INVESTIGATION OF THE PROPERTIES OF CAST HIGH-SPEED STEELS

G. V. BYCHKOV, V. P. SABUROV

(Presented by Professor, Doctor A. N. Dobrovidov)

Phase and structural transformations in metals and alloys that occur during crystallization and cooling largely determine the properties of the castings obtained. The character of cooling curves and the dependence of electrical properties on the phase and structural state of alloys make it possible to solve a number of scientific and practical problems of metallurgical engineering by experimental means.

During crystallization and cooling, volumetric and linear changes in the dimensions of a casting occur. The magnitude of shrinkage depends on the type of alloy and on technological factors. It causes internal stresses in the metal and the formation of shrinkage cavities in the casting. There is no method for calculating shrinkage as a function of the chemical composition of alloys.

Therefore, the study of physical and technological properties is of great importance in developing new alloy compositions and casting technological processes.

The purpose of the present work was to study the properties of cast high-speed steels with an addition of aluminum. The metal was poured into a dry cast-iron mold forming a cylindrical specimen. During cooling of the specimen, the curves of cooling, electrical resistance, and linear shrinkage were automatically recorded. The steel was melted in an induction furnace, in an acidic crucible. The melt weight was 7 kg. Discharge of the metal from the furnace into the ladle was carried out at a temperature of 1580–1600°. The temperature of the metal when pouring the mold was 1530–1550°. The metal temperature was monitored with an immersion thermocouple. Aluminum was introduced into the ladle before pouring the mold.

For the investigation, steels with medium (R9 type) and high (R18 type) tungsten content were selected. The carbon content was increased to 1.0%. In both steel compositions, aluminum was introduced in amounts calculated from 0.1–0.15 to 1.5–1.7%.

The setup was based on the method described in work [1]. However, the main part of the setup was modified and supplemented. The diagram of the setup is shown in Fig. 1 and consists of two parts. Along frame 1, a carriage 2 moves on ball bearings. Support bearings prevent the carriage from lateral displacement. The frame, measuring 180×650 mm, is welded from angle iron. The carriage travel is 25 mm.

A dry mold 3, 600 mm long, is placed on the carriage. The through cylindrical channel of the mold, 30 mm in diameter, is closed on both sides by copper water-cooled plugs. Plug 4 is immobile and is rigidly fastened to frame 1, while plug 9, connected to rod 10, is movable and can be displaced. The sprue, installed in the central part of the mold, serves as the riser.

Fig. 1.

A portion of metal is poured into sprue cup 7. The vertical channel of the sprue cup is initially closed by graphite plug 8. During pouring, the graphite plug is torn out and the metal enters the mold cavity. The use of such a gating system ensures constancy of head and prevents slag from entering the mold cavity.

In the upper part the mold has three openings. A platinum–platinum-rhodium thermocouple 13 with a quartz tip 3 mm in diameter is inserted into one of them, and contacts 5 are installed in the other two.

The curves of temperature, electrical resistance, and shrinkage are recorded on the tape of a self-recording electronic potentiometer EPP-09.

To measure and record the electrical resistance, an ammeter–voltmeter circuit was used. Alternating electric current is passed through an STE-250 stabilizer, a LATR, a step-down transformer (250/6 V), a rectifier, and a rheostat. The secondary voltage of the transformer and the magnitude of the rectified current are monitored by instruments connected into the circuit.

The direct current obtained is supplied to the terminals of the water-cooled plugs 4 and 9 and, through single-use contacts screwed into their ends (copper pins 5 mm in diameter), passes through the metal poured into the mold. The ends of the single-use metal contacts protruding from the plugs are reliably welded to the metal of the specimen poured into the mold. The design of the water-cooled copper plugs with screwed-in single-use contacts ensures stability of readings regardless of the temperature of the metal poured into the mold.

The voltage drop at the ends of the specimen is taken off by contacts 5, in the form of thin iron wire 1 mm in diameter, connected to copper leads through coolers 6. The use of coolers eliminated the possibility of thermoelectric EMFs of parasitic couples arising during heating of the wire contacts.

The change in the linear dimensions of the specimen is recorded by means of a linear-displacement transducer. The transducer is made in the form of an unbalanced resistance bridge 14, with a double slide-wire rheochord. Movable

the contacts of rheochord 11 are rigidly connected to rod 10, which moves in guides made of dielectric material. The use of such a circuit made it possible to obtain a bridge unbalance of 16 millivolts when the rheochord contact was displaced by 4 mm.

