CAST BLANKING DIES AND THEIR HEAT TREATMENT

I. O. Khazanov

(Presented by Professor, Doctor A. N. Dobrovidov)

Obtaining high-quality blanks for blanking dies is determined by the technological requirements imposed on the steel. The steel used for blanks must be readily forgeable, have sufficiently high thermal conductivity, and have a uniform distribution of carbides throughout the entire volume.

Steels of the Kh12 type, used for manufacturing the working parts of dies, while having high wear resistance, nevertheless in a number of cases do not possess the above-listed properties.

Forging, as a method of improving the structure of steel, does not always ensure an increase in the durability properties of dies, because even after forging the steel structure is insufficiently homogeneous and banded. In addition, blanks for large solid matrices are especially poorly forged, and in the structure of these blanks a coarse carbide network is very often observed, sometimes reaching a score of 7–8.

Usually the heating temperature of steel for forging should not exceed 1200°; overheating to 1220–1250° leads to melting of the eutectic and to the formation of cracks during forging. At high degrees of reduction of small blanks, carbide bands are very often encountered in the structure; these are frequently the cause of chipping of the cutting edges of dies. Forging of blanks for large solid matrices changes the structure practically only to a slight depth—10–15 mm from the edge of the forging. As a rule, at a distance of 10–15 mm from the surface toward the center, the metal of these forgings has a cast structure with a coarse carbide eutectic.

In the present work, heat-treatment regimes were investigated for blanking dies cast in a metal mold, which to a certain extent have advantages over forged dies because of their more homogeneous structure.

The method of producing dies by casting into a metal mold is new. Until now, die parts have not been cast in metal molds, and the existing foundry technology does not yield castings with a fine-grained structure (casting in sand, by investment patterns, in shell molds).

Obtaining blanks for dies by the permanent-mold casting method makes it possible to obtain a fine-grained homogeneous structure, and this improves the service properties of the dies. Cast blanks were used by us as finished punches and as blanks for matrices.

blanking dies without preliminary forging. Producing blanks by casting makes it possible to use tool-steel scrap and worn-out dies in the manufacture of die tooling. Remelting the scrap preserves up to 90% of scarce die steel.

Experimental heats of the steel were carried out in an AZ-43 high-frequency unit in an acid crucible with a capacity of 2.5 kg. The surface of the molten metal was protected from oxidation with ordinary glass. After complete melting of the charge, before pouring, deoxidation with aluminum was carried out at the rate of 0.1% aluminum by weight of the heat. The pouring temperature (1440–1460°) was measured with an OPPIIR-09 optical pyrometer. When titanium was added, the temperature of the steel during melting was raised to 1500°; the steel was deoxidized with aluminum, and then ferrotitanium was introduced. After the ferrotitanium had melted, the temperature of the steel was lowered and the steel was poured into a rotating chill mold directly from the crucible. To prevent the crucible from cracking, after completion of the heat sand was poured into it; the crucible thereby cooled more slowly, which prevented the formation of cracks. We cast punches and blanks for sectors of assembled dies in a rotating chill mold mounted on a centrifugal machine with an adjustable number of revolutions. To protect the chill mold from burning and melting of its walls, its working surface was dusted with marshalite suspended in a 0.5% aqueous solution of water glass at a chill-mold temperature of 250–300°. After dusting, it is desirable to coat the working surfaces of the chill mold with a layer of soot (smoke them over a gasoline flame); this improves the quality of the castings. Pouring is carried out into a chill mold preheated to 200–250°. The castings obtained are annealed.

To determine the percentage loss by burning of alloying elements in the steel, experimental heats were carried out.

The table gives data averaged from several heats.

Table

As is seen from the table, the initial steel before remelting corresponds in its composition to steel grade Kh12F1. Judging from the remelting results, in the case of rapid melting of the charge (20–30 minutes), burn-off is insignificant.

As a result of chemical analyses of a large number of heats, it was found that without recharging after the first remelting, all the blanks obtained corresponded to an average composition between Kh12F and Kh12F1. The vanadium content in all heats ranged from 0.40 to 0.65%. Consequently, the first remelting of steel can be carried out without recharging.

To refine the structure, 0.27% titanium was introduced into the steel of all heats. According to the analysis data, the titanium content in all heats fluctuated within the range of 0.12–0.16%.

