Permanent Mold Casting of Slitting Cutters

N. A. Erofeev and V. A. Chernikov

The present work is one of the topics of a general problem in the field of cast cutting tools being carried out by the Department of Metal Science of the Tomsk Polytechnic Institute named after S. M. Kirov under the direction of Professor A. N. Dobrovidov, Doctor of Technical Sciences.

The authors have been engaged in the manufacture and investigation of the durability of slitting cutters 140 mm in diameter and 5 mm thick, cast from high-speed steel in a permanent mold. In addition to its theoretical significance, the work was of purely practical interest, since it addressed the problem of manufacturing the above-mentioned tool at a machine-building plant.

The authors hope that their experience will be useful to production workers engaged in the manufacture of cast tools.

Technology of Melting, Casting, Heat Treatment, and Mechanical Treatment

For casting the cutters, a mixture of scrap high-speed steels of grades R18 and R9 was remelted. Such inaccuracy in the chemical composition of the initial material was allowed in view of the fact that, under the production conditions of plants manufacturing tools for their own needs, it is hardly possible to organize reliable separation of all grades of high-speed steels sent for remelting. On the other hand, it was of interest to investigate how differences in the chemical composition of the remelted steels, as compared with the composition of standard grades of forged steels, would affect the durability of the tool.

The melting of high-speed-steel scrap was carried out in a high-frequency generator AZ-46 at a voltage of 8–9 kV and 2.5–3 A of anode current, with reduced frequency owing to the additional introduction of a bank of air capacitors. A crucible made from a mixture of 80% quartz sand and 20% refractory clay, reinforced with asbestos-cement plates, was inserted into the inductor of the installation. The crucible capacity was 2 kg. After thorough drying and firing, such a crucible is capable of withstanding up to 60–70 melts. The cutters were cast in a permanent mold made of steel 3, since the durability of the mold itself was not of interest to us.

To avoid the formation of cracks in the casting, the ratio of the weight of the casting to the weight of the permanent mold should be within the limits \( \frac{1}{3} \div \frac{1}{4} \). The cavity to be filled in the mold was coated with a thin layer of graphite or soot, thereby preventing burn-on and ensuring satisfactory cleanliness of the casting surface. Before pouring, the permanent mold was heated to 250–300°. Casting was carried out by the centrifugal method at 500–600 revolutions per minute on a centrifugal casting machine with a vertical spindle, designed by the Department of Metal Science. At the same time, 1.2–1.5 kg of steel was melted. To protect against the burning out of carbon and alloying elements, the surface of the molten metal was covered with a layer of flux—glass.

Carbon losses due to burnoff were compensated by adding high-speed cast iron to the melt. Silicon-calcium and aluminum, taken at the rate of 0.1% of the weight of the melt, served as deoxidizers.

Ferrotitanium and ferroboron were used as modifiers, at the rate of 0.003% boron and 0.18% titanium. The cast cutter was removed hot from the permanent mold and, to prevent cracking, was transferred to a furnace heated to 300–350°, where it cooled slowly.

The heat treatment of the cast cutter consisted only of tempering. The experience of the department and literature data show that the tempering temperature for a tool made of cast high-speed steel should be taken as 600–620°. Taking into account the specific nature of the cast products, a careful study was made of the tempering of cast cutters, the results of which are expressed by the graph (Fig. 1).

Fig. 1. Effect of tempering temperature on the hardness of cast steel.

As follows from this graph, the behavior of cast high-speed steel during tempering is analogous to its behavior during tempering of hardened forged high-speed steel. The difference is that the temperature interval of tempering at which maximum hardness is attained is shifted by 50–60° toward higher temperatures. This phenomenon is explained by the increased alloying of the austenite of the cast steel in comparison with the austenite of forged steel, as a result of which its decomposition is impeded at temperatures corresponding to the decomposition of the austenite of forged steel. Multiple tempering at 600–620° did not reveal any noticeable decrease in hardness, which indicates an increased red hardness of cast high-speed steel in comparison with forged steel (Fig. 2). On the basis of the indicated results, the optimal

Fig. 2. Dependence of hardness on the number of tempers

For the tempering regimen, triple heating at 600–620° was adopted, with a one-hour hold each time and subsequent cooling in air. The hardness of the cutters after such tempering was 62–65 Rockwell units.

During machining of the cutters, the sprue was cut off, the side surfaces were ground, and the teeth were cut and sharpened. The grinding allowance was 0.5 mm per side, but it is possible to reduce the allowance to 0.3 mm.

Chemical Analysis of the Cast Cutters

Chemical analyses of the remelted steel gave, on average, the following results¹:

As follows from the analysis, the composition of the cast cutters does not correspond exactly to the composition of standard high-speed steels, but the difference is insignificant.

Structure of the Cast Steel

Since high-speed steel belongs to the ledeburitic type of steels, in the cast state it is characterized by the presence of a ledeburitic eutectic, which adversely affects the mechanical properties of this steel, in particular its impact toughness.

However, first, forged and hardened steel also has extremely low impact toughness; and, second, the reduction in impact toughness due to the presence of a ledeburitic eutectic depends on the size of the ledeburitic inclusions.

Owing to the high cooling rate of the casting in permanent-mold casting, the ledeburitic network does not develop extensively; the ledeburitic interlayers are very thin. The steel obtained is fine-grained, which is further promoted by the introduction of boron and titanium as modifiers. The structure in the cast state is: cryptocrystalline martensite with a large amount of retained austenite and ledeburitic eutectic in the form of a fine network. The hardness of the steel in the cast state is 62–65 Rockwell units.

During tempering at 600–620°, the martensite softens and the retained austenite decomposes, transforming into martensite. As a result of these mutually opposing processes, the hardness remains at a high level, thereby ensuring the necessary cutting properties of the tool. The structure of the tempered steel is troostomartensite with ledeburitic eutectic in the form of a network.

Testing the Cutters for Durability

The durability testing of the cutters was carried out in the shop of a machine-building plant. The materials used for the test were steel 3 and steel Kh12.

Comparison of the durability results of cast and forged cutters, with identical tooth geometry and identical cutting conditions, showed that the durability of the cast cutters was not lower than that of the forged cutters, and some cutters showed increased durability in comparison with forged cutters.

Conclusion

The problem of manufacturing cast tools has, in the main, been solved both theoretically and practically. The expediency of work in this direction is beyond doubt. The manufacture of cast tools simplifies the technology of both mechanical and heat treatment, reduces the consumption of scarce and expensive steels, and makes it possible to use scrap, while at the same time not impairing the quality of the tool in comparison with tools made from forged steel.

The inevitable drawback of cast tools—namely, a somewhat increased brittleness—is successfully compensated for by increased wear resistance and better cutting properties.

1. In this case, steel R9 was remelted. ↩