CARBON IN HIGH-CHROMIUM LOW-ALLOY CAST HIGH-SPEED STEEL

N. A. EROFEEV

(Presented by Professor Doctor A. N. Dobrovidov)

The properties of steel products are determined by the composition, nature, quantitative ratio, and distribution of phases. These factors change during alloying or heat treatment of steel. A large number of works have been devoted to establishing the influence of quenching temperature, cooling conditions, and various types of tempering on the nature and alloying of forged and cast high-speed steel.

A. P. Gulyaev [1] established the dependence of the phase composition on the quenching temperature of forged steels R18, R9, and R4. I. S. Kvater [2] studied these same questions as applied to cast high-speed steel of standard and modified composition. E. P. Starova [3] investigated the dependence of the alloying of the solid solution on the quenching temperature of low-alloy high-speed steel with nitrogen.

I. I. Gornov [4] clarified the influence of the cooling rate on the distribution of alloying elements among the phases of cast quenched steel R18.

There are comparatively few works in which the dependence of the indicated factors on the carbon content has been investigated, although carbon is the cheapest and at the same time a very important component of high-speed steel.

Solving this question is of great significance in the search for new steels for tools and machine parts.

The present work was carried out in order to establish the influence of the carbon content in steel on the alloying of phases in the quenched, tempered, and annealed states. The investigation was conducted on three grades of high-chromium low-alloy cast high-speed steel with different carbon contents (Table 1).

Table 1

Chemical composition of the steels studied and their hardness in the cast (quenched) state

Research Methods and Discussion of Results

The heats were carried out in a high-frequency induction furnace. After completion of melting, the steel was deoxidized with aluminum, introduced in an amount of 0.1%. The weight of each heat was 1 kg. The mold was poured with steel directly from the crucible at a metal temperature of 1500–1540°C, monitored with an optical pyrometer. Owing to the rapid cooling during casting into a metal mold (chill mold), specimens of small cross section were hardened.

The steels investigated were subjected to metallographic and carbide analyses in the hardened (as-cast), triply tempered at 560°, and annealed states. In addition, successive tempering in the temperature range 100–700°C and multiple tempering at 560°C were carried out.

The structure of all the investigated steels in the hardened state consists of a carbide constituent in the form of a fine network, residual austenite, and cryptocrystalline martensite. All steels in this state are poorly etched by a 10% solution of nitric acid in alcohol, which indicates the high alloying of the metallic matrix with chromium, the principal alloying element in them.

Good results in differentiating the phases, their distribution, and revealing intracrystalline heterogeneity were obtained by applying color oxidation of polished sections, carried out at 400–420° for 10–15 min.

To establish the dependence of etchability solely on the nature and composition of the phases, all polished sections were etched simultaneously.

The clearest difference in the degree of oxidation of parts of the grain on passing from its center to its boundaries is observed in hardened steel 1.0–4–12–2 (Fig. 1). The greater concentration of chromium near the grain boundaries is indicated by the change in the color of the oxide film from blue in the center to light yellow at the boundary. The difference in the degree of oxidation decreases as the carbon content in the steel increases, although the alloying of the solid solution also decreases in this case (Table 2). The latter is clearly visible in the structure of steel 2.0–4–12–2 (Fig. 2) and is explained,

Fig. 1. Microstructure of steel 1.0–4–12–2 in the hardened state. ×660.

apparently, a more uniform distribution of chromium within the grain under the action of carbon. This result is in agreement with the experimental data of A. S. Zavyalov and B. I. Bruk [5].

The use of color oxidation of polished sections made it possible to establish the phase composition of ledeburite. In all the steels it consists of a blue metallic matrix, which indicates its considerable enrichment with chromium; light chromium carbides; brown vanadium carbides; and light-yellow carbides of tungsten with iron.

Fig. 2. Microstructure of steel 2.0—4—12—2 in the hardened condition. ×660.

Thus, the use of color oxidation made it possible to determine the influence of carbon on intracrystalline liquation and to determine the phase composition of the primary carbides.