The distance between the specimens and the start of recording are set by means of the falling plate 12. As a result of pre-shrinkage expansion, the specimen increases in size; the movable specimen 9 and rod 10, overcoming the tension of weight 15, move to the right, and plate 12, which fixes the beginning of the reading, falls. Depending on the magnitude of the linear shrinkage, the size of the plate is selected and, accordingly, the sensitivity of the transducer is set by variable resistances.

In Fig. 2 the following curves are shown: I—cooling, II—electrical resistivity, III—change in the linear dimensions of specimens of steel of the P18 type: a) without aluminum and b) with 0.5% aluminum. In Fig. 3 curves are shown for analogous heats of steel of the P9 type.

Fig. 2.

Fig. 3.

Analysis of the cooling curves obtained shows that crystallization of the metal poured into the mold occurs with significant supercooling—

cooling. On the cooling curves of all specimens of the melts there are two inflections corresponding to temperature arrests. The first inflection corresponds to temperatures of 1380–1370°, the second to 1270–1250°. The temperature arrests are noted at 1 and 2 minutes after the metal is poured into the mold.

In specimens of P9-type steel without aluminum (Fig. 3, a), the second temperature arrest corresponds to 1120° and is recorded considerably later—4 minutes after the mold is poured.

After the second temperature arrest, the cooling curves of all specimens have no inflections; the temperature drop per unit time decreases, and the curves have a smooth character.

The specific electrical resistivity of specimens of P18-type steels with aluminum (Fig. 2, b), after a certain decrease, increases, which in time corresponds to the second temperature arrests on the cooling curves. In a steel specimen without aluminum (Fig. 2, a), the decrease in electrical resistivity continues, but less sharply.

With further cooling of the specimens, the electrical resistivity decreases; however, in the temperature intervals 1100–1000° and 800–750°, the drop in electrical resistivity decreases sharply. The same phenomenon is observed at 650–600° and 550–500°, where the resistance of specimens of some melts even increases. Below temperatures of 550–500°, the electrical-resistivity curves have no substantial inflections.

The drop in electrical resistivity of specimens of P9-type steel (Fig. 3) at the second temperature arrest decreases and then increases up to temperatures of 1000–900°, where its magnitude does not change. Further cooling is accompanied by a decrease in resistance down to 720–700° in steels with aluminum (Fig. 3, b) and down to 630–620° in steel without aluminum (Fig. 3, a). A horizontal segment of the curve corresponds to temperatures of 560–540°, and a certain increase in resistance is observed in the interval 490–470°. In steels with aluminum, an increase in electrical resistivity was noted at 650–620°.

The specific electrical resistivity of the specimens at room temperature increases with increasing aluminum content in the steels.

Recording the curves of change in the linear dimensions of the specimens shows that, with an increase in the aluminum content in the steels, preshrinkage expansion decreases, while the magnitude of linear shrinkage increases. This agrees with published data [2].

The time during which preshrinkage expansion increases is shortened as the aluminum content in the steel increases. After completion of the crystallization process, in the temperature interval 1100–1000°, a considerable increase in the linear dimensions of specimens of all melts is observed. It ends in specimens of P18-type steel 5 min after the mold is poured and in specimens of P9-type steel 6 min after pouring.

The investigations carried out showed the reliable operation of the apparatus and good reproducibility of the experimental results.

The cooling curves make it possible not only to obtain certain ideas about the process of crystallization of the metal in the mold, but also to consider changes in the electrical resistivity and linear dimensions of the specimens as functions of temperature and time.

The increase in electrical resistivity at individual stages of cooling apparently cannot be explained only by the state of the solid solution and the quantitative redistribution of phases. Electrical resistivity is a structure-sensitive property, dependent on the mutual arrangement of the structural constituents. Therefore, this question requires further investigation.

Changes in linear dimensions, like changes in electrical resistivity, characterize internal transformations, whose volumetric effect is considerably more noticeable than the thermal one. Linear shrinkage must be taken into account when designing technological processes for casting cutting tools.

REFERENCES

1. V. P. Chernobrovkin. In: Development of the Vacuum Process. Mashgiz, 1961.
2. B. G. Livshits. Physical Properties of Metals and Alloys. Mashgiz, 1959.