STRENGTH ISSUES IN THE PROCESSING OF CAST HIGH-SPEED STEEL INTO BAINITE

Yu. A. Evtyushkin

(Presented by Doctor of Technical Sciences, Prof. A. N. Dobrovidov)

The isothermal transformation in the intermediate (bainitic) region was first noted by S. S. Steinberg in the 1930s, but more complete studies were carried out by Bain, G. V. Kurdyumov, A. P. Gulyaev, and V. D. Sadovskii.

However, as a number of authors note, [1, 2] the bainitic transformation has still been insufficiently studied, and the literature contains a number of contradictory data and various proposals for explaining the features of this transformation [3].

The products of this transformation have no generally accepted name. In the metallurgical literature various names are assigned to them—“acicular troostite,” “bainite,” “pseudomartensite,” “Steinbergite” [1, 4].

Fig. 1.

In the C-shaped diagram the bainitic region lies above the temperatures corresponding to the martensitic transformation (Fig. 1).

Bainite is a highly dispersed mixture of ferrite supersaturated with carbon and special carbides. Bainite differs from troostite by the greatest fineness of its phases, reaching even molecular dispersion. The alpha phase that precipitates during the formation of bainite contains in solution a considerable amount of alloying elements and carbon; in this case the ferrite is not fully equilibrium, and consequently there must also be some distortion of the crystal lattice.

It has been established that the structure in this state is obtained not in the form of rounded precipitates, as in troostite, but in the form of acicular formations, similar to martensite. These needles differ from martensite in that they contain the finest carbide formations.

in a mixture with ferrite. The internal structure of bainite can be distinguished only with the aid of an electron microscope.

There are two methods of obtaining bainite, according to which primary and secondary bainite are distinguished. Primary bainite is obtained as a result of the isothermal decomposition of supercooled austenite. Secondary bainite is obtained as a result of decomposition, during tempering, of retained austenite, if the cooling of the steel is interrupted at temperatures close to the point \(M\), and an isothermal holding is given.

It is considered that bainite is only slightly softer, but definitely tougher and more ductile, than martensite [2,4]. There are also indications in the literature that the presence of bainite in high-speed steel can increase tool life. Sometimes it is expedient, as Yu. A. Geller [3] has shown, to apply bainitic hardening in order to reduce deformation and warping.

The improvement of the mechanical properties of steel treated to bainite is a consequence of the fact that the formation of the bainite structure proceeds with a longer holding at higher temperatures and with a smaller increase in volume than martensite; therefore phenomena that relieve phase work-hardening (since, during the formation of a martensitic or bainitic crystal, local plastic deformation arises) have time to occur. As a result, internal stresses in the hardened steel and the danger of microcrack formation are reduced.

As is known, the principal drawback of high-speed steel cast in metal molds is its low impact toughness and strength. The low strength of cast steel is due not only to the presence of a brittle eutectic in the structure, but also to large internal stresses and microcracks.

When high-speed steel is cast in metal molds by the centrifugal method, in addition to grain refinement, the eutectic is also refined; the structure of the steel becomes more homogeneous than with other casting methods. However, even with this casting method the strength of the steel remains low, owing to the presence of internal stresses.

As is known, when high-speed steel is cast in metal molds, owing to the high cooling rate and the temperature gradient across the section of the casting, large thermal stresses arise. When the elastic limit is exceeded, plastic deformation and work-hardening develop. Since the elastic limit of austenite is very low, the occurrence of local plastic deformation is unavoidable. Recovery processes, because of the high cooling rate, do not have time to take place. With further cooling of the steel, structural stresses, arising as a result of the transformation of austenite into martensite, are added to the thermal stresses. As a result, the bending strength of cast high-speed steel is only \(\sigma_{\text{b.u.}} = 65\ \text{kg}/\text{mm}^2\), whereas forged steel has a strength \(\sigma_{\text{b.u.}} = 240\ \text{kg}/\text{mm}^2\).

The hardness of the steel after casting in a chill mold, because of the presence of work-hardening, is very high (\(H_{Rc} = 62—65\)), despite the large amount of retained austenite.

By giving the cast steel a prolonged holding in the bainitic temperature range, we thereby relieve residual stresses and work-hardening. In addition, by replacing the martensitic transformation with a bainitic one, we obtain phases with a smaller difference in volume. All this makes it possib-

to reduce internal stresses, deformations, and the danger of microcrack formation.

The main objective of the work was to increase the strength of cast steel by means of bainite treatment.

Description of the Experiments

Cast high-speed steel of the R18 type was investigated. A tube-type high-frequency generator was used for melting the steel. The steel was melted in a rammed removable crucible with a capacity of 2 kg. Carbon burn-off was compensated by adding “high-speed cast iron”—carburized high-speed steel. The casting of specimens was carried out in metal molds by the centrifugal method.

The heat treatment of the specimens was carried out according to the following schedules:

1. Normal hardening (comparison variant).
2. Hardening to primary bainite.
3. Treatment to secondary bainite.

All these heat-treatment schedules are shown graphically in Fig. 2.

Fig. 2.

After heat treatment, mechanical tests were carried out, the results of which are summarized in Table 1.

A comparison of the test results shows that specimens treated to secondary bainite have higher mechanical properties than specimens in which a primary-bainite structure was obtained. This can be explained by the fact that secondary bainite forms from retained austenite, when part of the austenite has already transformed…

Table 1

into martensite, and further formation of martensite would occur in an almost continuous martensitic mass. This would produce large internal stresses and work hardening along the grain boundaries. This is why it is advantageous to replace these last portions of martensite with bainite.

Primary bainite, however, is not as effective, because it replaces the first portions of martensite, which form in soft austenite. In addition, the martensite formed during tempering from residual austenite again creates internal stresses and phase work hardening and thereby eliminates the beneficial effect of holding during primary bainitization.

It is also evident from the table of test results that the extreme strength values for specimens treated to secondary bainite lie within much narrower limits than under other treatment conditions. Deviations from the mean value of the ultimate strength amount to ±9% for normal hardening, ±8% for primary bainite, and ±2% for secondary bainite. Such fluctuations in strength values result from the presence of internal defects that arise during heat treatment. It is clear that such considerable fluctuations (especially with normal hardening) cannot ensure uniform durability of a cast tool, particularly if it operates under impact.

To reveal more clearly the effect of bainitic treatment on the strength of cast steel, some of the specimens were tempered to a bainite hardness of $H_{Rc}=60$. However, the strength of these specimens proved to be significantly lower: $\sigma_{\text{b.u.}}=65\div84\ \text{kg}/\text{mm}^2$.

Conclusions

Treatment of high-speed steel cast in metal molds to secondary bainite made it possible to eliminate internal stresses and remove work hardening. As a result, the strength of the steel increased significantly.

For steel cast in sand molds, bainitic treatment is of little effectiveness, since here the main influence on strength is exerted not by internal stresses and not by phase transformations, but by the presence of a coarse, brittle eutectic network.

Heat treatment to secondary bainite will be useful for cast tools operating in bending or under impact loads, since the hardness of bainite is slightly lower than that of martensite, but its toughness is much higher.

References

1. A. P. Gulyaev. Heat Treatment of Steel. Mashgiz, 1953.

2. S. G. Koop. Heat Treatment of High-Speed Steel. Metallurgizdat, 1956.

3. Yu. A. Geller. Tool Steels. Metallurgizdat, 1955.

4. A. P. Gulyaev, K. A. Malinina. “Isothermal Transformation of Austenite in High-Speed Steel.” Metallovedenie i obrabotka metallov [Physical Metallurgy and Metal Treatment], No. 12, 1956.