SPHEROIDIZATION AT TEMPERATURES CLOSE TO THE REGION OF THE PERITECTIC REACTION

Yu. A. Evtyushkin

(Presented by Professor Doctor A. N. Dobrovidov)

From an analysis of a number of studies [1, 2, 3] that have appeared recently, one may conclude that the question of the possibility of obtaining a cast structure without a carbide network by repeated heating to the temperature of the peritectic reaction is of interest and requires further study.

Under real cooling conditions, especially when steel is cast in metal molds, the equilibrium corresponding to the phase diagram (Fig. 1) is not attained for two reasons. First, because of the high cooling rate the reaction does not have time to proceed to completion; second, the products formed by the peritectic reaction \(Ж + Ф \to А + К\) separate the reacting phases, and further progress of the reaction can occur only through diffusion across a layer of austenite. Therefore, when cast steel is reheated to the melting temperature of the eutectic and held at this temperature, a reaction will proceed between the liquid phase and the solid phase, as a result of which the amount of eutectic will decrease [2, 3]. The remaining carbides will be fragmented and will assume a rounded form.

However, in accordance with the phase diagram, in steels with a carbon content greater than 0.7%, after the peritectic reaction some amount of liquid phase (corresponding to segment \(i - j\)) will always remain in excess.

In cast high-speed steels, a considerably higher carbon content than in forged steels is most favorable, and, consequently, the formation of eutectic in them is inevitable.

According to the data of work [4], it follows that an increase in alloying elements shifts the points \(g, h, i, j\) (Fig. 1). It has been noted that vanadium has an especially strong influence in this direction. An increase in the vanadium content in cast high-speed steels is desirable for other reasons as well. As is known, vanadium has a greater affinity for carbon than tungsten, and therefore vanadium carbides are formed instead of tungsten carbides. The total volume of carbides is thereby reduced, since vanadium carbides have a minimal volume. In addition, vanadium increases the solubility of carbon in the gamma-phase region, which improves the quality of high-speed steel. Vanadium increases the resistance of martensite to tempering, thereby significantly increasing red-

ductility; it causes dispersion hardening, which gives an increase in hardness at elevated temperatures. In addition, vanadium forms carbides possessing high resistance to wear. E. Gulyaev [5] notes that vanadium carbide VC is the hardest \((H_{Rc}=83.5—85)\) of the carbides formed in high-speed steel; it is even harder than tungsten carbide WC \((H_{Rc}=82.5)\), used in hard alloys.

Fig. 1. Polythermal section of the Fe—W—Cr—C diagram. Sections at 18% W and 4% Cr [4].

On this basis, in steels with an increased carbon content, by increasing the vanadium, an alloy corresponding to point \(i\) can be obtained.

In the microstructure of steel of heat No. 1 (Table 1) there were only signs of \(\delta\)-ferrite (Fig. 2a). The microstructure of steel 18—4—1—1.6 does not contain \(\delta\)-ferrite, since the vanadium content in this steel is somewhat lower than in steel 18—4—1—1.9, with the same amount of carbon. The microstructure of steel 18—4—1—1.6 is shown in Fig. 2b.

Table 1

Chemical composition of the steels studied

From examination of Figs. 2a and 2b it is seen that dark regions are present in the structure of both steels; however, the nature of these constituents is different. In Fig. 2a the dark regions represent a \(\delta\)-eutectoid formed from \(\delta\)-ferrite. Such a transformation of \(\delta\)-ferrite, as the discussion showed,

[3, 4], is possible. Confirmation of the formation of $\delta$-eutectoid from $\delta$-ferrite is the presence of distinct boundaries between the dark constituent and austenite. These boundaries formed as a result of the existence of two different phases (ferrite and austenite).

Fig. 2. a) ×660, b) ×440, c) ×440.

The dark regions in steel 18—4—1—1.6 formed as a result of the decomposition of enriched austenite. A similar structure, formed as a result of manifold transformations, is not always distinguishable in detail under an optical microscope. However, confirmation that in steel 18—4—1—1.6 the dark constituent formed from austenite, and not from ferrite, is the absence of distinct boundaries between the dark regions and austenite, since these two structural constituents at higher temperatures represented one phase (austenite).

Figure 2c shows a photograph of the microstructure of steel 18—4—0.9—1. The structure consists of coarse-needle martensite and retained austenite.

To determine the influence of temperature and holding time on the structure of cast steel, quenching was carried out from various temperatures (Table 2). Cooling was carried out in oil.

Table 2

Analysis of the microstructure shows that at a temperature of 1300° already after a two-minute hold, spheroidization of the ledeburite network begins. Increasing the holding time at this temperature leads to growth of the austenite grain and to coagulation of carbides.

Holding the steels under investigation at 1320° gives a considerably greater effect in terms of fragmentation of the ledeburite carbide network. Figures 3a and 3b show photographs of the microstructure of the steels after a one-minute hold at 1320°. Examination of the microstructure shows that the carbide network has broken up; in this case the carbides are located partly along grain boundaries and partly in the centers of austenite grains. This is especially clearly visible at the higher magnification in Fig. 3b.

From an examination of the microstructure shown in Fig. 3, it is evident that the carbides have begun to be distributed uniformly over the entire area of the austenite grain; at the same time, some of them acquire an angular shape, which, of course, is undesirable.

Fig. 3. a) ×750, b) ×660, c) ×1350.

Holding for 3 min at 1320° is sufficient for the complete course of the unfinished processes and for spheroidization of the carbides (Fig. 4) (steel 9—4—1—2). The structure of the steel consists of austenite and rounded carbides, which are distributed uniformly over the entire area of the austenite grains. The ledeburitic network has disappeared.

Fig. 4. ×750.

After holding at the appropriate temperature, quenching in oil, and triple tempering at 560°, the red hardness was determined. No decrease in red hardness was observed in comparison with the cast and tempered condition. On the contrary, with a holding time of 3 min (1320°) in steel 18—4—1—1.6, a tendency toward an increase in red hardness was observed.

Testing of the mechanical properties of cast steel 18—4—1—1.6, subjected to high-temperature holding for 3 min at 1320°, showed that the bending strength increased somewhat in comparison with the cast and tempered condition. An increase in impact toughness is also noticeable.

Conclusions

1. It is most advantageous to subject steels whose chemical composition corresponds to point i (Fig. 1) to high-temperature holding in the region close to the peritectic reaction.

2. Such a heat-treatment operation can be successfully used to improve the structure and, consequently, the properties of cast tools.

3. It is especially expedient to subject tools cast in metal molds to this operation. Such steel has the greatest deviation from the equilibrium state owing to the high cooling rate.

References

1. P. M. Yushkevich and S. A. Fedorova. On the microstructure of cast high-speed steel. Metallovedenie i obrabotka metallov, No. 3, Mashgiz, 1955.

2. N. S. Kreshchanovskii. Foundry Production. No. 8, 1960.

3. Kuo Kehsin. The Journal of the Iron and Steel Institute. 181, No. 2, pp. 128–137, 1955.

4. V. S. Meskin. Fundamentals of Steel Alloying. Metallurgizdat, 1959.

5. E. Gudremon. Special Steels. Part 2, Metallurgizdat, 1960.