STRUCTURE AND PROPERTIES OF HIGH-SPEED STEEL WITH ALUMINUM IN THE AS-CAST STATE

G. B. Bychkov, V. G. Gultyaev, V. I. Karasev

(Presented by Professor Doctor A. N. Dobrovidov)

The available literature data [1, 2, 3] indicate broad possibilities for improving the quality of cast tools made of high-speed steel by additional alloying and by changing the chemical composition.

The use of aluminum as a partial substitute for tungsten in tool carbon steels is known, as is the improvement of the service properties of low-alloy high-speed steels when aluminum is added [4].

No detailed studies have been carried out on the influence of aluminum on the properties of cast high-speed steels.

The aim of the present work was to study the influence of aluminum, from 0.1 to 3.0%, on the structure and properties of cast high-speed steel of the R9 type with different carbon contents (1.1–1.3%). The chemical composition of the steels is given in Table 1.

The steel was melted in an A3-43 high-frequency installation in an acid crucible. Casting was carried out into a metal mold (chill mold) on a centrifugal machine. The pouring temperature was 1460–1480°. The design of the chill mold made it possible to cast eight specimens measuring \(11 \times 11 \times 120\) mm. The casting weight was 1.8 kg.

The method of centrifugal casting into metal molds combines a high cooling rate of the casting with increased pressures during crystallization, which makes it possible to obtain a cast tool of good quality with high mechanical properties.

The advantages of this casting method also include a shortening of the technological cycle, since the tool is hardened during cooling in the mold.

In the course of the investigations, the microstructure of the steels, hardness, and electrical and magnetic properties were studied.

Electrical and magnetic properties—above all the coercive force—are structurally sensitive properties. The phases constituting the steel possess magnetic interaction, which depends in a complex way on their quantitative ratio and mutual distribution, i.e., on the specific structure of the steel.

Therefore, the study of the electrical and magnetic characteristics of steel when various amounts of alloying element are introduced, in parallel

with the study of structure, hardness, and other properties, makes it possible to determine its influence more fully and may be used in developing methods of nondestructive quality control for cast tools.

Table 1

For measuring the electrical resistivity of specimens of the steels under study, the double-bridge method was used. The circuit of the combined bridge of type MTB-2 includes the test specimen, which was carefully ground beforehand. The high reliability of contact between the specimen and the current-carrying clamps made it possible to reduce losses in the circuit to a minimum and to avoid heating of the specimen during the investigation.

For magnetic tests, specimens measuring \(10 \times 10 \times 60\) mm were prepared. In their magnetic characteristics, the specimens tested occupy an intermediate position between magnetically hard and magnetically soft materials, with a greater tendency toward the group of magnetically hard materials (see test results, Figs. 4, 5).

The magnetic characteristics of specimens of the steels under study were determined on a U-541-type installation of the Tochielektropribor plant for testing magnetically hard materials with a coercive force up to 1500 oersteds in a constant magnetic field by the ballistic method. The data obtained in the tests confirm the correctness of the chosen method of investigation.

The microstructure of steels of heats 01–08 consists of martensite, retained austenite, and a network of carbide eutectic of very fine structure. With an increase in the aluminum content in the steel, the amount of retained austenite decreases somewhat, while the amount of martensite increases, which is confirmed by austenometer readings. The hardness of the steels increases, but only slightly, from 62 to 63 RC.

As can be seen from Fig. 1, the hardness of the steel with 2.0% decreases, while the readings of the austenometer increase. At the same time, no noticeable changes are observed in the microstructure of heats 06 and 07.

It may be assumed that the slight decrease in hardness of steel heat 07 occurs as a result of narrowing and wedging-out by aluminum of the gamma region. The increase in the austenometer readings is then explained by an increase in the amount of the ferromagnetic constituent.

In Fig. 2 the microstructure is presented of steels with 1.3% carbon and different aluminum contents (heats 10–17), and in Fig. 3—the dependence of hardness and of the amount of retained austenite on the aluminum content in these steels.

Fig. 1.

For steels of heats 10–14, the microstructure consists of martensite, retained austenite, and ledeburitic eutectic, which crystallized last from the liquid solution enriched with carbon and alloying elements. With an increase in the aluminum content in the steel, the amount of martensite increases, while that of retained austenite decreases, which is confirmed by the increase in the hardness of the steel (Fig. 3).

Fig. 2.

In the structure of heats 10 and 11, coarse acicular martensite is observed. Such a structure in forged high-speed steels appears simultaneously with grain coarsening after quenching from high temperatures. Coarse acicular martensite in the structure of cast steels cannot cause a sharp decrease in mechanical properties, since during casting

in a chill mold the steel has a fine-grained structure. The large amount of retained austenite determines the reduced hardness of these steels, which is 55–56 RC.

