HIGH-TEMPERATURE THERMOMECHANICAL TREATMENT OF 9KhS STEEL

L. A. VERNIK

(Presented by Prof. A. N. Dobrividov)

In recent years many interesting studies have been carried out on the influence of high-temperature thermomechanical treatment on the mechanical properties of various steels, and attempts have also been made to clarify the causes of steel strengthening as a result of thermomechanical treatment.

In the present work, the influence of high-temperature thermomechanical treatment on the properties of industrially melted 9KhS tool steel was investigated. The change in shape during high-temperature thermomechanical treatment was produced by forging.

Free forging was carried out under shop conditions on MB412 model hammers. The specimen blanks were heated in a furnace to the specified temperature for 45 min. The temperature at the start of forging was \(1100^\circ\), and at the end of forging, \(900^\circ\). The specimens were forged with different degrees of deformation (30, 40, 50, 60, and 75%) and then quenched in oil. The degree of deformation was calculated by the logarithmic formula

\[ \varepsilon = 2 \ln \frac{F^\circ}{F}, \]

To prevent recrystallization of the deformed austenite, the time interval between the end of forging and the beginning of quenching did not exceed 15–20 seconds.

Making use of the “heredity” of thermomechanical treatment [1]—the preservation of high strength properties after thermomechanical treatment, high tempering, and repeated quenching—the specimen blanks were subjected to high tempering (\(680^\circ\)) in order to improve machinability. After fabrication, the specimens were quenched at \(870^\circ\) and tempered at \(180^\circ\).

The effect of high-temperature thermomechanical treatment of 9KhS steel on its mechanical and physical properties was investigated.

The results of the study of the change in hardness as a function of tempering temperature are presented in graph 1 (Fig. 1).

The process of softening after thermomechanical treatment proceeds considerably more slowly than after ordinary quenching.

Thus, specimens treated with degrees of deformation of 50, 60, and 75% retain high hardness (60 RC) after tempering at a temperature of \(320^\circ\), and with 40% deformation after tempering at \(370^\circ\), whereas untreated specimens already after tempering at \(250^\circ\) show a hardness of 59 RC.

Such a difference in the hardness of the specimens is probably associated with the energy threshold of recrystallization. High-temperature thermomechanical treatment raises the energy threshold of recrystallization [2]. The sharp drop in hardness in specimens with 30% deformation after tempering at 250° and above is explained by the attainment of the recrystallization threshold and the more intensive softening that follows.

Fig. 1

The effect of the degree of deformation on hardness and the amount of retained austenite is shown in graph 2 (Fig. 2).

Fig. 2

A substantial influence on the transformation of retained austenite is exerted by the diffusion of carbon, which increases with increasing degree of deformation [3]. At small degrees of deformation, which do not cause significant—

…of carbon precipitation from the solid solution, the austenite is strengthened. With increasing strength, the ability of austenite to transform decreases. This leads to an increase in retained austenite (30% deformation). At higher degrees of deformation, the decisive role is played by the precipitation of carbon from the solid solution. The ability of austenite to form slip bands, which are centers of martensite crystallization, increases. The amount of retained austenite in specimens with degrees of deformation of 40, 50, 60, and 75% decreases.

The strengthening caused by high-temperature thermomechanical treatment is explained by an increase in the density of structural imperfections in the initial phase that are retained in the martensite.

Fig. 3

(In the figure: ordinate labels include “Retained induction, gauss,” “Coercive force, oersted,” and “Specific electrical resistance, ohm·mm²/m”; abscissa: “Degree of deformation, %.” Curves are labeled \(B_r\), \(H_c\), and \(\rho\).)

The increase in the dislocation density in steel after high-temperature thermomechanical treatment, as compared with the total dislocation density after quenching, can be judged from the change in such quantities as the specific electrical resistivity \(\rho\), the coercive force \(H_c\), and the retained induction \(B_r\).

All types of defects in the crystal structure affect conductivity in steel and hinder the processes of magnetization and demagnetization as a result of scattering at defects.

The change in specific electrical resistivity, coercive force, and retained induction as a function of the degree of deformation during high-temperature thermomechanical treatment is shown in graph 3 (Fig. 3).

As the degree of deformation increases, the coercive force \(H_c\) rises from 42 oersteds for untreated specimens to 54 oersteds at 50% deformation. The residual induction \(B_r\) decreases, and the electrical resistivity increases.

Conclusions

1. High-temperature thermomechanical treatment raises the softening temperature.

2. High-temperature thermomechanical treatment with deformation up to 30% leads to an increase in retained austenite, while above 40% it leads to a decrease in retained austenite.

3. The strengthening effect of thermomechanical treatment is associated with an increase in the density of structural imperfections.

References

1. M. L. Bernstein, E. P. Demina, K. E. Safonova. Metallovedenie i termicheskaya obrabotka, No. 1, 1962.

2. L. I. Mirkin. Metallovedenie i termicheskaya obrabotka, No. 1, 1962.

3. A. P. Gulyaev, A. S. Shigarev. Metallovedenie i termicheskaya obrabotka metallov, No. 4, 1963.