The specimens for carbide extraction had a diameter of 10 mm and a length of 60 mm. Owing to the considerable corrosion resistance of the metallic matrix of the steels in the hardened and tempered states, the separation of carbide precipitates was carried out in a chemically active electrolyte of the following composition: 100 ml of hydrochloric acid, 10 g of thiosulfate, and 75 g of potassium chloride per 1 l of water [6]. When this electrolyte is used, uniform dissolution of the specimens occurs, with no residues of a shiny metallic film present on their surface. To isolate the carbide precipitate from annealed specimens, a one-normal solution of potassium chloride with the addition of 5 g of citric acid per liter of solution was used [7]. With both electrolytes, dissolution was carried out at a current density of 0.02–0.03 A/cm² and a temperature of 12–14°C. The dissolution time was 3–4 hours for hardened specimens and 2–3 hours for tempered and annealed specimens.

The results of determining the amount of carbides, their chemical composition, and the distribution of alloying elements between the phases of the steels studied are given in Table 2.

The course of the curves \(W_p\), \(V_p\), and \(Cr_p\) in Fig. 3 indicates that, with an increase in the carbon content in the steel, the solid solution becomes depleted in alloying elements as a result of their transfer into carbides. This also explains the continuous rise of the curve for the amount of carbides and of the content of alloying elements in them \((W_k, V_k, Cr_k)\).

Data of carbide analysis

Table 2

*) \(p\) — fraction of the element in the solid solution;
\(k\) — fraction of the element in the carbide residue.

The transfer of alloying elements into carbides is due to their greater chemical affinity for carbon than for iron. The solid solution is depleted most strongly in vanadium and tungsten and, to a lesser extent, in chromium. The smaller the affinity of an element for carbon, the

Fig. 3. Dependence of the amount of carbides, and of the alloying of the solid solution and carbides in quenched steel, on its carbon content.

less is the depletion of the solid solution in this element. This is confirmed by comparing the ratios of the concentrations of alloying elements in the carbide precipitate \((W_{\mathrm{co}}, V_{\mathrm{co}}, Cr_{\mathrm{co}})\) to their concentration in the solid solution \((W_{\mathrm{tr}}, V_{\mathrm{tr}}, Cr_{\mathrm{tr}})\). With an increase in the concentration of carbon in the steel, these ratios increase after quenching and after tempering (Table 3).

Table 3

Effect of carbon content on the ratios of the concentrations of alloying elements in the carbide precipitate to their concentrations in the solid solution

Continuation of the table

*) co — concentration of the alloying element in the carbide precipitate,
ss — concentration of the alloying element in the solid solution.

These ratios also show that the solubility in the solid solution of chromium, which has a lower affinity for carbon than tungsten and vanadium, is greater than the solubility of tungsten and vanadium. Thus, the concentration of chromium in the solid solution after quenching is, on average, 2.5–4 times lower, and after tempering 4.5 times lower, than its concentration in the carbide precipitate. The content of tungsten and vanadium in the solid solution after quenching and after tempering is four or more times lower than in the carbides. In the annealed state the solubility in the solid solution of chromium, tungsten, and vanadium is approximately the same. Analysis of the carbide precipitate of the annealed steels for carbon showed that all of it is contained in the carbides (Table 2).

The concentration ratios given, as well as the distribution coefficients (Table 2), characterize the effect of the carbon content in the steel on the redistribution of alloying elements between the phases during tempering. Comparison of the indicated ratios for the quenched state and after tempering shows that, as the carbon content in the steel increases, the degree of depletion of the solid solution in alloying elements decreases. This is clearly seen if one compares the distribution coefficients of alloying elements for steels 1,0—4—12—2 and 2,0—4—12—2 (Table 2). The distribution coefficients of the first steel decreased for tungsten from 3,2 to 0,9, for chromium from 5,8 to 1,5, and for vanadium from 3,6 to 1,1. These same coefficients for steel 2,0—4—12—2 after tempering and before it do not differ from one another. This is also confirmed by the fact that, with increasing carbon content in the steel, the difference between the concentration of the alloying element in the quenched solid solution and in the solid solution after tempering decreases (Fig. 4). In parallel with this, the amount of secondary carbides after tempering also decreases.