At the plant, dies were made from cast blanks and underwent production tests. All the dies that passed the tests showed a service life between regrindings significantly greater than that of the plant’s standard dies.

The cast dies were tested in blanking rotor and stator iron for electric motors.

For example, the service life between regrindings reached 33 thousand cuts, which exceeds the service life between regrindings of the plant’s dies made from forged steel by approximately a factor of three.

On the basis of the operating results of dies made from cast blanks, it may be stated with confidence that they can fully compete with forged dies. Further refinement of the technology of cast dies and the possibility of changing the chemical composition of the steel during casting may substantially increase the overall service life of the dies.

Judging from the results of plant tests, it may be concluded that cast blanks for blanking dies will find application in the tool shops of plants.

However, it should be noted that the service life of dies made from cast blanks depends to a large extent on the correct selection of their heat-treatment conditions. Proceeding from these considerations, we selected heat-treatment conditions for blanks cast in a permanent mold from Kh12F1T steel, which differ from the heat-treatment conditions for the forged steel used for manufacturing dies at the plant.

Annealing of Cast Steel

Since after casting the blanks are subjected to machining, good machinability by cutting is a necessary requirement. The hardness of a blank cast in a permanent mold fluctuates between 45–50 HRC.

By casting in a permanent mold it is difficult to obtain a finished part (a die segment) with minimum grinding allowances and high mechanical properties. It is especially important to obtain good impact toughness.

It is known that annealing improves the mechanical properties of cast steel. For die tooling operating under impact loads, even a small increase in impact toughness is very valuable. In annealing cast blanks we pursued two objectives:

1. To change, as far as possible, the structure of the ledeburitic eutectic in order to break it up and spheroidize it.
2. To reduce the hardness of the cast steel to 217–250 HB in order to improve its machinability.

For these reasons, we investigated three annealing conditions for cast steel, which are summarized in the graph in Fig. 1.

The hardness of the blanks after annealing:

under the first condition, HB = 258 kg/mm²;
under the second condition, HB = 241 kg/mm²;
under the third condition, HB = 241 kg/mm².

After annealing, a comparative analysis was made of the microstructures obtained under different conditions. Comparison of these microstructures showed that annealing under the first condition did not change the structure of the ledeburitic eutectic at all; the ledeburitic carbides were not spheroidized at all. Under the second and third annealing conditions, the carbide ...

the network proved to be fragmented, the carbides became spheroidized, which led to a reduction in hardness to HB = 241 kg/mm² (Fig. 2). The advantages of the steel annealed according to the second and third schedules were obvious. Under production conditions, the second schedule should be considered the most rational for annealing cast blanks, since it does not require—

Fig. 1. Annealing schedules for cast Kh12F1 steel.
Vertical axis: temperature, °C. Horizontal axis: time, hours. Curves: 1st schedule, 2nd schedule, 3rd schedule.

Fig. 2. Microstructure of steel annealed according to the second schedule.

—the use of high-temperature furnaces. As for the difference in hardness between annealed wrought and cast steel of analogous chemical composition, it will always be present, since in cast steel annealing cannot eliminate the skeletal orientation of the carbide eutec-

even at a sufficiently high spheroidizing temperature. Therefore, the primary eutectic carbides of cast steel, which have a directed orientation (relative to the walls of the mold), increase the hardness by 15–20 HB in comparison with forged steel. The increased hardness after annealing of steel cast in a metal mold can be explained by the lower ability of the eutectic constituent in the steel to deform; it serves, as it were, as reinforcement taking up the load during indentation by the indenter. This same property of the eutectic in cast steel may evidently be the reason for the increase in wear resistance in comparison with forged steel.

Hardening of Cast Annealed Steel

The specimens were heated in a high-temperature silite furnace over a temperature range from 900 to 1200°. The specimens were quenched in oil.

Analysis of the results of quenching specimens of cast steel showed that quenching from 900° does not give sufficient hardness, since at this temperature the solubility of carbides in austenite is low. The microstructure of steel quenched from 900° consists of cryptocrystalline martensite, carbide eutectic, and in places undissolved secondary carbides are visible.

Quenching from temperatures in the range from 950 to 1050° gives a sharp jump in the increase in hardness, with the maximum hardness obtained when quenching from a temperature of 1000°. The change in hardness as a function of the quenching temperature is shown in Fig. 3. The microstructure of such steel consists of cryptocrystalline martensite against a background of retained austenite and a carbide network.

