Carbon, Vanadium, Tungsten, and Chromium in High-Speed Steel

A. N. Dobrovidov, N. A. Erofeev

(Presented by Professor A. N. Dobrovidov)

For a rational approach to questions of alloying, it is necessary to know the regularities of the distribution of carbon and alloying elements among the phases of steel in the quenched and tempered states. The determination of the influence of the quenching temperature, cooling conditions

Table 1

Chemical composition of the steels investigated

and the various types of tempering on the alloying and the nature of the phases in forged and cast high-speed steel [1–8]. However, there are clearly not enough works in which the dependence of the composition, nature, quantitative ratio, and distribution of phases on the content of any one element entering into the composition of high-speed steel has been studied, despite the fact that solving this question is of great importance in the search for new steels for tools and machine parts, and that this question is fundamental in the theory of alloying.

We have carried out an investigation of the dependence of the alloying of the solid solution, the amount of carbides, their phase composition, and certain properties of steel on the content in it of tungsten, vanadium, chromium, and carbon, using specimens of twenty-one grades of steel with different contents of tungsten, vanadium, chromium, and carbon (Table 1).

The steel heats were made in a high-frequency induction furnace. The weight of a heat was 1 kg. Glass was used as the flux. After the completion of melting, the steel was deoxidized with aluminum and silicocalcium, added in an amount of 0.1% (of each). The steel was poured into the mold directly from the crucible at a temperature of 1500–1540°C, controlled by an optical pyrometer. Casting was carried out into a sectional metal mold. The cast specimens were cooled with the mold to room temperature. The dimensions of the specimens for carbide analysis were: diameter 10 mm, length 80 mm. Owing to rapid cooling during casting, the small-section specimens were quenched.

Method for isolating the carbide precipitate

To isolate carbides from the steels studied, the electrolytic method developed by N. M. Popova [9] was used. Before dissolution, the specimen was cleaned of the scale layer with an emery wheel. Instead of the copper or iron plates usually used as the cathode, a coil made of a copper tube 4 mm in diameter was used, which simultaneously served both as the cathode and as the cooler. The specimen, in a bag of parchment paper, was lowered into the electrolyte so that its upper part was above the electrolyte level. Dissolution was carried out simultaneously in 10 baths connected in series. The electrolyte was a normal solution of potassium chloride with the addition of 5 g of citric acid per liter of solution. The current density during electrolysis was 0.03–0.05 A/cm²; the temperature of the electrolyte during dissolution was 12–14°C. The duration of electrolysis was 4–5 hours for quenched specimens and 2–3 hours for tempered specimens. When this regime was observed, the carbide precipitate remained on the specimen in the form of a firmly retained thin layer.

After the completion of electrolysis, the specimens were quickly transferred into hot (80–70°C) water to remove adsorbed gases. Treatment with hot water immediately after electrolysis is an addition to the usual procedure for carrying out carbide analysis. The absorption of gas by the carbide precipitate is associated with the presence in it of free carbon, which is liberated from the solid solution during electrolysis. Amorphous carbon behaves like strongly activated charcoal and adsorbs the gases evolved at this time [10].

After treatment with hot water, the precipitate was carefully removed from the specimen and filtered through a Gooch crucible, followed by washing. Since the tungsten of the metallic matrix is oxidized during electrolysis, the precipitate of steels containing tungsten was additionally washed

with a 10% sodium hydroxide solution. The filtered precipitate together with the crucible was dried in a thermostat at a temperature of 90–100°C to constant weight.

The precipitate obtained and treated in this way was subjected to chemical analysis for alloying elements—tungsten, chromium, and vanadium. The amount of alloying elements in the solid solution was found from the difference between their content in the steel and in the carbide precipitate. This method for determining the alloying of the solid solution was used in works [2–8]. A. P. Gulyaev [10] determines the composition of the solid solution on the basis of chemical analysis of the electrolyte into which the alloying elements pass from the solid solution during electrolysis. However, in the opinion of Yu. A. Klyachko et al. [11], analysis of the electrolyte does not accurately reflect the composition of the metallic phase, since the composition of the electrolyte, owing to secondary reactions occurring during electrolysis, does not correspond to the composition of the principal metallic phase. There is no consensus regarding the advisability of determining the composition of the solid solution from analysis of the electrolyte.

Because of the absence of a well-developed and sufficiently simple method for separating free carbon from combined carbon quantita-

Table 2

Concentration of alloying elements in the phases of quenched steels

*) CP — concentration of the element in the precipitate.
SS — concentration of the element in the solid solution.