The decrease in the degree of depletion of the solid solution in alloying elements after triple tempering at 560° proves an increase in the stability of retained austenite, while the decrease in the hardness of the steel after quenching (Table 1) indicates an increase in its amount when carbon is introduced into the steel.

The increase in the amount of retained austenite is associated with a lowering of the temperature at which the martensitic transformation begins under the action of carbon dissolved in the solid solution. In this case, the effect of the additionally dissolved carbon in austenite on lowering the temperature at which the martensitic transformation begins is more effective than its increase due to the total decrease in the concentration of all alloying elements in the austenite. For example, the total decrease in the alloying of the austenite of steel 2,0—4—12—2 in comparison with steel 1,0—4—12—2 is 1,05% W + 2,95% Cr + 0,73% V = 4,73%. This should raise the temperature at which the martensitic transformation of steel 2,0—4—12—2 begins. However, increasing the carbon content in steel 1,0—4—12—2 by

0.82% (steel 2.0—4—12—2), part of which dissolves in austenite, lowered the temperature of the onset of the martensitic transformation in comparison with its initial position.

Fig. 4. Degree of enrichment of the solid solution with alloying elements during tempering as a function of the carbon content in the steel.

The increase in the stability of retained austenite can be explained by the direct and indirect action of carbon. The direct action may be associated with the fact that, being in solid solution, carbon retards the diffusion processes accompanying the precipitation of secondary carbides during tempering. The latter is possible only with an increase in the interatomic bonding forces in the austenite lattice. The indirect action of carbon may be attributed to a decrease in the amount of martensite, which promotes more rapid decomposition of austenite [8]. It is believed that the greater the amount of martensite in the steel, the faster the retained austenite decomposes. Thus, carbon is, on the whole, responsible for increasing the amount and improving the stability of retained austenite. Both of the indicated characteristics are directly dependent on the concentration of carbon in austenite.

The change in hardness during multiple tempering (Table 4), the magnitude of the secondary and maximum hardness and the corresponding temperature during successive tempering (Table 5) confirm the results of phase analysis. These data also show that, with an increase in the carbon content in the steel and, correspondingly, in the quenched solid solution, the stability and amount of retained austenite increase. For example, the hardness of steel 2.0—4—12—2 after triple

Table 4

Change in steel hardness at a tempering temperature of 560°

Table 5

Magnitude of the secondary and maximum hardness and the corresponding temperature during successive tempering of steels

tempering at 560° did not change in comparison with the hardened state; therefore, the distribution coefficients of the alloying elements also changed very little. The same tempering of steels (1.0—1.5)—4—12—2 led to considerable structural changes—in both steels the hardness reached a maximum value—and therefore a substantial redistribution of the alloying elements occurred (Fig. 4).

Conclusions

The experimental data obtained in the present work indicate the following regularities:

1. An increase in the carbon content in steel leads to continuous depletion of the solid solution in tungsten, chromium, and vanadium after hardening and after annealing. At the same time, the amount of carbides and the content of alloying elements in them continuously increase.

2. The increase in the amount and in the stability of retained austenite is directly dependent on the carbon concentration in it.

3. During redistribution of the alloying elements in the course of triple tempering at 560°, the solid solution (austenite) with the lowest carbon concentration in it is depleted of alloying elements to the greatest extent.

REFERENCES

1. A. P. Gulyaev. Steel, No. 3, 1946.
2. I. S. Kvater. Cast High-Speed Steels. Mashgiz, 1952.
3. E. P. Starova. Low-Alloy High-Speed Steels with Nitrogen. Mashgiz, 1954.
4. I. I. Goryunov. Casting of tools in metal molds. Cast and Surfaced Tools. Collection of articles, Mashgiz, 1951.
5. A. S. Zavyalov and B. I. Bruk. Physics of Metals and Metallography, vol. 8, issue 3, 1959.
6. N. F. Lashko, N. I. Eremin. Phase Analysis and Structure of Austenitic Steels. Mashgiz, 1957.
7. N. M. Popova. Carbide Analysis. Mashgiz, 1949.
8. P. M. Yushkevich. Journal of Technical Physics, vol. 24, issue 4, 1954.