EFFECT OF ALUMINUM ON THE PROPERTIES OF CAST HIGH-SPEED STEEL

G. V. Bychkov

(Presented by Prof. Dr. A. N. Dobrovidov)

From the literature it is known that aluminum is used as a substitute for tungsten in tool steels [1], and also that the service properties of low-alloy high-speed steels are improved by the addition of aluminum [2].

The addition of 1.0% aluminum to cast high-speed steel of the R9 type helps to increase cutting durability [3]. More detailed studies of the effect of aluminum on the properties of cast high-speed steel have not been carried out.

The purpose of the present work was to study the effect of aluminum, from 0.1 to 3.0%, on the properties of cast high-speed steel of the R9 type.

The steel was melted in a high-frequency installation in an acid crucible with a capacity of 2 kg. Casting was carried out into a metal mold on a centrifugal machine. Specimens measuring \(11 \times 11 \times 120\) mm were quenched during cooling in the mold. The chemical composition of the heats is given in Table 1.

Specimens from each heat were subjected to fivefold tempering for 1 hour at 560, 580, 600, 625, and 650°. The different regimes made it possible to determine the optimum temperature and number of temperings, and also to judge the red hardness of the steel as a function of the aluminum and carbon content.

The hardness of specimens of R9 steel in the cast quenched state, with aluminum additions from 0.1% to 2.0%, practically does not change and amounts to 62–63 RC. As the carbon content in the steel is increased to 1.3%, the effect of aluminum additions on hardness after casting becomes more pronounced. When the aluminum content is increased from 0.1 to 1.0%, the hardness of the specimens in the cast state increases from 55 to 60°RC, which may be explained by a decrease in the amount of retained austenite. A further increase in aluminum content leads to a decrease in hardness in the cast state, and at 2% aluminum—to a sharp drop to 44–45 RC, which is explained by aluminum narrowing the gamma region.

In cast R9 steel with an aluminum addition of up to 1.5%, the maximum hardness (66 RC) was obtained after fivefold tempering at 560° or after threefold tempering at 580° (65 RC).

The first tempering at 625° gives an increase in hardness; during the second tempering the hardness decreases and after the fourth amounts to 61–62 RC. Fourfold tempering at 650° leads to a decrease in hardness to 54–58 RC,

Table 1

Chemical composition, in %

moreover, specimens of heats with 0.35–0.67% aluminum had a hardness of 58 RC (Fig. 1).

In steels with an increased carbon content, when aluminum is added up to 1.0%, fivefold tempering at 625° raises the hardness to

In Fig. 1: ordinate—HRC; panels—heat 02, heat 03, heat 04, heat 05, heat 06, heat 07; abscissa—number of temperings; legend—560°, 580°, 600°, 625°, 650°.

Fig. 1.

64–65 RC, while fivefold tempering at 650° does not reduce the hardness below 60 RC. After triple tempering, the maximum hardness in these steels is attained at 580 and 600° (Fig. 2).

Further increase in the aluminum content leads to lower hardness values after tempering; this is especially noticeable at 625 and 650°. The introduction of more than 2.0% aluminum into the steel leads to a sharp drop in the hardness of tempered specimens, and with 3.0% aluminum the hardness does not rise above 54–56 RC for all tempering temperatures investigated. This can be explained by the fact that the steel becomes ferritic and the hardness increases only as a result of dispersion hardening.

Repeated tempering at different temperatures showed that cast R9 steel with 0.35–0.67% aluminum has increased red hardness. Specimens of this steel with 0.67% aluminum (heat 0.4), after fourfold

Fig. 2.

tempering at 650°, had a hardness of 58 RC. In steel with increased carbon content and an addition of up to 1.0% aluminum, under the same tempering conditions the hardness is retained up to 60 RC. Increasing the aluminum content worsens the red hardness of the steels.

Conclusions

1. On the basis of the investigation carried out, it was established that an addition of 0.1–1.0% aluminum to cast steel of the R9 type does not change the hardness in the as-cast hardened state.

2. With an increased carbon content in the steel, the introduction of from 0.1 to 1.0% aluminum increases the hardness in the as-cast state from 55 to 60 RC.

3. In cast steels with an addition of up to 1.0% aluminum, the maximum hardness is reached after threefold tempering at 580–600°.

4. Cast high-speed steel with an aluminum addition of about 0.7% has sufficient red hardness when heated to 650°. The red hardness of the steel increases with increasing carbon content.

References

1. S. L. Keiz and K. R. Van Horn. Aluminum in Cast Iron and Steel. Metallurgizdat, 1959.

2. A. P. Garashchenko. Metallography and Heat Treatment, I. Metallurgizdat, 1954.

3. K. M. Stroeva. The influence of cobalt, aluminum, and boron on the properties of cast high-speed steel. Proceedings of the NPI, vol. 46/60, Publishing House of the Novocherkassk Polytechnic Institute, 1958.