HARDENING OF IRON–COBALT–TUNGSTEN ALLOYS DURING TWO-STAGE TEMPERING

A. D. Klement’ev

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

In the experimental development of tempering regimes for cast cutting tools made of precipitation-hardening iron–cobalt–tungsten–molybdenum alloys, some variants of step tempering showed good results [1, 2]. The alloys were tempered in the initial as-cast state, since they were hardened during casting into a metal mold. It was observed that, for individual alloys, under certain step-tempering regimes the preceding low-temperature stage had a stimulating effect on subsequent hardening at a higher temperature. However, in the studies of that period the features of alloy hardening under different tempering variants were not studied specifically, since the main criterion for evaluating tempering regimes was cutting properties. This article considers some results of investigations of the hardening process of iron–cobalt–tungsten alloys during two-stage tempering.

The regularities of step tempering are revealed most clearly in alloys with pronounced precipitation strengthening. Convenient in this respect is an alloy with 25% Co and 25% W. The hardness of the alloy in the as-cast, hardened-during-casting state is 40 $HRC$; the maximum hardness after tempering is 68 $HRC$. An important feature of this alloy is the homogeneity of its structure (solid-solution grains), which makes it possible to obtain stable results in repeated hardness measurements, since in some cases it is necessary to differentiate hardness within very narrow ranges of values. It proved possible here to use hardness measurement by $HRC$, which is also important for the continuity of the results obtained in the overall plan of investigation of the alloys studied. In necessary cases, refinement by microhardness was carried out.

Figure 1 gives a graph of the change in hardness of the alloy at various tempering temperatures. Hardness was determined for each tempering temperature on the same specimens, since intermediate cooling and heating for measuring hardness have no influence on the course of strengthening. This eliminates scatter in hardness values due to differences between individual specimens and makes it possible to observe the smallest changes in hardness, depending only on the tempering conditions. Duplicate specimens were used to prevent accidental errors.

Figure 2 presents a graph of the effect of preliminary tempering at 500° for 30 minutes on subsequent tempering at temperatures from 600 to 750°C. For comparison, curves of the change in hardness at the corresponding temperatures without preliminary tempering are also plotted there.

Fig. 1. Change in hardness of an alloy with 25% Co, 25% W as a function of tempering temperature and duration

Attention is drawn to the change in relative hardness during complex and simple tempering as a function of temperature–time conditions, which is graphically manifested in the intersection of the corresponding curves. A change in the hardening conditions during two-stage tempering affects not only the initial stage of strengthening, but is associated with the entire subsequent course of the process. At some stages of hardening, an increase in hardness is observed during two-stage tempering compared with simple tempering.

Let us consider the features of the change in hardness as a function of the temperature and duration of complex tempering. Hardening during tempering at 600°C (Fig. 2a) indicates an initial retardation of strengthening during complex tempering compared with simple tempering. Subsequently, the hardening process during complex tempering intensifies, and the hardness curves intersect. The hardness during two-stage tempering proves to be somewhat higher (an increase in hardness).

During tempering at 650°C (Fig. 2, б), the process is analogous to the preceding one, but the point of intersection of the curves shifts toward shorter tempering times. A somewhat greater increase in hardness is observed.

The regularity that has emerged in the mutual arrangement of the curves at 700°C (Fig. 2, в) at first glance appears to be violated: the point of intersection of the curves is shifted far to the right, and the initial stage of hardness decrease, noted in the preceding cases, is absent, or it has moved to the right-hand part of the graph. In reality, the analogy here with the preceding course of the hardening process is preserved, but the picture is supplemented by new details. If—

Legend in figure:
○ — single-stage tempering
△ — two-stage tempering
First stage: 500°C, 30 min

Fig. 2. Effect of preliminary tempering at 500°C for 30 min on the hardness of the alloy as a function of the temperature and duration of subsequent tempering

—in accordance with the tendency that has appeared, the shift of the curves toward shorter tempering times is continued, then the point of intersection of the curves should lie outside the limits of the graph, which is what occurs during tempering at 700°C. The distinctive feature here consists in the appearance of another point of intersection of the curves in the right-hand part of the graph. As can be seen from the graph, the region of hardness increase during complex tempering shifts toward shorter tempering times, while the magnitude of the hardness increase (the interval in the course of the curves) reaches 1.5 HRC.

Tempering at 750°C (Fig. 2, г) is characterized by a further shift of the curves to the left, following the established regularity. Beyond the point of interse—

...of the curves clearly shows a stage of accelerated softening during complex tempering.

It may be assumed that the stage of initial softening, manifested in a slowing of the precipitation-hardening process, exists both at temperatures of 600 and 650°C and at higher tempering temperatures. At temperatures of 600 and especially 650°C this stage is not fully revealed because shorter holding times are absent. However, the presence of such a stage follows from the relative values of the initial hardnesses and the mutual arrangement of the hardness curves. Hardening during complex tempering begins from a higher hardness (53.5 HRC, Fig. 1, tempering at 500° for 30 min) than in simple tempering (40 HRC). To resolve the question definitively, special experimental studies at shorter tempering durations will be required.

Thus, the change in hardness during two-stage tempering, as compared with single-stage tempering, proceeds in the form of the following successive stages: a stage of initial softening, a stage of intense hardening accompanied by an increase in hardness, and a final stage of accelerated overaging.

These regularities can be interpreted on the basis of known concepts of the precipitation-hardening process. The dispersed phase formed at the low tempering stage proves insufficiently stable under higher heating; this may lead to its dissolution (the reversion phenomenon) and to the formation of a new phase, or to rearrangement of the original phase in accordance with the higher tempering temperature and its duration. The latter circumstance evidently explains the stage of initial softening during complex tempering.