With a further increase in the aluminum content in the steel (heats 12–14), the amount of retained austenite also decreases, while the amount of martensite increases. The hardness of the steels of these heats continues to rise and reaches 60 units (heat 14).

Fig. 3.

The noted changes in the structure of the steels of heats 10–14, together with the increase in hardness, indicate the effect of the aluminum addition on raising the temperature of the martensitic transformation of the steels studied.

The microstructure of the steels of heats 15–17 differs considerably from that of the steels of heats 10–14. An increase in the aluminum content in heats 15–17 to 3.0% causes the appearance, in place of martensite, of a new structural constituent, which differs from it in its nature but is also etched strongly. This constituent has a troostite-like structure and is located in the central part of the grains.

Areas of the new dark constituent have a rounded shape, occupy a large area of the grains, and are separated by ledeburite. Moreover, as the aluminum content increases (heat 17), the boundaries of this constituent become more diffuse. The dark central parts are surrounded by a gray rim that separates them from the light ledeburite.

The hardness of the specimens of heats 15–17 drops sharply to 46 RC, which, together with the large area occupied by the dark constituent, confirms its troostite-like nature. This is also confirmed by the data of magnetic analysis—the readings of the austenitometer increase, likewise indicating an increase in the amount of the ferromagnetic constituent.

If the rising portion of the hardness curve of the steels studied (Figs. 1, 3) can be explained by a decrease in the amount of retained austenite in the steels, then further investigations are required to explain the sharp drop in hardness of the steels of heats 07, 16, and especially 17. An explanation of this phenomenon by a narrowing and wedging-out of the gamma region by aluminum cannot be considered the only correct one, since in that case the dark structural areas should be a delta eutectoid formed from delta ferrite, and should have distinct boundaries between the dark constituent and austenite—two different phases.

The results of microstructural analysis and the study of the hardness of the steels of the heats investigated are confirmed by the data on the determination of the physical properties of heats 01–08, presented in Fig. 4, and of heats 10–17—in Fig. 5.

The specific electrical resistivity of specimens of all the steels increases with increasing aluminum content.

The decomposition of austenite and the formation of martensite cause internal stresses and lattice distortion, as a result of which the electrical resistivity increases.

The increase in the electrical resistivity of the steels of heats 07 and 15–17 is explained by a change in the structure of these steels: the formation of a troostite-like constituent of various dispersity, surrounded by ledeburite.

The nonuniform etchability of this constituent indicates heterogeneity of the structure within a single grain and different distortion of the lattice along grain boundaries. As a result, electron scattering increases and conductivity decreases.

The curves of the change in saturation induction, maximum magnetic permeability, and coercive force of steel specimens as a function of carbon and aluminum content are presented in Figs. 4 and 5.

When considering these curves and the hardness curves of the steels (Figs. 1, 3), one should note the parallelism in the change of hardness and coercive force. The literature contains indications of the possibility of such a coincidence [5].

The increase in coercive force is due to high magnetic anisotropy and considerable nonuniform stresses during the formation of martensite. This is observed up to the appearance in the steel structure of a new troostite-like constituent—less hard in magnetic terms than martensite.

Fig. 4.

Fig. 5.

Conclusions

The investigations carried out showed that the structure of high-speed steel of the R9 type, cast in a chill mold with an aluminum content of up to 1.5%, consists of martensite, retained austenite, and a network of carbide eutectic. With an aluminum content above 1.5%, instead of martensite there appears a new structural constituent of troostite-like type.

structure, whose regions occupy a large area of the grains and are separated by ledeburite.

The temperature of the martensitic transformation increases under the influence of aluminum, and the amount of retained austenite in the structure of the hardened steel decreases.

The hardness of the steel increases until a troostite-like constituent appears in the structure, and then drops sharply.

With an increase in aluminum content to 3.0%, the electrical resistivity of the steel increases.

The change in the magnitude of the coercive force with increasing aluminum content in the steel corresponds to the nature of the change in hardness of the steel specimens from all heats.

The study of electrical and magnetic characteristics, in parallel with the study of the structure and other properties of steels, may be used in developing methods for nondestructive quality control of cast cutting tools.

REFERENCES

1. A. T. Penskii and Z. L. Razubaeva. Cast Tool Requiring No Heat Treatment. Mashgiz, 1946.

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

3. I. S. Kvater. Cast High-Speed Steels. Mashgiz, 1952.

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

5. B. G. Livshits. Physical Properties of Metals and Alloys. Mashgiz, 1959.