EFFECT OF ANNEALING CONDITIONS ON THE STRUCTURE AND PROPERTIES OF CAST HIGH-SPEED STEELS

I. O. Khazanov, Yu. M. Lozinskii, V. F. Zhidovin

(Presented by Prof. Dr. A. N. Dobrovidov)

At the present time, both in the domestic and foreign literature, works are appearing that are concerned with investigating the possibility of using steels of the high-speed type for die tools. Especially promising in this respect may be the use of such steels for the manufacture of active parts of hot-forging dies in the cast state [1, 2]. The high heat resistance of high-speed steels, combined with increased strength at high temperatures, can be used in the manufacture of punches and dies from these steels, whose working edges during operation experience high specific pressures and heat up to high temperatures.

However, a significant drawback of cast high-speed steels is their reduced plastic properties \((\delta, a_k)\), caused by the features of the cast structure and by the presence in the steel of a considerable amount of carbides.

The purpose of the present work was to investigate the effect of annealing conditions (temperature and holding time), as well as the temperature of subsequent hardening and tempering, on the structure and properties of the steels studied. In the present work, steels of the following composition were investigated (Table 1).

Table 1

Steel melting was carried out in a high-frequency unit in an acid crucible. The heat weight was 2 kg. The specimens were cast into metal mold trees, which ensured the production of castings without shrinkage defects in them. With this method of casting specimens, shrinkage porosity is concentrated in the middle section of the mold—

* Engineer G. N. Ovcharenko took part in carrying out the experimental part of the work.

and specimens for testing are cut from the sound part of the casting. To reduce the depth of the decarburized layer on the surface of the specimens during heating, they were covered with fine chips of high-speed steel. After annealing, the surface of the specimens was machined to size, thereby completely eliminating the possibility of a decarburized layer being present.

The literature contains recommendations for selecting annealing temperatures for high-speed steels, for example, from 790 to 900°C [3, 4, 5]. Some investigators point out that the hardness of cast high-speed steel decreases only after high-temperature heating, and recommend, in order to reduce hardness, heating to 1260–1280°C in salt baths with air cooling.

N. T. Gudtsov and S. A. Kazeev [7] showed that by prolonged holding of cast high-speed steel at temperatures close to the quenching temperature, its structure can be changed very substantially, bringing it closer to the structure of forged steel. Yu. A. Geller [8] indicates that, in order to prevent deterioration of heat resistance, which becomes more pronounced with increasing annealing temperature, tungsten steels should preferably be annealed at no higher than 860°C. The deterioration of heat resistance is explained by the fact that, as the temperature and holding time are increased, carbides of an intermediate type, for example, \(M_{23}C_6\), transform into stable carbides (for example, \(M_6C\) or \(MC\)). During heating for quenching, these carbides do not dissolve in austenite even at very high temperatures, as a result of which its degree of alloying is reduced [9].

Annealing at a comparatively low heating temperature does not dissolve the primary carbides, but, by causing coagulation, can improve the structure [8]. In the present work, the following annealing temperatures were used for heats 1–4: 1150, 1200, and 1250°C. The holding time at 1150° was 3 hours, and at 1200° and 1250° it was 1, 3, and 6 hours. For heats 5–8 the following annealing temperatures were used: 1200, 1250, and 1280°C. The holding time at each temperature was 1, 3, and 6 hours. After high-temperature annealing, an additional isothermal anneal was carried out, standard for high-speed steels.

As can be seen from Table 2, cast steels of the composition studied have a general tendency toward a decrease in hardness with increasing annealing temperature and increasing holding time. A second anneal following the high-temperature anneal, according to the normal standard schedule for forged steels of similar composition, leads to a certain additional decrease in hardness. The data of Table 2 show that, with increasing carbon and alloying-element contents in the steel, the hardness of the cast steel after standard annealing is considerably higher than the hardness of steel annealed according to the double schedule.

As follows from a comparison of hardness at different holding times at the temperature of the high stage, despite a substantial decrease in hardness with increasing holding time, repeated standard annealing does not produce the same effect, and the hardness after repeated annealing changes only within very small limits (up to 20–25 HB), mainly in steels with an increased carbon content. The difference in hardness after standard and double annealing reaches 70 HB and more. As studies of the microstructure of steel annealed only according to the standard schedule and according to the double annealing schedule have shown, the difference in the microstructures is very substantial.

Figure 1 presents a microphotograph of the structures of steel of heat 7 in the cast condition, annealed only according to the standard schedule, and annealed according to the double schedule (heating to a high temperature with holding times from 1 to 6 hours, followed by standard annealing with heating ...

Table 2

Continuation of Table 2

to 860°C, holding for 3 hours, cooling to 750°C, holding for 5 hours, and then slow cooling at a rate of 20–30°/h to 600°C, followed by tempering in air).