Table 2a

Distribution of alloying elements between the phases of quenched steels

*) \(P\) — fraction of the element in the solid solution.
\(K\) — fraction of the element in the carbide precipitate.

The distribution of carbon between the phases of the steel was not determined in the present work.

The weight of the precipitate, referred to the difference in the weight of the specimen before and after electrolysis, gives the total percentage content of carbides in the steel. The amount of carbides was determined as the arithmetic mean of 4–5 determinations.

The isolated carbide precipitates from quenched steels were subjected to X-ray analysis by the powder method. In the precipitates of all steels containing more than 1.5% vanadium, vanadium carbide VC was detected, represented by all interference lines. The latter indicates a considerable amount of this carbide in the precipitate. In the precipitates of steels containing more than 4% tungsten and more than 1.5% vanadium, in addition to vanadium carbide VC, the double iron–tungsten carbide \(Fe_3W_3C\) is present. The intensity and number of lines of this carbide on the X-ray diffraction patterns increase with increasing tungsten content in the steel, which is associated with an increase in the amount of carbide in the precipitate. The absenc-

The absence of interference lines of chromium carbide on the radiographs, despite the presence of chromium in the precipitate, should be explained by the low sensitivity of the method [12]. In addition, chromium, apparently, is partially dissolved in the carbides found.

The results of determining the amount of carbides and the distribution of alloying elements between the solid solution and the carbide phase in quenched and tempered steel are given in Tables 2 and 3.

Carbon in high-speed steel

The results of carbide analysis of the steels studied give an idea not only of the distribution of alloying elements between the phases of the steel, but also of the distribution of carbon between them and of the effect of this distribution on the properties of the steel.

Table 3

Concentration of alloying elements in the phases of steels after triple tempering at 560°

*) KO — concentration of the element in the carbide precipitate.
TR — concentration of the element in the solid solution.

In Fig. 1 the dependence is given of the hardness, heat resistance, amount of carbides, and alloying of the solid solution of hardened steel on its carbon content. The same dependence of the indicated characteristics on the carbon content also holds for other compositions of the steels investigated (Tables 2a and 3a).

Table 3a

Distribution of alloying elements between the phases of steels after triple tempering at 560°

*) P — fraction of the element in the solid solution.
K — fraction of the element in the carbide precipitate.

The course of the curves \(W_{\mathrm{p}}\), \(V_{\mathrm{p}}\), and \(Cr_{\mathrm{p}}\) in Fig. 1 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 transition into carbides. This also explains the continuous rise of the curve for the amount of carbides. The transition 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 lower the affinity of an element for carbon, the less the solid solution is depleted in this element. This is confirmed by comparing the ratios of the concentrations of alloying elements in the carbide precipitate (\(W_{\mathrm{k}}\), \(V_{\mathrm{k}}\), \(Cr_{\mathrm{k}}\)) to their concentration in the solid solution (\(W_{\mathrm{tr}}\), \(V_{\mathrm{tr}}\), \(Cr_{\mathrm{tr}}\)). With an increase in the carbon content,

of carbon in the steel, increase both after quenching and after triple tempering at 560° (Table 3). 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 much greater than the solubility of tungsten and vanadium. Thus, the chromium concentration

Fig. 1. Dependence of hardness, heat resistance, amount of carbides, and alloying of the solid solution in quenched steel on its carbon content. Steel: 1.0–4–4–3; 1.2–4–4–3; 1.3–4–4–3

in the solid solution after quenching is on average 1.5–2.5 times lower than its concentration in the carbide precipitate. The content of tungsten and vanadium in the solid solution after quenching is seven or more times lower than in the carbides.

The ratios of concentrations given also characterize the influence of the carbon content in the steel on the redistribution of alloying elements between phases during tempering. In steels with a lower carbon content, the solid solution remains more saturated with alloying elements even after tempering. Thus, after tempering, the ratio of the concentrations of alloying elements for steel 1.0–4–4–3 increased to a lesser extent than for steel 1.3–4–4–3. For example, \(W_{\text{carb}} : W_{\text{ss}}\) for tungsten in steel 1.0–4–4–3 increased from 8.3 to 11 times, whereas for steel 1.3–4–4–3 it increased from 11 to 19 times.

The depletion of the solid solution in alloying elements in the iron–carbon–alloying element system (\(V, Ti\)) with increasing carbon content was considered by N. S. Fastov and B. N. Finkelstein [13]. On the basis of thermodynamic calculations, the authors established an inversely proportional relationship between the concentration of the alloying element in the solid solution and the carbon content in the steel. Our experimental data confirm the theoretical propositions of this study and show that an analogous dependence exists not only for ternary equilibrium systems, but also for nonequilibrium multicomponent systems with complex carbides that are solid solutions with a defective crystal lattice.

