EFFECT OF BORON MODIFICATION ON CHANGES IN THE AMOUNT AND COMPOSITION OF CARBIDES IN CAST HIGH-SPEED STEEL

N. D. TYUTEVA and V. T. SVISHCHENKO

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

Melts for the Investigation

Specimens of cast tools were investigated—knives for assembled end mills measuring \(35 \times 24 \times 8.5\) mm. The chemical composition of the melts is given in Table 1.

The steel was melted in a coreless induction furnace. Boron was added in the form of ferroboron. The ferroboron was introduced at the last moment of melting, before the metal was poured into the molds. The steel was preliminarily deoxidized with aluminum, calculated for an addition of 0.09% aluminum. The tools (knives) were cast in metallic rotating molds, i.e., under pressure. The advantages of this method consist, first, in the greater productivity of the process and, second, in the further shortening of the tool-manufacturing cycle, since a tool of high-speed steel cast in a chill mold is hardened during the cooling of the casting, and the heat treatment of the tool consists only of triple tempering.

For the modified and unmodified knives the following tempering schedule was selected:

The holding time at the tempering temperature was one hour.

A preliminary investigation [1] established that modification with boron increases the durability of cast knives by a factor of 3–4 in comparison with the average durability of knives from unmodified melts when operating under identical conditions. The optimal addition of boron in the form of ferroboron to cast high-speed steel with various tungsten contents lies within the range of 0.015–0.025% boron, provided that the steel is deoxidized with aluminum, calculated for an addition of 0.09% aluminum.

It was also established [1, 3] that, under the influence of boron modification, the microhardness of the structural constituents of the cast steel and the volume occupied by the carbide phase change. Data on the change in the microhardness of the structural constituents and the change in the volume occupied by the carbide phase made it possible to conclude that boron modification causes a redistribution of alloying elements between the phases of cast high-speed steel.

In the present work this conclusion is confirmed by phase analysis (Table 1).

Method of Phase Analysis

The method of phase analysis was chosen on the basis of the work of N. A. Saverina [2].

The objects of Saverina’s investigation were chromium steel Kh30 and austenitic chromium-nickel steel Kh18N9, both in the hardened and in the tempered condition. Saverina carried out an experiment with “the use of a current density of \(1.2\ \text{A}/\text{cm}^2\), i.e., 60 times higher than that usually employed” [2].

In dilute solutions, as Saverina indicates, it was impossible to create such a current density on the specimen; therefore, for the corresponding experiment she chose as the electrolyte a saturated solution of potassium chloride, acidified with hydrochloric acid (to prevent precipitation of the hydrated oxides of the dissolving metals).

In Saverina’s experiments it is also stated that an increase in temperature does not affect the decomposition of carbides.

Our experiments showed that, when working at high current densities (\(1.2\ \text{A}/\text{cm}^2\)) without cooling, strong heating of the specimen occurs. If an electrolyte containing an admixture of hydrochloric acid is used, then with a considerable rise in temperature in the bath (the temperature of the electrolyte in the bag reached \(60\text{–}80^\circ\text{C}\)) in the presence of hydrochloric acid, partial destruction of unstable carbides is possible. This occurred in our work at high current densities and without cooling. The results of analysis of the carbide phase were not constant.

It was not possible to discern any regularity in the change in the chemical composition of the carbides under the influence of modification. Therefore we adopted the following variant: the electrolyte composition was a saturated solution of potassium chloride with an addition of \(0.5\%\) citric acid. The current was direct, from storage batteries. The current density was \(0.2\ \text{A}/\text{sq cm}\). The duration of anodic dissolution was from one to two hours. Dissolution was carried out with continuous cooling by running water. The cathode was a copper coil used for cooling the electrolyte. The temperature of the water passing through the cooling coil was \(+2^\circ\text{C}\); the temperature of the electrolyte in the bag near the specimen was \(10\text{–}12^\circ\text{C}\).

To determine the ratio of the weight of carbides to the weight of the entire dissolved part of the specimen, the latter was weighed before and after anodic dissolution. The precipitate from the bag and from the specimen was transferred to a Schott filter (the Schott filter was first dried to constant weight) and washed with hot water.

Then the filter with the precipitate was dried in a drying oven at a temperature of \(105^\circ\text{C}\), likewise to constant weight. The difference in weight between the ignited filter and the filter with precipitate gave the weight amount of carbide precipitate. The difference in the weight of the specimen gave the amount of metal dissolved.

Before removal of the precipitate, the specimen was carefully placed in a beaker with hot water; vigorous evolution of gases adsorbed by the surface of the specimen was observed. With a large number of specimens the gas evolution was so energetic that it gave the impression that the water was boiling. Such liberation of the specimen from oxygen, in our opinion, should reduce the tendency of the carbide powder to oxidize.

The percentage content of carbides in the alloy was determined from the data on the weight of the isolated carbides and the weight of the dissolved part of the specimen.

The change in the composition of the carbide phase with respect to carbon, chromium, tungsten, and vanadium under the influence of modification is shown in Table 1.

Table 1

Conclusions

1. From Table 1 it follows that the amount of carbides in the tempered modified melts increases, in comparison with the amount of carbides in the tempered unmodified melts, on average by 50%.

2. The chemical composition of the carbide precipitates of the modified and unmodified melts is different.

3. Taking into account the change in composition and the change in the total amount of carbides, it may be concluded that the $\gamma$-solid solution (the light constituent of cast steel), under the influence of boron modification, is depleted in carbon and chromium. This conclusion is in good agreement with the data on the decrease in the microhardness of the light constituent ($\gamma$-solid solution) under the influence of boron modification [1].

4. From the table it follows that the $\gamma$-solid solution is also depleted in tungsten. However, the change in the concentration of tungsten in the $\gamma$-solid solution cannot be judged solely from the change in the chemical composition of the carbide phase. Microstructural analysis has established [1] that, under the influence of boron modification, the amount of the dark constituent containing a significant amount of tungsten sharply decreases in the structure of cast steel. The increase in the total amount of tungsten in the carbide phase of the modified steel may occur not at the expense of depletion of the $\gamma$-solid solution in tungsten, but at the expense of a decrease in the amount of the tungsten-rich dark constituent.

5. The data of phase analysis confirm the assumption expressed by us earlier [1, 3] that, under the influence of boron modification, a redistribution of alloying elements between the phases of cast high-speed steel occurs.

References

1. Svishchenko V. T. Modification of cast high-speed steel with boron. Proceedings of the Tomsk Polytechnic Institute, vol. 75, 1954.

2. Saverina N. A. A method for isolating carbides and nitrides from stainless steel. Factory Laboratory, 1952, No. 8.

3. Svishchenko V. T., Tyuteva N. D. Some questions in the theory of modification. Proceedings of the Tomsk Polytechnic Institute, vol. 75, 1954.