Fig. 1. ×340. Change in the structure of cast steel of heat 7 after standard and double annealing

From the micrographs (Fig. 1) it is clearly seen that standard annealing has only a very slight effect on the microstructure of cast steel. The main effect of standard annealing is reduced only to spheroidization of secondary carbides and diffusional equalization of the composition of the solid

solution. After standard annealing the carbide network remains practically unchanged, which during subsequent heating for hardening reduces the solubility of the primary carbides in the solid solution. As can be seen from the left part of the micrograph in Fig. 1, the structure of the cast steel annealed by the double annealing schedule has almost no traces of skeletal orientation of the primary carbides. The structure is uniform in composition, and the carbides are fully spheroidized. A similar

Table 3

microstructure was observed in all heats annealed according to the double schedule. Moreover, increasing the holding time at the high stage does not improve the microstructure of the steel. With an increase in holding time at high temperature, a significant coarsening of the carbides is observed, which cannot improve the mechanical properties of the steel and its red hardness [8].

Table 3 gives the results of comparing the hardness dependence of cast steel previously annealed according to the standard regime, and also annealed according to the double regime, then quenched and tempered. The same data are also given in the graphs for heats 1, 3, and 7 (Figs. 2, 3, 4).

Fig. 2. Effect of the tempering temperature, with a duration of 1 h, on the hardness of steel of heat 1, previously annealed according to the indicated regimes (S.A. = standard annealing)

As is evident from the graphs and Table 3, steels of heats 1, 3, 7, and 8, when the holding time during annealing at the temperature of the high stage is increased, tend to show a decrease in red hardness after their final heat treatment. The most acceptable treatment for steels of these heats may be double annealing with a short holding time (1 h) during the first high heating and subsequent standard annealing. From the graphs (steels of heats 1, 3, 7) and Table 3 (steels of heats 4, 5), it is evident that after one standard annealing the red hardness of cast steels is even somewhat lower than the red hardness of the same steels previously annealed according to the double regime.

A study of the mechanical properties of steels of heats 1, 3, 4, 7, and 8, previously annealed only according to the standard annealing regime and according to the double regime (Table 4), showed that the use of preliminary double annealing for cast steels of a composition similar to heats 1, 3, 4, 7, and 8 makes it possible to increase somewhat their impact toughness and ultimate strength. Strength and impact toughness were determined in steel with a hardness of 49–52 HRC, since our aim was to use these steels for the manufacture of hot-forming tools. As is evident from Table 4, the impact toughness and ultimate strength of steel annealed according to the double regime, although only very slightly, nevertheless increased in comparison

Fig. 3. Effect of tempering temperature for a duration of 1 h on the hardness of steel from melt 3, preliminarily annealed according to the indicated regimes. (S. A. — standard annealing)

Fig. 4. Effect of tempering temperature for a duration of 1 h on the hardness of steel from melt 7, preliminarily annealed according to the indicated regimes. (S. A. — standard annealing)

compared with steel annealed according to the standard regime. Tests were carried out at each point on 5 specimens. Impact toughness was determined on standard specimens \(10 \times 10 \times 55\) mm with a notch; the tensile strength was determined on specimens with a working-section diameter of 6 mm.

Table 4

As follows from the results obtained in the present work, preliminary double annealing of cast steel increases its red hardness and mechanical properties. The present work confirmed the results obtained earlier in the work of O. N. Tanicheva [6] on determining the effect of high-temperature annealing on the red hardness of high-speed steel. In our case, any improvement in the mechanical properties of cast steel is even more valuable than for forged steel, since cast high-speed steel, compared with forged steel of analogous composition, has lower strength and impact toughness.

As production tests of piercing punches made of steel from melt 1 showed, for stamping bearing rings of types 304 and 208, their service life averaged 55 thousand punches, which considerably exceeds the service life of a similar tool made of forged steel 3Kh2V8.

To determine the effectiveness of using steels of the high-speed type for making cast dies for hot deformation of metals, it is advisable to conduct service-life tests of them. The most promising for this purpose may be steels from melts 3, 5, 7.

Conclusions

As the results of the investigations carried out have shown, the use of double annealing for cast high-speed steels has the following advantages over standard annealing:

a) the hardness of the steel is substantially reduced (by 50–80 HB);

b) the red hardness of the steel is noticeably increased, by 50°C and more, depending on the composition of the steel;

c) the use of double annealing leads to improved machinability of cast blanks.

References

1. Yu. A. Geller. Modern die steels for hot stamping, pressing, and drawing. Proceedings of the seminar on improving the durability of dies for hot and cold stamping. Moscow, 1965.

2. V. N. Saikina. A new technology for heat treatment of punches made of high-speed steel. Metallovedenie i termicheskaya obrabotka, No. 5, 1964.

3. S. G. Koopp. Heat treatment of high-speed steel. Metallurgizdat, 1956.

4. V. M. Doronin. Heat treatment of carbon and alloy steel. Metallurgizdat, 1956.

5. A. P. Gulyaev. Properties and heat treatment of high-speed steel. Mashgiz, 1939.

6. O. N. Tanicheva. Metallovedenie i termicheskaya obrabotka metallov, No. 9, 1964.

7. N. T. Gudtsov, S. A. Kazeev. On the question of the nature of high-speed steel. Metallurgizdat, 1930.

8. Yu. A. Geller, Yu. I. Karavanov. Stanki i instrument, No. 7, 1960.

9. E. Goudremont. Special Steels, vol. 2, 1960.