EFFECT OF BORON MODIFICATION ON CHANGES IN THE COMPOSITION OF CARBIDES IN CHROMIUM ALLOYS

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

N. D. TYUTEVA

Melts for the investigation

For the investigation, melts of chromium cast iron were obtained with approximately identical chromium and carbon contents and with different contents of the modifier—boron. The cast iron was melted in a coreless induction furnace in a rammed crucible. Boron was added in the form of ferroboron in amounts of hundredths and thousandths of a percent.

Ferroboron was introduced at the last moment of the melt, 80–100 sec before pouring the metal into molds. The cast iron was preliminarily deoxidized with aluminum, based on an addition of 0.1% aluminum.

When casting melts without boron, the crucible charge was changed in order to eliminate the possible entry of boron from the metal and slag remaining in the crucible. Cooling of the castings was carried out in different ways:

1. In sand molds, rods with a section of 30–35 mm were cast.
2. In a thick-walled chill mold, which was a cylinder with an outside diameter of 75 mm, a thin rod 7–8 mm in diameter was cast.

Such a sharp difference in cooling rate was specified deliberately, in order to determine the effect of cooling rate on modification.

When casting a thin rod from chromium cast iron (on average with 7% chromium), accelerated cooling and the high chromium content made it possible to obtain only a two-phase system with a structure of primary carbides and austenite. We obtained the alloy in the condition of austenite hardening without decomposition of the solid solution. In such a two-phase alloy it was easier to trace the change in the chemical composition of the carbide phase during modification with boron.

When casting rods 30–35 mm in diameter in sand, the alloy structure consisted of carbides and products of austenite decomposition. In this case the change in the composition and quantity of carbides was determined not only by the influence of the modifier, but also by decomposition of the solid solution during slow cooling.

Table 1

Chemical composition of the melts

The boron content in the melts was determined by the method of spectral analysis. In the castings, the microstructure and microhardness of the individual structural constituents were studied, and in thin rods quenched during the casting process the change in the composition of the carbide phase was additionally investigated by the method of phase analysis.

Method of Phase Analysis

The method of phase analysis was chosen on the basis of the work of N. A. Saverina¹. In N. A. Saverina’s work it is stated: “In the anodic dissolution of stainless and heat-resistant steels containing a large amount of chromium, special difficulties arise.”

The high chemical resistance of these steels, associated with the formation of a passive film, considerably hinders the dissolution process. Owing to the nonuniform, pitting or intercrystalline character of dissolution, not only the carbide phase precipitates into the sediment, but also particles of the metallic matrix surrounding it. This circumstance made it impossible to use the methods of anodic dissolution known up to the present time for the phase analysis of stainless steels.

The purpose of the present work is to select electrolysis conditions under which uniform dissolution of steel with a high chromium content occurs and quantitative separation of the excess phases and their stability during electrolysis are ensured. The objects of investigation were ferritic chromium steel Kh30 and austenitic chromium-nickel steel Kh18N9, both in the quenched and in the annealed state.

We carried out an experiment using a current density of \(1.2\ \mathrm{A/cm^2}\), i.e., 60 times higher than that usually employed. In dilute solutions it was impossible to create such a current density on the specimen; therefore, for carrying out the corresponding experiment, a saturated solution of potassium chloride, acidified with hydrochloric acid (to prevent precipitation of hydrates of oxides of the dissolving metals), was chosen as the electrolyte.

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

Our experiments showed that, when working at high current densities of \(1.2\ \mathrm{A/cm^2}\) without cooling, strong heating of the specimen occurs. If an electrolyte containing hydrochloric acid is used, then with a considerable rise in the temperature in the bath (the temperature of the electrolyte in the bag reached \(60\text{–}80^\circ\mathrm{C}\)), partial destruction of unstable carbides is possible in the presence of hydrochloric acid. This occurred in our work at high current densities and without cooling. The results of analysis of the carbide phase were variable. It was not possible to note any regularity in the change of the chemical composition of carbides under the influence of modification. Therefore we settled on the following variant: the composition of the electrolyte was a saturated solution of potassium chloride with \(0.5\%\) citric acid. The current was direct, from storage batteries. The current density was \(0.2\text{–}0.3\) ampere per \(\mathrm{cm^2}\). The duration of anodic dissolution was from 1 to 2 hours. Dissolution was carried out with continuous cooling by running water. The cathode was a copper coil for cooling the electrolyte. The temperature of the water passing through the cooling coil was \(+2^\circ\mathrm{C}\); the temperature of the electrolyte in the bag near the specimen was \(10\text{–}12^\circ\mathrm{C}\).

To determine the ratio of the weight of carbides to the total weight of the 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.

(preliminarily, the Schott filter was 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 \(105^\circ\mathrm{C}\), again bringing it to constant weight. The difference in weight gave the weight amount of the carbide precipitate. The difference in the weight of the sample gave the amount of metal that had dissolved. From this it is not difficult to determine the weight amount of carbides in the alloy.

The change in the composition of the carbide phase with respect to carbon and chromium under the influence of modification is shown in Table 2.

Table 2

Change in the composition of the carbide phase under the influence of modification with boron

Conclusion

Under the influence of boron, the amount of carbon and chromium in the carbide phase decreases.

1. N. A. Saverina, “Method for Separating Carbides and Nitrides from Stainless Steels,” Zavodskaya laboratoriya, 1952, No. 8. ↩