Effect of Cooling Rate During Quenching on the Strength and Toughness of Die Steels 4Kh8V2 and 3Kh2V8

V. F. Zhidovin, I. O. Khazanov

(Presented by the scientific seminar of the cluster of departments: physical metallurgy, equipment and technology of heat treatment of metals, equipment and technology of welding production, and technology of metals.)

The increased requirements imposed on die steels for hot deformation of metal have prompted numerous studies undertaken in this field in recent years. A whole series of new steels has been proposed [1, 2], possessing high strength, toughness, heat resistance, and in most cases being more economical. However, the die steels 4Kh8V2 and 3Kh2V8 still remain the most widely used for the manufacture of tools employed in hot working [3].

Production experience shows that, with identical heat treatment of steels intended for the manufacture of hot-working tools, substantial differences may be observed in the toughness of specimens cut from large sections and of specimens made from small blanks and heat-treated individually. The reason for this may lie in the different degree of forging reduction, which affects the macrostructure and grain size, and also in the different cooling rate during quenching. In the present work, the influence of the latter factor was studied on standard die steels 4Kh8V2 and 3Kh2V8.

Table 1

Samples were made from bar stock and, after heating for quenching to temperatures of 1050–1150°C for steel 4Kh8V2 and 1100–1200°C for steel 3Kh2V8, were cooled in oil, in air, and in sand. In addition, specimens heated for quenching to 1100°C for steel 4Kh8V2 and 1150°C for steel 3Kh2V8, which are optimal for the steels studied [4], were cooled in water and subjected to interrupted quenching (cooling in molten NaOH alkali at a temperature of 400–450°C followed by transfer to air). Tempering of the quenched specimens was carried out at two temperatures: 200 and 550°C for steel 4Kh8V2, and 200 and 620°C for steel 3Kh2V8. Tempering at 200°C was carried out in order to exclude the influence of quenching stresses on the mechanical properties. No substantial changes in the microstructure occur in this case, since the mobility of atoms

carbon at this temperature is close to zero [5]. The tempering time at 200°C was 4 h and 2 h at 550 and 620°C. Specimens quenched in water, and specimens subjected to step quenching alkali → air, were tempered to a hardness of 50 HRC.

Strength and impact toughness were determined on specimens with a cross section of 10×10 mm. Static bending tests were carried out on a 10-t tensile-testing machine. The specimens were loaded at a rate of 10 mm/min. The distance between supports was 100 mm. Specimens for determining impact toughness had a round notch with a radius of 1 mm and were tested on an MK-30 pendulum impact tester. The hardness, bending strength, and impact toughness of the steels studied after various heat-treatment regimes are given in Table 2.

Table 2

The structures obtained under the selected cooling conditions were assessed on the basis of thermokinetic diagrams of the transformation of supercooled austenite given in the literature [6], on which the cooling curves for the cores of specimens with a cross section of 10×10 mm during quenching in oil and in air are plotted. Analysis of the diagrams shows that, after quenching in oil, specimens of the steels studied have a completely martensitic structure. Lower cooling rates (quenching in air and in sand) cause in steel 3Kh2V8, which has a low chromium content, a partial transformation in the bainitic region. Specimens of steel 4Kh8V2 acquire a completely martensitic structure at all the selected cooling rates, which is explained by its high chromium content. The results of metallographic analysis

satisfactorily agree with the conclusions drawn on the basis of analysis of thermokinetic diagrams for the transformation of supercooled austenite in steels close in composition to those investigated.

As can be seen from Table 2, the hardness of steel 4Kh8V2 in the quenched state depends little on the cooling conditions. The difference in hardness of steel 3Kh2V8 quenched in oil and in sand reaches 10 Rockwell units. The hardness of steel 3Kh2V8 quenched in air occupies an intermediate position. This indicates that the hardenability of the steels decreases as the cooling rate is reduced, as a result of the formation of bainite with an increased amount of retained austenite.

