IMPROVEMENT OF THE STRUCTURE OF CAST HIGH-SPEED STEELS

Yu. A. Evtyushkin, D. M. Likhosherstov

(Presented by Professor Doctor A. N. Dobrovidov)

Cast tools made of high-speed steel, owing to their simplicity and low cost of manufacture, as well as their higher heat resistance, which ensures good cutting properties, are becoming ever more widely used. However, the increased brittleness inherent in the cast structure causes concern among production workers, and for this reason the introduction of cast tools is being hindered.

The use of cast tools in production shows that these fears are unnecessary and that, in most cases, a cast tool can replace a forged one. In addition, the reliability of a cast tool can be increased by eliminating its brittleness.

Indeed, cast high-speed steel has higher red hardness than forged steel because of the increased alloying of the solid solution obtained during quenching in the casting process [1, 2]. Its principal shortcoming—brittleness—can be eliminated by changing the composition, by modification, and by appropriate heat treatment.

The brittleness of cast high-speed steel is due mainly to the presence of a coarse carbide network, located predominantly along the grain boundaries of the solid solution (Fig. 1). Moreover, its influence on strength proves to be so great that it sometimes predominates over casting defects.

The amount of the carbide phase that determines the character of the ledeburitic eutectic, and its skeleton-like arrangement, can be reduced by decreasing the amount of alloying carbide-forming elements or by using elements that form carbides with a small specific volume, for example vanadium or niobium [1].

A reduction in the amount of alloying elements does indeed reduce the carbide eutectic. Thus, forged R9 steel, as a rule, has carbide heterogeneity 1–2 grades lower than R18 steel.

The decrease in alloying of the solid solution caused by the reduced content of alloying elements can be compensated to some extent by quenching during casting, as is observed in the practice of producing cast tools. However, reducing the amount of the carbide phase still does not ensure a significant increase in the strength of cast steel. An additional increase in the strength of cast

in high-speed steel can be achieved by obtaining a discontinuous carbide network. This can be accomplished by alloying, accelerated cooling during casting, and modification [3].

Alloying cast high-speed steel of the R9 type (1.18% C; 10.05% W; 4.11% Cr; 2.11% V) with niobium in an amount of 6.34% sharply changes the structure and increases strength. Instead of a continuous network with a dendritic character of arrangement (Fig. 2), the structure exhibits a uniform distribution of carbides without a network (Fig. 3). The results of phase analysis also show that niobium in this steel lowers the total volume of the carbide phase. The latter phenomenon is associated with the ability of niobium to form carbides with a small specific volume (for example NbC) and with its strong carbide-forming capacity.

Fig. 1. Structure of cast high-speed steel R18, ×600.

Thus, niobium in steel of the R9 type, by reducing the total volume of the carbide phase and eliminating the continuous carbide network, increased the bending strength in the cast, tempered (550°—1 hr, 3 times) condition by a factor of 3–4, to 240 kg/mm², in comparison with similar steel without niobium.

Accelerated cooling, for example, casting into metal molds, reduces the thickness of the carbide network. Such a structure is more readily improved by annealing. In addition, annealing is necessary to soften the steel during machining of the cast tool.

We investigated two annealing schedules, presented in Table 1.

Table 1

Fig. 2. Structure of cast high-speed steel of the R9 type (1.24% C; W—9.56%; Cr—4.54%; V—2.19%). Cast in a metal mold. ×120.

Fig. 3. Structure of cast high-speed steel of the R9 type with niobium (1.18% C; 10.05% W; 4.11% Cr; 2.11% V and 6.34% Nb). Cast in a metal mold. ×120.

Annealing No. 1 was recommended by Yu. A. Geller and Yu. I. Karavanov [4] for cast R9 steel. The annealing temperature is taken higher than for wrought R9 steel.

As is known, the higher the temperature, the greater the dissolution of carbides in austenite and the more active the processes of carbide coagulation. To break up the carbide eutectic and homogenize the metallic matrix, we investigated a high-temperature annealing regime (No. 2).

The annealing regimes were investigated on cast high-speed steels of the R9, R14, R18, and R22 types. The chemical composition of the steels is given in Table 2.

Table 2

Chemical composition of the steels

Annealing was carried out on specimens measuring \(12 \times 12 \times 120\) mm, cast in a metallic chill mold, and on specimens 40 mm in diameter, cast in shell molds.

Annealing No. 1 had the most effective influence on the structure of steels R9 and R14, and a less pronounced influence on R18. The carbide network in the structure of steel R9 almost completely dissolved (Fig. 4), and the metallic matrix became more uniform.

Fig. 4. Structure of cast high-speed steel R9 (1.5% C; 9.10% W; 4.20% Cr and 3.12% V) after annealing according to regime No. 1. ×440.

Regime No. 2 caused rupture of the network and its coagulation even in steel R18 cast in shell molds (Fig. 5).

After annealing, hardness was measured.

Analyzing the results obtained, it may be noted that both annealing regimes give a sufficient reduction in hardness to HRC = 23–26 in steels R9 and R14 and to HRC = 24–29 in steels R18 and R22.

Fig. 5. Structure of cast high-speed steel R18 (1.20% C; 18.42% W; 3.78% Cr and 1.40% V) after annealing according to regime No. 2. ×440.

After annealing and hardening, the specimens were tested for red hardness by the method of A. P. Gulyaev [5]. None of the annealing regimes studied reduces red hardness. As a rule, the red hardness of the cast steels under investigation is 10–15° higher than that of the corresponding forged steels.

To study the effect of annealing on the strength of cast steel, specimens measuring 10×10×100 mm, after annealing according to regime No. 1 and hardening from a temperature of 1270° for steels R14, R18, and R22 and 1220° for steel R9, followed by triple tempering at 560°, were subjected to bending tests (Table 3).

Table 3

The data obtained show that annealing followed by hardening increases the ultimate bending strength by a factor of 2–2.5. It is also characteristic to note that annealing and subsequent hardening improve the fracture of cast steel. In the as-cast and tempered condition the fracture shows signs of naphthalene-like appearance and transcrystallinity. After annealing, hardening, and tempering, however, the fracture is porcelain-like, the same as in forged high-speed steel.

References

1. T. A. Lebedeva and I. A. Reibis. Structure and Properties of Cast Tools Made of High-Speed Steel. Mashgiz, 1949.
2. I. I. Gorbunov. In: Cast and Surfaced Tools. Mashgiz, 1951.
3. Z. D. Nevzorova. In: Cast and Surfaced Tools. Mashgiz, 1954.
4. Yu. A. Geller and Yu. I. Karavanov. Stanki i instrumenty, No. 7, 1960.
5. A. P. Gulyaev, M. N. Fadyushina. In: Methods and Practice of Metallographic Investigation of Tool Steel. Mashgiz, 1961.