EFFECT OF ULTRASONIC TREATMENT ON THE CRYSTALLIZATION PROCESS OF ShKh15 STEEL

G. G. Krevskii, G. V. Simonov, N. D. Tyuteva

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

A whole series of researchers [1, 2, 3, 4, 5, 6] have established that intense sonic and ultrasonic vibrations are a very effective factor in acting upon a substance.

In both domestic and foreign technology, ultrasound has become widely used in metallurgical processes to improve the quality of cast metal by creating vibrations in it during melting, pouring, or crystallization.

The first works on the use of ultrasound in metallurgy were published in 1939. Meanwhile, the imperfection of transducers and other devices did not make it possible to obtain sufficiently convincing data on the positive influence of ultrasound on metallurgical processes.

Most of the work on the use of ultrasound in metallurgy is devoted to the sonication of low-melting metals and metals with an intermediate melting temperature. A considerably smaller number of works is devoted to the use of elastic vibrations for treating refractory metals and alloys.

Whereas, in the sonication of the first two groups of metals, the transmission of vibrations into the melt occurs comparatively simply, in the sonication of molten metals having a high melting temperature the transducer rod can no longer be brought into direct contact with the melt.

The first attempts [7] to change the crystallization conditions of steel specimens by means of elastic vibrations gave no results. At present there are a number of reports on laboratory studies in which this problem has been successfully solved.

The aim of the present work was to study the influence of ultrasonic vibrations on the crystallization of ShKh15 tool steel.

As the source of electrical energy of ultrasonic frequency, an ultrasonic generator of the UZG-6 type was used. To convert electrical energy of ultrasonic frequency into the energy of ultrasonic mechanical vibrations of the same frequency, magnetostrictive transducers with directed radiation were used. As the magnetostrictive material, an alloy of the permendur type—K50F2—was employed. It possesses the highest heat resistance and

withstands heating up to 800°. The Curie point for alloys of this type lies within the range of 980° [2].

For the effective action of ultrasound on the molten metal, the acoustic transformer of the transducer was equipped with a concentrator, which contributed to an increase in the amplitude of the oscillations arising in the Permendur stack.

Three types of concentrators were used in the work: cylindrical ($D = 100$ mm, $d = 38$ mm), exponential ($D = 100$ mm, $d = 19$ mm), and conical ($D = 100$ mm, $d = 16$ mm). The ultrasonic oscillations were introduced into the melt from below through the bottom of the chill mold. In this case the concentrator was in direct contact with the molten metal (Fig. 1).

Fig. 1.

Steel melting was carried out in a high-frequency melting-and-quenching unit of the A3-46 type, in an acid crucible under a layer of slag. The dimensions of the cast billets were: diameter—38 mm, height—100–120 mm.

The temperature of the metal being poured was monitored with an OPPIR-09 optical pyrometer and was maintained in all experiments within the range 1560–1580°.

The metal was poured into chill molds with different wall thicknesses and into ceramic molds. In order to increase the duration of the crystallization period, some of the ceramic molds were preheated to 700°.

To eliminate the influence of differences in the chemical composition of the steel, each heat was poured into two molds of similar parameters, in one of which the metal was sonicated.

The regimes for sonication of the ingots were adopted as follows:
a) fundamental resonant frequency 19.4–19.45 kHz,
b) magnetizing current 30 amperes,
c) excitation voltage 400 volts,
d) consumed power 2.8–2.6 kW,
e) electroacoustic efficiency of the transducer 47.7–46.4%.

Sonication of the ingot continued throughout the entire crystallization period, until the casting had cooled to within 500°. The acoustic contact between the concentrator and the melt was sufficiently reliable in almost all experiments.

Table 1

The initial charge for melting consisted of ball-bearing parts. To compensate for the loss of elements, additions with ferroalloys were made. In all, 12 heats were carried out with different concentrators, four heats for each type of concentrator. The chemical composition of the control heats of each group is given in Table 1.

The results of the investigations showed that in all cases the sonicated ingot proved to be denser. The shrinkage cavity-

it forms only in the upper part of the ingot (Fig. 2, б) and is small in size, whereas in the untreated ingot it extends over more than half the height (Fig. 2, а).

Fig. 2.

Fig. 3. (×200).          Fig. 4. (×200).

The effect of ultrasonic vibrations on the crystallization of ShKh15 tool steel is the elimination of carbide heterogeneity in the ingot and the refinement of the grain.

Figure 3 shows the microstructure of untreated cast steel. The figure shows that the structure has a clearly pronounced heterogeneity. Figure 4 shows the microstructure of an ultrasonically treated ingot, where it is evident that the ultrasonic vibrations made the structure homogeneous.

The effect of ultrasonic treatment over the height of the ingot changes substantially depending on the type of concentrator. Data on the height of ultrasonic treatment are given in Table 2.

Table 2

The low results of the ultrasonic-treatment effect in melts 8 and 10 can be explained by poor acoustic contact between the concentrator and the melt being treated. During ultrasonic treatment of these melts, welding of the emitter face to the casting metal did not occur.

Ultrasonic treatment of metal poured into a heated ceramic mold (up to 700°) showed that, along with a sharp change in structure, the latter proves to be rather coarse-grained in comparison with the structure of steel cast in metal molds. In this case the height of ultrasonic treatment of the metal increases by approximately 1.5 times compared with castings in similar cold molds.

Conclusions

1. The greatest ultrasonic-treatment effect over the height of the ingot is observed when a cylindrical concentrator is used.

2. It may be considered that the greatest effect of structural improvement for steels of this type under the action of ultrasound is obtained when casting into metal molds.

3. During crystallization in heated ceramic molds under the action of ultrasound, along with an overall improvement in the steel structure, grain growth is noticeable.

REFERENCES

1. Ya. B. Gurevich, V. I. Leont’ev, I. I. Teumin. Effect of ultrasound on the structure and properties of a steel ingot. Stal’, No. 5, 1957.

2. A. E. Crawford. Ultrasonic Engineering. Foreign Literature Publishing House, Moscow, 1958.

3. A. P. Kapustin. Effect of Ultrasound on Crystallization Kinetics. Publishing House of the Academy of Sciences of the USSR, Moscow, 1962.

4. N. P. Nikolaichik, E. N. Nikolaichik. Ultrasound in the National Economy. Udmurt Book Publishing House, Izhevsk, 1959.

5. G. I. Eskin. Ultrasound in Metallurgy. Metallurgizdat, Moscow, 1957.

6. R. Milcheva. Effect of ultrasound on the rate of crystallization of high-speed steel. Tekhnika, No. 10, 1962.

7. E. A. Hiedeman. Metallurgical effects of ultrasonic waves, Acoust. Soc. America, No. 5, 26, 1954.