INVESTIGATION OF THE STRUCTURE AND PROPERTIES OF DIE STEELS IN THE AS-CAST STATE

I. O. Khazanov, Yu. M. Lozinskii

(Presented by Prof. A. N. Dobrovidov)

At present, special importance is being attached to the question of using tool steels in the as-cast state for certain types of tools. According to data available in the literature, die tools made from cast blanks are not inferior in durability to forged ones, and in many cases their durability is considerably higher \[1, 3, 4\]. Thus, some authors indicate that the durability of cast dies is 1.5–2.5 times higher \[1, 3, 8\], while the cost of manufacturing them is 4–7 times lower than that of forged dies. Such high durability of cast tools is associated with certain features of the cast metal.

It is known that steel in the as-cast state has more uniform strength in different directions, whereas forged steel has reduced strength in the direction perpendicular to the fibers. In addition, it has been established that cast steel has wear resistance 1.5–3.0 times higher than forged steel \[2, 5\], possesses a higher tensile strength limit and increased heat resistance \[6\].

Along with these advantages, cast steel has a number of drawbacks, such as, for example, reduced plastic properties. Therefore, when assessing the advisability of using cast dies, one should take into account the specific operating conditions of the tool and the properties of the given steel in the as-cast state.

The aim of the present work was to investigate the structure and certain properties of die steels 4Kh8V2 and 3Kh2V8F in the as-cast state. The steel was melted in an induction furnace in an acid crucible. The melt weight was from 2 to 6 kg. Aluminum was used as a deoxidizer at a rate of 0.1% of the melt weight.

The metal was poured into metallic trefoil molds, making it possible to obtain specimens without shrinkage porosity.

The chemical composition of the melted steels is given in Table 1. After casting, specimens of steel 4Kh8V2 had a hardness of 55–56 HRC, and those of 3Kh2V8F had HRC = 46. The steels under investigation were annealed according to the following schedule: heating to a temperature of 860°C, holding for 3 hours, then cooling to a temperature of 750°C with a holding time of 3 hours, after which cooling to a temperature of 500°C at a rate of 30–40°/hour, followed by air cooling. The hardness after such annealing was, for steel 4Kh8V2, HB = 183 kg/mm², and for steel 3Kh2V8F, HB = 192 kg/mm². The structure

...of these steels in the as-cast state consists of martensite and of a “light constituent” arranged in the form of a network (Fig. 1). After annealing, the main structure is granular pearlite, and in place of the “light”

Table 1

constituent, products of its decomposition with a large quantity of finely dispersed carbides were formed (Fig. 2). In the present work, the microhardness of the structural constituents of the steels under investigation was measured in the as-cast and annealed states.

Fig. 1.

The measurements were made on a PMT-3 instrument at loads of 20 and 50 g. The hardness of the “light constituent” of as-cast 4Kh8V2 steel was \(HV = 741\ \text{kg/mm}^2\), and that of 3Kh2V8F steel was \(HV = 738\ \text{kg/mm}^2\), whereas the hardness of the martensite in them was, respectively, \(HV = 510\ \text{kg/mm}^2\) and \(HV = 496\ \text{kg/mm}^2\).

After annealing, the hardness of the main structure, granular pearlite, in 4Kh8V2 steel was \(HV = 231\ \text{kg/mm}^2\), while the more finely dispersed regions—the decomposition products of the “light constituent”—had a hardness of \(297\ \text{kg/mm}^2\). For 3Kh2V8F steel these values were, respectively, \(268\ \text{kg/mm}^2\) and \(316\ \text{kg/mm}^2\). Apparently, the “light constituent” is carbon-enriched, non-etching martensite. In some

in places, in the central part of the “light constituent,” inclusions of primary carbides are noticeable.

The presence in the structure of the “light constituent” is, in all probability, connected with the phenomenon of segregation of carbon and other alloying elements during the crystallization of the steel. After annealing, this structural constituent decomposes into a more finely dispersed ferrite–carbide mixture with a larger amount of carbides, which leads to a somewhat increased hardness of these regions in comparison with the main structure. Thus, the increase in the wear resistance of cast steel is connected with the presence in its structure of a hard framework of more highly alloyed, segregated martensite (the “light constituent”).

Fig. 2.

In the work, the optimum heat-treatment conditions for cast steels 4Kh8V2 and 3Kh2V8F were determined. The study of quenching-heating temperatures was carried out in the temperature interval 980–1200°C for steel 4Kh8V2 and 1080–1250°C for steel 3Kh2V8F. Heating for quenching was performed in a salt bath, and cooling in a molten alkali heated to 400–450°C, followed by transfer of the specimens to air. The optimum quenching temperature was taken to be that temperature at which, upon quenching, the steel acquired maximum hardness and the greatest heat resistance during subsequent tempering, without the presence of overheating in the steel.