Fig. 3. Hardness of cast steel Kh12F1T as a function of quenching and tempering temperature.

A further increase in the heating temperature leads to a decrease in primary hardness and to the manifestation of the effect of secondary hardness characteristic of steels of this type.

In determining the optimum quenching temperatures for secondary hardness, we noted that steel cast in a metal mold, even when heated to 1200°, does not give a significant increase in the austenitic grain size during quenching.

It may be noted that the range of quenching temperatures for obtaining secondary hardness in this steel can be expanded to 1180°; however, in this case the holding time of the specimens at this temperature should be selected rationally, based on their dimensions. Incre-

increasing the holding time during heating for hardening is equivalent to overheating.

The structure of steel hardened from 1200° consists of retained austenite surrounded by a fine eutectic constituent.

The optimum hardening temperature for secondary hardness in Kh12F1T steel cast in a chill mold should be considered 1150–1180°, with tempering at 550°. Such regimes give a hardness of 62–63 HRC, which ensures good tool life (Fig. 4).

Fig. 4. Dependence of the hardness of cast Kh12F1T steel on hardening temperatures and tempering temperatures, and also on the number of temperings at a temperature of 550°.

From the graph in Fig. 3 it is evident that cast Kh12F1T steel can be heated for hardening to primary hardness within the temperature range 950–1050°, whereas in standard forged steel of the same composition such temperature fluctuations are inadmissible. The wide interval of hardening temperatures for cast steel is explained by the greater alloying of the solid solution in cast steel, by the somewhat elevated carbon content (in our case), and by the presence of titanium in the steel under investigation, which protects the steel from overheating.

Tempering of Cast Steel

Specimens hardened to primary hardness from temperatures of 950–1050° were tempered at temperatures of 150–550° at intervals of 50°. Pro-

The holding time at the tempering temperature was 1 hour. Specimens hardened to secondary hardness were tempered in the temperature range 450–600° at every 50°, in order to determine the optimum temperature for the manifestation of secondary hardness. The results of tempering the specimens are presented in the graph in Fig. 4.

The microstructure of steel hardened to secondary hardness and tempered at 550° consists of finely dispersed tempering carbides, cryptocrystalline tempered martensite, and primary carbides surrounding the grains of the solid solution.

The microstructure of steel hardened to primary hardness is analogous to the microstructure of the steel after hardening and tempering to secondary hardness, with the difference that the network of primary carbides in the steel after hardening and tempering to secondary hardness is finer.

From the data obtained it follows that:

a. As the tempering temperature of cast steel X12F1T increases, the hardness decreases smoothly. For hardening from 950°, the optimum tempering temperature is 200° at a hardness of HRC = 62. To increase toughness, the tempering temperature may be raised to 400° and HRC = 58–59 obtained.

b. When hardened from 1000°, the optimum tempering temperature is 400° at the upper hardness limit HRC = 58–59. To increase toughness, the tempering temperature may be raised to 450°; in this case the hardness will decrease to HRC = 56–57.

c. For hardening from 1050°, the optimum tempering temperature lies in the range from 350 to 450° at a hardness of HRC = 58–60.

d. From the graph in Fig. 4 it follows that, when hardened to secondary hardness, the optimum tempering temperature is 550°. The hardness after such treatment is 62–63 HRC.

To determine the number of tempering operations after hardening to secondary hardness, fivefold tempering was carried out at 550° for one hour each. As is seen from the graph in Fig. 4, after the very first tempering at 550° the hardness of the steel reaches its maximum value.

Conclusions

1. The manufacture of dies from cast blanks makes it possible:
   a) to eliminate the operation of forging the blanks,
   b) to improve the structure of the steel through rapid cooling of the castings and appropriate heat treatment,
   c) to use up to 90% scrap of scarce die steel.

2. The service life between regrindings of cast dies is considerably higher than that of forged dies and, on average, is equal to twice the service life of forged dies.

3. The total service life of cast dies is equal to the service life of forged dies and can be significantly increased through the available reserves in producing blanks by the casting method (change of composition, use of modifiers, etc.).

Literature

S. V. Belynsky. Investigation of Cast and Forged Steel. Mashgiz, 1952.