Mechanical Properties of Cast Steel Kh6VF

V. S. Golovacheva, N. A. Erofeev

(Presented by the Section of Metallurgy and Heat Treatment of Metals of the Scientific and Technical Conference of Machine Builders of Tomsk, 1967)

At present, certain types of cutting tools are manufactured by casting. At the Tomsk Tool Plant, blanks for woodworking milling cutters made of Kh6VF steel are cast in shell molds. Manufacturing cutters by casting reduces their cost by more than half in comparison with forged ones. The service life of such cutters is no lower than that of cutters made of forged steel of the same composition.

In seeking the optimum casting regime for blanks of woodworking tools, the mechanical properties of cast Kh6VF steel in various states were determined as a function of its carbon content, the modifying additions introduced, and the temperature at which the liquid steel was poured into the shell molds.

Melting was carried out in a high-frequency unit of the MGP-102A type with a crucible capacity of 150 kg. The temperature regime of melting and pouring of the steel is an important technological factor determining the life of the crucible, electric-power consumption, structure, quality, and strength properties of cast blanks; therefore, much attention was devoted to it.

To measure the temperature of the liquid metal, a PR30/6 immersion thermocouple was used. Measurements were taken both in the crucible and directly in the ladle.

The chemical composition of Kh6VF steel was varied in terms of carbon content. Specimens of this steel with 0.85, 1.0, and 1.3% carbon were cast and tested. Modifiers were added to steel of standard composition in the following amounts: titanium 0.15–0.3%, boron—0.005–0.01 and 0.05%. The modifying additions were introduced in the form of lumps of ferroalloys into the stream of liquid metal as it was poured into ladles. Tests of the mechanical properties of Kh6VF steel were carried out on proportional specimens, in accordance with GOST 1497-61, cast in shell molds. After casting, the specimens were subjected to abrasive cleaning to remove casting fins and to the appropriate heat treatment. For the purpose of comparing the properties of cast Kh6VF steel with forged steel, specimens of the same dimensions were made from rolled stock of this steel.

In carrying out the work, hardness measurements and tests for impact toughness and bending were performed. The latter is the most expedient and widely used of all the others for solving most problems requiring determination of the influence of chemical composition and heat-treatment conditions on the strength of tool steels.

In bend testing, the load was applied at the middle of the specimen (concentrated bending). Because of the nonuniformity of the cast structure and possible fluctuations in mechanical properties, 4–5 specimens were tested for each point. The result was taken as the arithmetic mean, without taking into account large deviations caused by visible casting defects (cavities, gas pores, slag inclusions, tears, etc.). The specimens were tested in the as-cast, annealed, quenched, and tempered states.

Annealing of specimens with different carbon contents was carried out according to several variants. Along with the annealing regime of 880°C with an 8-hour holding time, annealing was carried out with 12- and 6-hour holding times at the same temperature and with a 5-hour holding time at a temperature of 750°C. Further cooling in all cases was performed at a rate of 30–40°/h.

The results of hardness measurements showed the optimality of the first annealing regime (880°—8 h) for cast woodworking tools made of Kh6VF steel. Depending on the carbon content, the hardness of the steel increases, under identical annealing conditions, as the amount of carbon increases from 0.85 to 1.3% (Fig. 1). In the as-cast, unannealed

Fig. 1. Change in hardness of Kh6VF steel as a function of holding time during annealing (880°C) and carbon content

state, such an increase in the amount of carbon significantly reduces the hardness of Kh6VF steel, which is a consequence of an increase in the amount of retained austenite in specimens whose quenching is combined with casting, since this steel is self-hardening.

In the quenched state, the specimens were tested after quenching from temperatures of 930, 950, 980, 1000, 1050, and 1100°C. Caustic soda at a temperature of 400–420°C or saltpeter at a temperature of 160–180°C was used as the intermediate cooling medium. Final cooling of the specimens was carried out in air.

The hardness of both cast and forged specimens varies depending on the quenching temperature (Fig. 2). The greatest value of hard-

...properties are exhibited by specimens of all chemical compositions with respect to carbon after hardening from \(1050^\circ\mathrm{C}\). A further increase in the hardening temperature to \(1100^\circ\) leads to a decrease in hardness because of the formation of a large amount of retained austenite in the steel structure.

The hardness of tempered specimens was determined over a tempering-temperature range from 200 to \(600^\circ\mathrm{C}\), with a holding time of one hour.

Fig. 2. Dependence of the hardness of Kh6VF steel with different carbon contents on the hardening temperature

For each tempering temperature, a freshly hardened specimen was taken. The general pattern for both forged and cast specimens of Kh6VF steel is a decrease in hardness with increasing tempering temperature and decreasing hardening temperature. The tempering regime adopted as optimal (\(350^\circ\mathrm{C}\)—1 hour) for woodworking tools provides a hardness of up to 58 Rockwell units. The highest hardness values are found in cast tempered specimens hardened from \(1050^\circ\mathrm{C}\) (Fig. 3).

The introduction of modifying additions during the casting of cutters made of Kh6VF steel has no substantial effect on its hardness in all states.

The test results showed that the strength properties of cast Kh6VF steel depend on the pouring temperature of the liquid metal into the molds. In practice it is not always possible to measure the temperature of the steel with a thermocouple; therefore, for such cases, knowledge may be useful of the behavior of Kh6VF steel during melting, as established as a result of practical observations and temperature measurements. Depending on the temperature of the liquid metal in the crucible and the condition of its surface, four periods should be distinguished in the behavior of liquid steel, each of which corresponds to a definite macro- and microstructure and, consequently, to the quality of the castings.