The optimum tempering temperature must be determined as a function of the operating conditions of the tool. For dies operating under large dynamic loads, tempering is carried out at a higher temperature than for a tool that fails because of wear of the working surfaces [7].

In the work, the optimum heating temperature for quenching was determined: for cast steel 4Kh8V2, 1120–1150°C; and for cast steel 3Kh2V8F, 1170–1200°C. The dependence of the hardness of quenched steels on the tempering temperature is presented in Fig. 3 (the holding time at the tempering temperature was 3 hours).

As can be seen from Fig. 3, the heat resistance of these steels depends to a considerable degree on their carbon content. Heat resistance was taken to mean the temperature of a 3-hour tempering after which the steel had a hardness of \( \mathrm{HRC} = 45 \). When the carbon content in steel 4Kh8V2 is reduced from 0.44% to 0.32%, the heat resistance falls from 600 to 555°C. In steel 3Kh2V8F, when the carbon content is reduced from 0.40 to 0.31%, the heat resistance decreases, respectively, from 640 to 600°C. This indicates that the carbon content in these steels must be kept closer to the upper limit of the range specified by the GOST standard. In manufacturing die tooling by the casting method, it is possible to regulate the carbon content in the steel within the required range.

Fig. 3. Dependence of the hardness of hardened steel on tempering temperature:
— steel 4Kh8V2 with 0.44% C,
— steel 4Kh8V2 with 0.32% C,
— steel 3Kh2V8F with 0.40% C,
— steel 3Kh2V8F with 0.31% C.

A comparison of some properties of the steels under investigation in the cast and forged states was carried out in this work. No differences in the heat resistance of cast and forged steel were found.

The mechanical and physical properties of these steels were studied. Impact-toughness tests were carried out on specimens measuring \(10 \times 10 \times 55\) mm with a notch; the tensile strength was determined on standard Gagarin specimens.

The results of the mechanical and physical tests of cast and forged steels are presented in Table 2. As can be seen from Table 2, the tensile strength of the cast steels is not inferior to the strength of the forged steels. However, the impact toughness of the steels in the cast state is significantly lower than that of the forged steel. The electrical resistance of cast steel 4Kh8V2 in the hardness range \( \mathrm{HRC} = 39.5\text{—}51.5 \) is lower than that of forged steel. Since electrical conductivity is the reciprocal of electrical resistance, it may be said that the electrical conductivity of steel 4Kh8V2 in the cast state is higher than that of the forged steel. Because electrical conductivity changes similarly to thermal conductivity [2], in all probability cast steel 4Kh8V2 has a higher thermal conductivity than forged steel. It should be noted that steel 3Kh2V8F has a lower—

has a lower electrical resistance and, accordingly, a higher electrical conductivity than steel 4Kh8V2.

Table 2

Conclusions

1. In the structure of cast steels 3Kh2V8F and 4Kh8V2 there is a “light constituent” arranged in the form of a network, which has high hardness (HV = 738–741 kg/mm²). The increased wear resistance of cast steel is to a considerable extent connected with the presence in its structure of the “light constituent,” which has high hardness.

2. The heating temperature for hardening cast steels should be selected somewhat higher than for forged steel (for steel 4Kh8V2, 1120–1150°C; and for 3Kh2V8F, 1170–1200°C).

3. A decrease in the carbon content from 0.44–0.40% in steels 3Kh2V8F and 4Kh8V2 by 0.1–0.12% leads to a decrease in the heat resistance of these steels by 40–45°C.

4. The tensile strength of cast steels is not inferior to the strength of forged steels, with a lower impact toughness.

5. Cast steel 4Kh8V2 has a lower electrical resistance and, accordingly, a higher electrical conductivity than forged steel.

References

1. E. Eminger, V. Koshelev. Cast Tooling, Mashgiz, 1962.
2. E. Houdremont. Special Steels, vol. 1, 11, Moscow, 1959.
3. Ya. M. Okhrimenko et al. Forging Dies and Their Operation, Moscow, 1963.
4. Harvey R. F. “Metal Treatment and Drop Forging,” No. 236, vol. 32, 1965.
5. H. Uhlitzsch. “Neue Hütte,” No. 1, 1958.
6. S. S. Dyachenko, R. S. Kaplan. Bulletin of the Kharkov Polytechnic Institute, issue 1, No. 5, 1965.
7. Yu. A. Geller. Tool Steels, Metallurgizdat, 1961.
8. I. O. Khazanov, V. I. Krasnov. Collection of Rationalization Proposals, No. 12, Moscow, 1962.