The stage of subsequent acceleration of hardening is explained by the stimulating influence of the preceding stressed state, the presence of clusters of alloying elements, and a more developed dislocation structure. In addition, a broader inclusion of the alloy volume in aging may be assumed. The latter circumstance is due to the fact that the hardening process across the section of the alloy proceeds nonuniformly and non-simultaneously in regions with a homogeneous matrix structure and along grain boundaries, mosaic boundaries, in regions of dislocation accumulation, etc. This should play an especially noticeable role owing to the pronounced intracrystalline segregation, which is associated with the alloy-production technology adopted in our studies (tempering after casting). Under the action of different temperatures and more complex conditions of precipitation hardening during two-stage tempering, conditions are created for more uniform hardening of regions of different structure in the alloy. These circumstances, by intensifying the precipitation-hardening process, evidently cause not only acceleration but also an increase in hardness during complex tempering.

The magnitude of the hardness increase grows as the tempering temperature is raised from 600 to 700°C (Fig. 2). The greater increase in hardness at high tempering temperatures may be explained by the fact that the manifestation of the factors of the softening stage is then more strongly overlapped by the action of the factors of the hardening stage. However, at an even higher temperature (750°C), coarsening of the precipitation-hardening structure probably begins to make itself felt, and the increase in hardness decreases somewhat.

At the overaging stage, the factors that stimulate the development of precipitation hardening under conditions of high diffusional mobility of atoms lead to an earlier completion of this process.

The acceleration of all the hardening stages considered above, the deformation, and the shift of the hardness curves to the left during complex tempering are also associated with the development of diffusion processes as the temperature is increased.

The subsequent hardening during two-stage tempering is influenced by the nature of the initial dispersed phase, which, in turn, depends on the temperature and duration of its formation. Figure 3 presents a plot of the effect of these factors on subsequent hardening at a temperature of 650°C, at which the regularities of interest to us are clearly revealed. In addition, this temperature is of the greatest practical interest because of the close correspondence between it and the operating conditions of the cutting tool and its heat treatment.

Hardening at 650°C after preliminary tempering at 500° for 30 minutes (Fig. 3, б) has already been considered (Fig. 2, б). Shortening the time

Fig. 3. Effect of the temperature and duration of preliminary tempering on the hardness of the alloy as a function of the time of subsequent tempering at 650°C

of the first tempering to 15 minutes should lead to the formation of a less developed dispersed phase, exerting a corresponding influence on the subsequent hardening, as follows from the graph (Fig. 3, б).

Preliminary tempering at 450°C (Fig. 3, а) for two hours has an effect analogous only to that of the 500°—30-minute tempering considered above. Reducing the holding time at the first tempering stage to 30 minutes at the same temperature leads to a further decrease in the stability of the dispersed phase, which exerts an even smaller influence

on the process of subsequent hardening. The graph does not even show the first intersection of the curves (it remains outside the graph, to the left).

A certain peculiarity arises with a considerable duration of preliminary tempering at 550°C (Fig. 3c). After four-minute tempering at the first stage, with an insignificant change, the process observed after tempering at 500°—30 minutes—650° is repeated (Fig. 3b). Increasing the duration of preliminary tempering at 550° to 15 minutes leads to results that are the logical continuation of the emerging regularity, consisting in a prolongation of the period of temporary softening. Seemingly in contradiction with this is the position of the hardness curve after preliminary tempering at 550°—1 hour: the intersection of the curves is shifted to the left (in the preceding cases it shifted to the right as the holding time increased). However, this circumstance may be explained by the more pronounced stimulating effect of the preliminary tempering stage and by the high hardness of the alloy before the second tempering stage (65 HRC, tempering at 550°—1 hour, Fig. 1).

It follows from the graph (Fig. 3) that raising the temperature of preliminary tempering is, to a certain extent, analogous to increasing its duration.

The influence of the first stage on the various stages of complex tempering may be represented as follows. As strengthening during preliminary tempering increases, the initial softening of the alloy increases. The increase in hardness at the stage of intensive development of the dispersion-hardening process grows with increasing dispersion strengthening at the first stage, but only up to a certain limit (Fig. 3c). The maximum value of the increase in hardness is attained at the second tempering stage at 650° after 1 hour following preliminary tempering at 550°—15 minutes, or, respectively, after 15 minutes at 550°—1 hour. As noted above, the maximum value of the increase in hardness is also determined by the optimum temperature of the second tempering (Fig. 2). A substantial acceleration of the overaging stage is observed after preliminary tempering with the highest degree of dispersion strengthening (Fig. 3c). Thus, obtaining the maximum increase in hardness without noticeable overaging is achieved at optimum values of the first and second stages of complex tempering.

Comparison of the conditions of preliminary tempering with the subsequent one makes it possible to outline the optimum regime of step tempering. A variant of accelerated tempering for attaining the maximum hardness of the alloy will be a two-stage tempering of 500–550°—15–30 minutes—650°—15–30 minutes. In the experimental development of a tempering regime for cast dispersion-hardening cutting alloys, good results were shown by three-stage tempering at 550°—15 minutes—650°—15–30 minutes—550°—1 hour [1, 2], which here receives a certain clear justification.

REFERENCES

1. A. D. Klement'ev. Investigation of cast dispersion-hardening cutting alloys. Izvestiya vuzov MVO SSSR, “Ferrous Metallurgy,” 1959, No. 3, 77–83.

2. A. D. Klement'ev. Cast dispersion-hardening cutting alloys. Contemporary design of cutting tools. Proceedings of the conference of toolmakers of Western Siberia. Collection I. Moscow, VNII, 1962, pp. 242–252.