EFFECT OF COBALT ON THE TRANSFORMATION DURING TEMPERING AND ON THE RED HARDNESS OF CAST HIGH-SPEED STEEL

V. A. KASHCHUK

(Presented by Professor Doctor A. N. Dobrovidov)

The manufacture of cutting tools by casting is simpler and cheaper than the manufacture of forged and rolled tools [1].

A tool made of high-speed steel, obtained by casting in metal molds (chill molds), is quenched during the cooling of the casting and does not require heat treatment other than tempering. The new manufacturing technology opens up new possibilities for the use of cobalt high-speed steels. Many types of cutting tools of simple shape, when manufactured by chill casting, can be obtained in finished form, requiring only sharpening along the cutting edges and triple tempering [2].

Thus, the study of the influence of cobalt on the tempering process of chill-cast high-speed steel, as well as the determination of its red hardness, is of practical and scientific interest. The results of the author’s investigations are given below.

The steels studied were melted in a high-frequency induction furnace. The charge consisted of scrap R18 steel, ferroalloys, and specially prepared cast iron. The steel was poured into metal molds on a centrifugal machine.

The chemical composition of the steels is given in Table 1.

To study the kinetics of transformation processes during tempering of the steel, specimens of all heats were tempered 3 times for 1 hour at 560°, and then subjected to repeated one-hour tempering at temperatures of 560, 600, 625, 650, and 700°C.

Separate specimens were used for each heating temperature.

The results of hardness measurements of cast specimens of steels from heats 1, 4, and 7 after different numbers of temperings at different temperatures are presented as graphs on the left in Figs. 1, 2, and 3. On the right in these figures are plotted curves for determining red hardness “according to Geller,” which is characterized by the maximum temperature of fourfold tempering after which the steel retains a hardness equal to 60 HRC. The vertical dashed line in the graphs on the right indicates the red hardness of the steels in °C.

It was established in this work that, in the process of repeated tempering at 560°, with increasing cobalt content the number of temperings,

Melt 1.

Fig. 1. Red hardness and change in hardness after multiple tempers at different temperatures for steel of melt 1.

Left plot:
Vertical axis: \(H_{RC}\), hardness.
Horizontal axis: number of tempers with a holding time of 1 hour.
Curves: \(560^\circ\), \(600^\circ\), \(625^\circ\), \(650^\circ\), \(700^\circ\).

Right plot:
Vertical axis: \(H_{RC}\), hardness.
Horizontal axis: tempering temperature.
Temperature marks: \(550^\circ\), \(600^\circ\), \(650^\circ\), \(700^\circ\), \(750^\circ\).

Melt 4.

Fig. 2. Red hardness and change in hardness after multiple tempers at different temperatures for steel of melt 4.

Left plot:
Vertical axis: \(H_{RC}\), hardness.
Horizontal axis: number of tempers with a holding time of 1 hour.
Curves: \(560^\circ\), \(600^\circ\), \(625^\circ\), \(650^\circ\), \(700^\circ\).

Right plot:
Vertical axis: \(H_{RC}\), hardness.
Horizontal axis: tempering temperature.
Temperature marks: \(550^\circ\), \(600^\circ\), \(650^\circ\), \(700^\circ\), \(750^\circ\).

Fig. 3. Red hardness and change in hardness after multiple tempers at different temperatures for steel of melt 7.

Left plot:
Vertical axis: \(H_{RC}\), hardness.
Horizontal axis: number of tempers with a holding time of 1 hour.
Curves: \(560^\circ\), \(600^\circ\), \(625^\circ\), \(650^\circ\), \(700^\circ\).

Right plot:
Vertical axis: \(H_{RC}\), hardness.
Horizontal axis: tempering temperature.
Temperature marks: \(550^\circ\), \(600^\circ\), \(650^\circ\), \(700^\circ\), \(750^\circ\).

Table 1

The number of tempers required to achieve maximum hardness increases steadily. If for the steel of heat 1 it was equal to 1, then in the steels of heats 4 and 5 it is already equal, respectively, to 4 and 5 tempers, and in the steel of heat 6—to 10 tempers. In the steel of heat 7, even as a result of tenfold tempering, it is not possible to raise the hardness above 50 \(H_{RC}\).

It should be noted that in the steel of heat 2, as in the steel of heat 1, the maximum hardness is attained after the 1st tempering; that is, here there is no need to increase the number of tempering operations.