Tempering in the indicated temperature ranges affects the hardness of the experimental steels in different ways. The hardness of steel 4Kh8V2 decreases continuously with increasing tempering temperature and, after tempering at 550°C, is 50–52 RC for all cooling conditions during quenching. The hardness of steel 3Kh2V8 after tempering at 200°C does not change in comparison with the hardness in the quenched state. Tempering at 620°C leads to a decrease in the hardness of specimens quenched in oil and to an increase in the hardness of specimens quenched in air and in sand. Such an increase in hardness is apparently associated with the formation of secondary martensite and is intensified by the effect of dispersion hardening.

Of the mechanical-strength indices of the steels investigated, impact toughness changes most noticeably with the cooling conditions during quenching. It is evident from Fig. 1 that a decrease in the cooling rate during quenching

Fig. 1. Dependence of the strength and toughness of the experimental steels on the cooling conditions during quenching.

Figure labels: strength \(\sigma_{\mathrm{b}}\), kg/mm\(^2\); impact toughness, kgm/cm\(^2\); 4Kh8V2; 3Kh2V8; cooling in NaOH solution; cooling in oil; cooling in water; cooling in air; cooling in sand.

leads to a reduction in impact toughness by more than a factor of 2, while the strength changes only insignificantly. Metallographic analysis showed that, with a decrease in the cooling rate during quenching, carbides precipitate along grain boundaries, preceding the transformation as a consequence of supersaturation of the austenite. These resul-

The results confirm the conclusions drawn in [6] that, even at an increased cooling rate during quenching in steels intended for the manufacture of tools used in hot working, it is impossible to avoid the decomposition of austenite along grain boundaries. This, apparently, explains the decrease in impact toughness as the cooling rate during quenching is reduced. The unequal embrittling effect of carbide precipitates along grain boundaries on the strength and toughness of the steels studied should be explained by the difference in the methods used to determine these properties. Impact-toughness tests, being dynamic, are more sensitive to the structural state of the object under study. As can be seen from Table 2, the tendency toward carbide precipitation along grain boundaries increases with an increase in the quenching temperature and a decrease in the cooling rate. Of interest is the fact that steel 3Kh2V8 after low-temperature tempering shows an increase in toughness as the cooling rate during quenching is reduced. This is associated with an increase in the amount of bainite and retained austenite in the structure of the quenched steel. The positive effect of an increased amount of bainite on the toughness of steel quenched without tempering is known from the literature [6]. After tempering at a temperature of 620°C, a marked decrease in the impact toughness of steel 3Kh2V8 is observed. The toughness of specimens of steel 4Kh8V2, which harden completely to martensite at all the selected cooling rates, changes less noticeably than the toughness of specimens of steel 3Kh2V8. At a comparable value of impact toughness, specimens of steel 4Kh8V2 tempered at temperatures of 200 and 550°C have different hardness (Table 2). This indicates that tempering intensifies the embrittling effect of carbide precipitates along grain boundaries.

Conclusions

1. Reducing the cooling rate during quenching lowers the toughness of the steels studied after tempering according to the regimes adopted in the practice of manufacturing hot-working tools by more than a factor of 2. The strength changes only slightly.

2. This effect of slowed cooling during quenching is apparently associated with the precipitation of carbides along grain boundaries as a result of supersaturation of austenite.

3. The embrittling action of carbide precipitates along grain boundaries increases with increasing quenching temperature and decreasing cooling rate. Tempering acts in the same direction.

References

1. V. M. Zuev, Yu. A. Geller, L. I. Reznichenko. Forging-and-stamping production. No. 12, Mashinostroenie Publishing House, 1968.

2. Scientific works of TsNIITMASh recommended for industrial implementation, sixth edition, Department of Scientific and Technical Information, Moscow, 1967.

3. I. N. Golikov, M. I. Gladshtein, I. I. Murzin. Vanadium in Steel. Metallurgiya Publishing House, Moscow, 1968.

4. E. V. Smirnov et al. Special Steels and Alloys. Collection of works of TsNIITsM, issue 52, Metallurgiya Publishing House, Moscow, 1967.

5. A. P. Gulyaev. Heat Treatment of Steel. Mashgiz, Moscow, 1960.

6. K. Bungardt, O. Mülders, R. Meyer-Rhotert. Archiv für das Eisenhüttenwesen, 37, No. 5, 1966.