From the results of our experiments it is evident that the redistribution of alloying elements between the phases of steels with different contents

the carbon is caused not by a change in the chemical composition of the carbides, but by an increase in their amount in the steel. This follows from a comparison of the composition of the carbides both after hardening and after tempering. The latter is possible only with the same phase composition of the carbide precipitate, which is confirmed by X-ray structural analysis data.

The heat resistance of the steel increases as the carbon content is raised (Fig. 1). Since this is accompanied by a decrease in the alloying of the solid solution, the increase in heat resistance can be explained only by the greater saturation of the solid solution with carbon.

The decrease in hardness (Table 1) of the hardened steel with increasing carbon content is associated with an increase in the amount of retained austenite, which is explained by a lowering of the martensitic transformation start temperature under the action of carbon dissolved in the solid solution. At the same time, the effect of the additionally dissolved carbon in austenite on lowering the martensitic transformation start temperature is more effective than its rise due to the total decrease in the concentration of all alloying elements in austenite. For example, the overall decrease in the alloying of the austenite of steel 1.3—4—4—3 in comparison with steel 1.0—4—4—3 is \(0.84\% W + 0.50\% Cr + 0.65\% V = 1.99\%\). This should raise the martensitic transformation start temperature of steel 1.3—4—4—3. However, the increase in carbon content in steel 1.0—4—4—3 by 0.35% (steel 1.3—4—4—3), part of which dissolves in austenite, lowered the martensitic transformation start temperature in comparison with its initial position. For the very same reasons, exactly the same dependence of the hardness of hardened steel on its carbon content is also observed at other contents of alloying elements (Tables 1 and 2). With a considerable reduction in the carbon content in steel below the optimum corresponding to a given content of alloying element in it, a steel with low hardness and poor hardenability is obtained. The reason for this is the low concentration of carbon in the hardened solid solution.

The distribution of tungsten, vanadium, and chromium between the phases of hardened steel depends on the carbon content (Table 2). When its content is above 1.2%, a larger fraction of tungsten and vanadium is found in carbides and a smaller fraction in the solid solution in all the steels investigated in the present work. When the carbon content is up to 1.2%, most of these elements remains in the solid solution, which should be regarded as “favorable,” since the heat resistance of the steel and the durability of a high-speed-steel cutting tool depend on the overall saturation of the solid solution with these elements and with carbon.

The regularity obtained for the influence of increasing carbon content in steel on the distribution of alloying elements between the phases of the steel, hardness, the amount of retained austenite, and its stability is also present for forged high-speed steels in the hardened condition [6—8]. Analysis of the literature and of our experimental data shows that the higher heat resistance of high-vanadium high-speed steels (R14F4, R9F5) and the durability of tools when cutting with these steels are explained only by an increase in the concentration of carbon in the solid solution, and not by an increase in the vanadium content in martensite or by growth in the amount of vanadium carbides \(VC\) in the steel. For example, steels R9 and R9F5 differ scarcely at all in the alloying of the solid solution with tungsten, vanadium, and chromium in the hardened condition, while the durability of a tool made of steel R9F5 is higher than that of a tool made of steel R9 [6]. The amount of carbides in these steels also differs by no more than 4—6%.

The closeness of the properties of chromium to iron and its lower chemical affinity for carbon, in comparison with tungsten and vanadium, promote only a “favorable” distribution \((G_p : Cr_k > 1)\) of chromium between the phases of hardened steel. However, its distribution between the phases, like that of tungsten and vanadium, depends on the carbon content in the steel: with an increase in the carbon content in the steel, the alloying of the solid solution with chromium also decreases.

A comparison of the chemical analysis of carbide precipitates of steels quenched and tempered three times at \(560^\circ\) shows that, after tempering, the chromium content in the precipitate increases by approximately a factor of 2 in comparison with its content in the precipitate of the hardened steel. At the same time, the difference between the amounts of tungsten and vanadium is no more than 2–3%. The latter may be explained by the identical nature of the primary and secondary carbides into which tungsten and vanadium enter. The increase in the concentration of chromium in the precipitate after tempering is probably associated with the precipitation of a special chromium carbide.

The rise in chromium concentration in the carbide precipitate after tempering is explained by the greater rate of its diffusion in the solid solution than the rate of diffusion of tungsten and vanadium. The low diffusion rate of these elements is connected with their large atomic radius in comparison with the atomic radius of chromium, and also, apparently, with the greater magnitude of the interatomic bonding forces between carbon, tungsten, and vanadium in the lattice of the solid solution in comparison with the bonding forces between atoms of carbon and chromium.

