable combination of alloy properties.

Figure 2 presents a graph illustrating two-stage tempering at 500–650°C with a range of durations strongly shifted toward short holding times. For convenience in placing the curves over the entire hardening range and for greater clarity, the hardness scale is taken as different in different sections of the graph.

Fig. 2. Effect of preliminary tempering at 500°C for 4 and 15 minutes and 1 hour on the hardness of the alloy as a function of the duration of subsequent tempering at 650°C

In the first stage (500°C), different specimens of the alloy were tempered for 4 and 15 minutes and for 1 hour, and were then subjected to the second stage of tempering at 650°C. For comparison, a curve is given for the change in hardness at 650°C without preliminary tempering.

The increase in maximum hardness during two-stage tempering as compared with single-stage tempering is noteworthy. In addition,

as a result, a change is observed in the hardening rate during complex tempering and, finally, the appearance of a minimum in the initial portion of the hardness curve after the longest heating in the first stage ($500^\circ\mathrm{C}$ — 1 h).

Let us examine in greater detail the features of complex tempering. The hardness curve of the alloy with a 4-minute preliminary tempering twice intersects the curve for simple single-stage tempering at $650^\circ\mathrm{C}$ and rises somewhat above it. Starting from a higher hardness level (45 $HRC$), the first curve initially runs somewhat more gently, intersecting the curve for simple tempering (which starts from a lower hardness level, 40 $HRC$), which indicates a slowing of the development of the dispersion-hardening process in the initial period of complex tempering. Subsequently, the hardening process during two-stage tempering intensifies, the curves intersect again, and with complex tempering a somewhat higher level of strengthening is attained.

The observed regularity is manifested even more clearly during tempering with an increased duration of the first stage. The higher initial hardness after 15-minute preliminary tempering (53.5 $HRC$) causes an even more noticeable slowing of the initial hardening.

A particular peculiarity in the change of hardness of the alloy is observed during complex tempering with a 1-hour hold in the first stage. The relatively high initial hardness (58.5 $HRC$) decreases at the very beginning of the second stage of tempering, and only after this do the stages noted above follow: slowed and then somewhat more accelerated hardening. Consequently, at the beginning of the second tempering stage, along with hardening, a softening process takes place, which, depending on the intensity of the preliminary tempering, is manifested as a slowing of hardening, as a delay in it, or even as its short-term decrease.

The increase in the hardness maximum with increasing duration of the low-temperature stage of complex tempering is accompanied by a tendency for this maximum to shift toward shorter durations, that is, toward accelerated overaging of the alloy.

The regularities considered indicate the expediency of studying more intensive preliminary-tempering regimes, which is reflected in the following series of experiments. Figure 3 presents a graph showing the results of analogous investigations of the same alloy during two-stage tempering at $550$—$650^\circ\mathrm{C}$. Holding at $550^\circ\mathrm{C}$ for 8, 15 min and 1 h leads to an increase in hardness (Figs. 1 and 3) respectively to 60.5, 62.5, and 65 $HRC$, which indicates a higher level of thermal action in the first tempering stage compared with the preceding series of experiments. In this case as well, for comparison, a curve is given for the change in hardness at $650^\circ\mathrm{C}$ without preliminary tempering.

The character of the change in hardness during complex tempering at $550$—$650^\circ\mathrm{C}$ for all three regimes of the first stage is analogous to the change in hardness during the corresponding tempering at $500$—$650^\circ\mathrm{C}$, but only for the case of the greatest duration of preliminary tempering ($500^\circ\mathrm{C}$ — 1 h).

The general regularities of the influence of preliminary tempering consist in the fact that, with an increase in its intensity, a more sharply expressed initial softening is revealed, as well as a subsequent acceleration of the process of dispersion hardening in the second tempering stage. At the same time, the maximum hardness of the alloy increases somewhat, and overaging is accelerated.

Some details of this process are as follows. The decrease in hardness during the initial softening, increasing as the intensity of the thermal regime of the first tempering stage is raised, reaches 3 \(HRC\) (with preliminary tempering at 550°C for 1 hour). The hardness curves for compound tempering intersect the hardness curve for single-stage tempering; that is, the hardness of the alloy in this case proves to be lower than with single-stage tempering, despite the higher initial level of hardness.