A comparison of the graphs shown in Figs. 1, 2, and 3 shows that, with increasing cobalt content in the steel, its hardness during repeated tempering at high temperature increases. Thus, after fourfold tempering at 700°, the hardness of the steel of heat 1 is 47 \(H_{RC}\), while for the steel of heat 7 it is 61 \(H_{RC}\).

The principal factor determining the cutting properties of high-speed steel is its red hardness, i.e., its ability to retain high hardness when heated to the temperatures at which red glow begins (about 600°).

The red hardness of the experimental steel was determined by the method developed by Yu. A. Geller. The specimens were tempered three times at a temperature of 560° and then subjected to fourfold heating at 560, 600, 625, 650, and 700°C, with a holding time of 1 hour. The red-hardness values are given in Table 2.

Table 2

To determine the best temperature interval for tempering, as well as the degree of stability of retained austenite and martensite with increasing tempering temperature, steel specimens from all heats were subjected to tempering at successively increasing temperatures in the interval 100–770°C, with a holding time of 1 hour at each heating temperature.

The changes in hardness during successive tempering are given in Table 3.

Table 3

Hardness, \(H_{R}C\)

When analyzing the data in Table 3, attention should be paid to the following features of high-speed steel with cobalt: if, in the steel of heat 1, the increase in hardness after its drop in the range \(300—400^\circ\) begins at \(400^\circ\) (\(63.5\ H_{R}C\)) and reaches a maximum at \(550^\circ\) (\(67.0\ H_{R}C\)), then in the steel of heat 7 it likewise begins at \(400^\circ\) (\(38.5\ H_{R}C\)), but reaches a maximum only at \(750^\circ\) (\(60.0\ H_{R}C\)). The increase in hardness after the drop in the range \(300—400^\circ\) may be explained by the transformation of retained austenite into martensite and partly by dispersion hardening of the \(\alpha\)-phase. The principal factor in the increase in hardness is, in our opinion, the transformation of retained austenite, since the increase in hardness by \(21.5\ H_{R}C\), which occurs in the steel of heat 7, is difficult to explain by dispersion hardening alone.

It is evident from Table 3 that, with an increase in the cobalt content in the steel, the maximum of secondary hardness shifts toward higher tempering temperatures, and the temperature interval in which secondary hardness is observed widens. The stability of retained austenite increases with increasing cobalt content.

It is true that the behavior of the steels of heats 2 and 3 during successive temperings should be noted. In these steels the transformation of retained austenite begins at the same temperature as in the steel of heat 1 and ends at \(500^\circ\). Such behavior of these steels indicates a decrease in the stability of retained austenite in them as compared with the stability of retained austenite in steel R18.

The different effect of cobalt on the stability of retained austenite (a decrease in stability in steels with a cobalt content up to 10%, and an increase in it in steels with a higher cobalt content) can be explained by the different temperature and different dilatometric effect of the martensitic transformation during tempering of steels with a cobalt content up to 10% and steels with a higher cobalt content.

In the study by Michel and Benaze [3], the temperatures and the dilatometric effect of the martensitic transformation during tempering of high-speed steel with different cobalt contents (up to 20%) are given, and it is noted that, under the same tempering conditions, the more cobalt there is in the steel, the lower the martensitic point will lie.

transformations during the first tempering, and the smaller the dilatometric effect will be. In addition, in the study by Michel and Benazet it is noted that high-speed steel without cobalt is more resistant to tempering than 5% cobalt steel, and that high-speed steel attains high resistance to tempering when the cobalt content is more than 10%.

The results of the present work agree with the above-mentioned data of Michel and Benazet’s study.

Since steels with different cobalt contents have unequal stability of retained austenite, this circumstance should be taken into account when selecting the optimum tempering conditions for these steels. We experimentally selected optimum tempering conditions for attaining maximum hardness (Table 4).

Table 4

Conclusions

1. Cobalt increases the amount of retained austenite during quenching of steel.

2. Different cobalt contents affect the stability of retained austenite in different ways. Up to 10% cobalt decreases the stability of retained austenite, while above 10% it increases it.

3. As the cobalt content in steel increases, during tempering at successively increasing temperatures the maximum of secondary hardness shifts to higher tempering temperatures, and the temperature interval in which secondary hardness is observed broadens.

4. Cobalt significantly increases the red hardness of cast high-speed steel.

References

1. Lebedev T. A. and Revis I. A. Structure and Properties of Cast Tools Made of High-Speed Steel. Mashgiz, 1949.

2. Tikhonov I. T. Proceedings of the Tomsk Polytechnic Institute, vol. 68, issue 1, 1951.

3. Michel A. et Benazet P., Revue de Métallurgie, 1932, No. 5.