The first period corresponds to heating the steel to temperatures not above \(1480^\circ\mathrm{C}\). During this period the surface of the liquid metal is rapidly covered with a continuous thick film of white oxides. Cold melting of the steel takes place.

The second period may be assigned to temperatures of \(1520\)—\(1540^\circ\mathrm{C}\). The oxide film on the surface of the bath is very thin, with a small number of separate dark spots. At these temperatures, as in the first period, the steel does not boil.

In the case when melting is carried out under a thick layer of slag, overheating of the metal is possible. One indication of this may be burning through of the crust

with torches of burning-out elements, and the steel begins to boil. This is observed at temperatures of 1580–1600°C.

The latter period corresponds to a temperature of 1600°C and higher, which is determined by intense boiling of the metal and strong spark formation.

The very highest strength properties are possessed by specimens poured at temperatures of 1500–1540°C, which corresponds to the second period in the melting process. Raising the steel pouring temperature to 1580–1620°C leads to a decrease in strength due to the formation of a coarse structure in the cast steel, with a columnar grain shape. Lowering the pouring temperature to 1420–1440°C promotes a reduction in bending strength because of the formation of considerable looseness in the core of the specimens.

Fig. 3. Change in hardness as a function of tempering temperature and preceding quenching

Tempering temperature
— cast
— — — forged

Tests of specimens poured at different temperatures, in the quenched state, confirm the established regularity, which is characteristic for all investigated quenching temperatures. The bending strength of specimens poured at temperatures of 1500–1540°C increases, in comparison with the strength of specimens poured at lower or higher temperatures, by 10–23% (Fig. 4). The cooling conditions of specimens from the quenching temperatures also did not change the influence of the metal pouring temperature on strength (Fig. 5). (Cooling was applied in caustic at a temperature of 400–420°C with holding for 3–4 and 16 min, and in saltpeter at a temperature of 160–180°C with holding for 3–4, 16 min, and 3 h.)

Specimens poured at temperatures of 1500–1540°C have a substantial advantage in mechanical properties over the others even after complete heat treatment. Their bending strength is greater than that of specimens poured at low and high temperatures by 15–17% (Fig. 5).

Fig. 4. Dependence of the bending strength of Kh6VF steel after quenching from various temperatures on the pouring temperature of the liquid steel

Fig. 5. Bending strength after complete heat treatment of Kh6VF steel as a function of the pouring temperature

The experimental data show that the optimum temperature for pouring Kh6VF steel into shell molds, in order to obtain maximum strength, is 1500–1540°C. This metal temperature must be maintained in the crucible by periodic reheating of the steel during pouring. In addition, to preserve fluidity, the ladle must be preheated before pouring to 800–900°C when casting tools of small cross section, and to 500–600°C when casting large blanks.

Depending on the chemical composition, the bending strength of cast Kh6VF steel in all its conditions decreases as the carbon content increases from 0.85 to 1.3%. An increase in the quenching temperature has a similar effect on this characteristic of the steel (Fig. 6). The decrease in strength with increasing quenching temperature and

Fig. 6. Bending strength of Kh6VF steel as a function of carbon content and quenching temperature

carbon content is explained by substantial grain growth, as well as by the adverse effect of the increasing amount of ledeburite and retained austenite. A comparison of forged specimens with cast ones shows that the latter have lower bending resistance after quenching from all temperatures. According to Badaeva’s data, the strength of forged Kh6VF steel begins to decrease when the amount of retained austenite increases above 15%, and decreases still more sharply when its content rises above 50%. A similar regularity is also observed in cast Kh6VF steel, which is confirmed by the reduction in its bending strength with increasing carbon content and rising quenching temperature. After complete heat treatment, the strength properties vary depending on the tempering temperature (Fig. 7).

The introduction of titanium and boron as modifying additions in the investigated amounts has a significant positive influ-

the bending resistance of cast steel Kh6VF. The addition of titanium contributes to increasing its strength by 12–15% in comparison with unmodified steel. It has an especially significant effect in an amount of 0.3%.

Tests of steel with additions of boron showed that in amounts of 0.005 and 0.01% it increases the bending resistance of the steel after complete heat treatment by 10–15%. The introduction of boron in an amount of 0.05% contributes to a decrease in strength because of the formation, along grain boundaries, of a large amount of a brittle eutectic constituent.

Legend:
quenching temperature 950°
quenching temperature 1000°
quenching temperature 1050°

Fig. 7. Ultimate bending strength of Kh6VF steel as a function of tempering temperature

Tests for impact toughness of cast steel Kh6VF established that this characteristic decreases as the pouring temperature of the liquid metal is raised from 1420–1440 to 1580–1620°C. In the as-cast and quenched state, the values of the impact toughness of the steel vary, depending on the chemical composition with respect to carbon, within the range of 1 kgf·m/cm². Complete heat treatment increases the impact toughness of cast steel Kh6VF to 2–2.5 kgf·m/cm². Its value increases with increasing tempering temperature and decreasing quenching temperature.

Modifying additions in this case do not have a substantial effect.

1. The mechanical properties of cast steel Kh6VF in all its states depend on its carbon content, the heat-treatment regimes, and the temperature at which the liquid steel is poured into the molds.

2. The optimum pouring temperature for this steel in the production of woodworking tools is 1500–1540°C.

3. Modifying additions of titanium and boron have a positive effect on the strength properties of cast steel Kh6VF. The following amounts have been adopted as optimal: titanium—0.15–0.3%, boron—0.005–0.01%.