Fig. 3. Effect of preliminary tempering at 550°C for 8 and 15 minutes and 1 hour on the hardness of the alloy as a function of the duration of subsequent tempering at 650°C

As for the increase in maximum hardening, it is reached after 15 or 8 minutes, respectively, under the regimes of preliminary tempering at 550°C for 15 minutes and at 550°C for 1 hour. Evidently, the overall gain in time is more advantageously obtained by a slight increase in the duration of the second tempering. In addition, as the level of thermal action of the first tempering stage is raised, the overaging process is accelerated, as is evident from the course of the curves in the final stage of tempering (Figs. 2 and 3). Obtaining almost as high a hardness, with a somewhat greater margin against overaging, is possible with preliminary tempering at 500°C (Fig. 2), but with a relatively longer tempering duration.

Thus, the optimal conditions for obtaining the maximum alloy hardness of 67.5 \(HRC\) are provided by compound tempering: 550°C for 15 minutes—650°C for 15 minutes.

The features of the hardening process under two-stage tempering found in this work—the stage of initial softening, the subsequent acceleration of hardening, the increase in maximum hardness, and, finally, accelerated overaging—can be explained on the basis of the known regularities of the process of dispersion hardening of alloys [4].

The stage of initial softening is associated with the well-known phenomenon of reversion. Each level of tempering temperature and time corresponds to a definite composition, structure, dimensions, and nature of the bond between the excess phase and the matrix. Subsequent higher heating leads to a rearrangement of this phase in accordance with the new temperature–time conditions. This process may be accompanied either by a further increase in hardening or, as in the present case, by varying degrees of softening.

The acceleration of hardening at the next stage of complex tempering, in comparison with simple tempering, is explained by different conditions for the formation of the strengthening phase: the presence of already formed particles, the concentration of elements at precipitation sites, the dislocation state, diffusion conditions, the nature of the stress state, etc. To a certain extent, a broader and more uniform coverage of the solid solution by aging should be manifested here.

These same factors must also determine the appearance of an increase in maximum hardness. It should be borne in mind that the alloys exhibit pronounced intracrystalline segregation, manifested, in particular, in a peculiar nonuniformity in the change of microhardness and integral hardness of the alloys studied during tempering [5]. The selectivity of hardening of different regions of the inhomogeneous alloy matrix at different tempering temperatures is also known. It is evident that a more complex thermal action during two-stage tempering, along with the complication of the nature of the excess phases and a more complete involvement of the alloy in hardening, leads to additional strengthening.

As for the acceleration of overaging, i.e., a more rapid decrease in hardness, an analogous phenomenon has often been observed when additions were introduced into the main composition of an alloy; these additions likewise act to accelerate the hardening process and increase strengthening [1]. Evidently, this circumstance has a broader basis. The change in the kinetic and thermodynamic conditions of aging, which had begun at the preceding stages of hardening and is accompanied by an increase in the stressed state of the alloy, leads to an overall acceleration of the aging process.

Conclusions

1. Under two-stage tempering (with a lower temperature of the first stage), in comparison with single-stage tempering, the following regularities of the aging process are observed: a stage of partial initial softening and subsequent more intensive development of the dispersion-hardening process, accompanied by an increase in the level of maximum hardness and acceleration of the overaging stage.

2. In the change of the kinetic and thermodynamic conditions of aging under two-stage tempering, a definite role must be played by sharply pronounced intracrystalline segregation, caused by the technology used to obtain the alloys (quenching during casting in metal molds and subsequent tempering).

3. In the alloys studied, the phenomenon of reversion, observed in a number of dispersion-hardening alloys of other systems, was found.

References

1. A. D. Klement’ev. Investigation of cast dispersion-hardening cutting alloys. Izv. vuzov, “Chernaya metallurgiya,” No. 3, 1959, pp. 77–83.

2. A. D. Klement’ev. Cast dispersion-hardening cutting alloys. Proceedings of the Conference of Toolmakers of Western Siberia. Collection 1. Moscow, VNII, 1962, pp. 242–252.

3. A. D. Klement’ev. Hardening of iron–cobalt–tungsten alloys during two-stage tempering. Izv. TPI, vol. 263, pp. 3–8.

4. S. Z. Bokshtein. Structure and Properties of Metallic Alloys. Moscow, Metallurgiya, 1971.

5. A. D. Klement’ev. Investigation of the nonuniformity of properties in the cross section of castings made of iron–cobalt–tungsten–molybdenum alloys. Izv. TPI, vol. 173, 1970, pp. 178–181.