Vanadium in High-Speed Steel

Figure 2 shows the effect of vanadium content in steel on its distribution between the steel phases, on hardness, the amount of carbides, and

Fig. 2. Dependence of hardness, heat resistance, amount of carbides, and distribution of vanadium between the phases of hardened steel on the vanadium content in it. Steel: 1.2-4-4-1.5; 1.2-4-4-3; 1.2-4-4-4

the heat resistance of the steel after quenching. The curves of vanadium distribution between the solid solution and carbides intersect at a content of 3.5% vanadium in the steel. With a smaller amount of vanadium, its curve...

the content in the solid solution lies above the curve of its content in the carbide phase. This proves that, when vanadium is present in steel up to 3.5%, the greater part of it goes into the solid solution, and the lesser part into the formation of carbides. Such a distribution of the alloying element may be called “favorable” in the sense that, under the stated conditions, the heat resistance of hardened steel increases. When the vanadium content in steel is above 3.5%, the greater part of it goes into the formation of carbides, and the lesser part dissolves in the solid solution. Such a distribution of the alloying element between the phases of the steel will be “unfavorable.”

If the portion of vanadium in the solid solution is denoted by \(V_p\), and that in carbides by \(V_k\), then at a vanadium content in steel of up to 3.5% the ratio is \(V_p : V_k > 1\); at 3.5%, \(V_p : V_k = 1\); and above 3.5%, \(V_p : V_k < 1\). The data of Table 2 show that, with an increase in the vanadium content in steel up to 3.0%, a proportional distribution of it between carbides and the solid solution occurs. With a further increase in the vanadium content in steel, the proportionality in the distribution is disturbed: a larger share of it goes into the carbides. For example, the solid solutions of steels 1.2—4—4—3 and 1.2—4—4—4 do not differ in vanadium content, whereas in the carbides of the second steel there is twice as much of it as in the first. The same regularity in the distribution of vanadium between the phases of hardened steel as a function of its content in the steel is also observed for the other steels investigated in the present work, and in forged high-speed steels [5—8]. It is interesting to note that in all steels of this work and in the steels investigated in works [5—8], the vanadium content in the solid solution of hardened steel does not exceed 2.5%, although its content in the steel increases to 5%. This amount of vanadium apparently determines its maximum solubility in the austenite of high-speed steels. The remaining part of the vanadium goes to the formation of excess vanadium carbides VC, which do not increase the heat resistance of the steel, while the solid solution becomes strongly depleted in carbon. At a certain vanadium content in the steel, in the region \(V_p : V_k < 1\), such a strong depletion of the solid solution in carbon will occur that the \(\gamma\)-region will be completely wedged out. Under this condition the structure of the steel will be formed by alloyed solid solution \(\alpha\) and the carbide phase. This also explains the worsening of the hardenability of forged high-speed steel if the vanadium content in it is increased without increasing the carbon content, as well as the high carbon content in high-vanadium high-speed steels, and also the experimentally established relationship between the concentration of vanadium and carbon in high-speed steel for obtaining high secondary hardness, heat resistance, and cutting properties [5—8].

The amount of vanadium required for its equal distribution \((V_p : V_k = 1)\) between the solid solution and carbides depends on the carbon content in the steel. The less carbon there is in the steel, the more vanadium will dissolve in the solid solution and the less will pass into carbides. If two steels with lower and higher carbon contents are taken and equal amounts of vanadium are introduced into them, then the ratio \(V_p : V_k\) for the first steel will be greater than for the second. This is clearly seen in the example of steels 1.2—4—8—2 and 1.0—4—8—2: for the first steel \(V_p : V_k = 2.4\), for the second \(= 3.1\). Therefore the curves \(V_p\) and \(V_k\) for the steel with the higher carbon content will intersect at a lower vanadium content in it. This dependence is shown schematically in Fig. 3. Exactly the same regularity in the distribution of vanadium between the phases of hardened steel as a function of the carbon and vanadium contents in it was obtained for other steels of the present work and in the works of Yu. A. Geller [7] and A. G. Ivanov [6].

Thus, the amount of vanadium required for equal distribution between the solid solution and the carbides is inversely proportional to the carbon content in the steel. Proceeding from such a dependence, on the basis of our own data and the data in the literature [6—8], one can substantiate the content of carbon and vanadium in all existing high-speed steels and choose ratios between these components in new high-speed steels with high secondary hardness, heat resistance, and cutting durability. This dependence is shown in Fig. 4, in which the dashed straight line corresponds to an approximately equal distribution of vanadium between the phases of the hardened steel. The solid straight line indicates the ratios between the carbon and vanadium contents necessary to obtain high hardness and heat resistance of high-speed steel [8, 17]. These straight lines divide the entire area into four regions, differing in the ratios \(V_p : V_k\), and, consequently, in the properties that the steel acquires at these ratios. Below the first and above the second region are located ratios between the carbon and vanadium contents in high-speed steel at which low heat resistance and cutting durability are obtained. In the first case, the decrease in the properties of the steel is explained by the reduced vanadium content in the steel and in the solid solution. In the second case, it is due to considerable depletion of the solid solution in carbon as a result of the formation of a large amount of carbon-rich vanadium carbides. Above the second region are located ratios between vanadium and carbon at which poorly hardenable steels are obtained. With an increase in carbon content and a constant amount of vanadium in the steel, within the limits of the third region the heat resistance of the steel increases. In the fourth region are located ratios between the carbon and vanadium contents at which high-speed steel has high heat resistance. However, for all ratios between these components in steels located in the third and fourth regions, the ratio \(V_p : V_k < 1\); it decreases—

Fig. 3. Scheme of the distribution of a carbide-forming alloying element between the solid solution and carbides of hardened steel as a function of the carbon content in it.
——— steel with a lower carbon content;
— — — steel with a higher carbon content.

increases as the carbon content in the steel increases. The relationships between the vanadium and carbon contents in forged high-vanadium high-speed steels R9F5 and R14F4 are located in the third and fourth regions. For these steels the ratio \(V_p : V_k < 1\) [6—8]. It is equal to 0.6 for the first steel, and 0.9 for the second. The reason for the difference is the higher content of carbon and vanadium in the first steel than in the second.

The increase in heat resistance, the increase in the amount of retained austenite and the increase in its stability in high-vanadium high-speed steels [8, 14], at the ratio \(V_p : V_k < 1\) and a high content (1.2—

Fig. 4. Distribution of vanadium between the phases of quenched high-speed steel as a function of the vanadium and carbon contents in it

1.5%) of carbon in them, is explained by the high concentration of carbon in the solid solution after quenching. This indicates the special role of carbon in increasing the heat resistance of such steels, since the overall alloying of their solid solution does not differ from the alloying of the solid solution of steel R9.

Thus, using Fig. 4, one can approximately select the relationships between the carbon and vanadium contents in high-speed steels at which high heat resistance and cutting-tool life are obtained, and also determine the role of each of these components in changing one or another characteristic of the steel. It should be noted that, as applied to forged high-speed steels, use of the established relationship will be the more accurate, the higher

the quenching temperature and prolonged holding at it [6–8]. From this graph one can also substantiate the deterioration of the grindability of steel when vanadium and carbon are introduced into it. The smaller the ratio \(V_p : V_k\), the more vanadium carbides \(VC\) there are in the steel and, consequently, the worse the grindability of the steel because of the high hardness of this carbide.

The region of “favorable” distribution of vanadium between the phases also contains its optimum content in the steel, corresponding to maximum red hardness. In steels 1.2—4—4—1.5, 1.2—4—4—3 and in steel with 1.2% C, 4% W, 4% Cr and 4.5% V, the optimum vanadium content is 3.0%. Further addition of vanadium to the steel lowers red hardness owing to intensive depletion of the solid solution in carbon.

The curves of the change in red hardness and hardness of the steel show that hardness increases with increasing vanadium content up to 4.5%, while red hardness increases up to 3.0%. The reason for the continuous increase in hardness and for the inflection on the red-hardness curve lies in the depletion of austenite in carbon when vanadium is added to the steel. The latter leads to an increase in the temperature of the beginning of the martensitic transformation and, consequently, to a decrease in the amount of retained austenite and an increase in the hardness of the steel. If a steel with a vanadium content above 4.5% is taken, then at the given carbon content a poorly hardening steel will be obtained, and the hardness, like the red hardness, will also decrease. The same applies to steels R14F4 and R9F5 if, at the same carbon content, vanadium is introduced into them above the permissible amount. But in these steels the decrease in all characteristics is explained by the formation of a large quantity of vanadium carbides, which are difficult to dissolve when heated for quenching.

When vanadium is introduced into steel, the alloying of the quenched solid solution with chromium increases, while that with tungsten remains almost unchanged. For example, for steel 1.2—4—4—1.5, \(Cr_p : Cr_k = 6\) and \(W_p : W_k = 1.4\), and for steel 1.2—4—4—4 these ratios are 8 and 1.3, respectively. The appreciable enrichment of the solid solution with chromium is explained by the law of carbide formation: instead of chromium carbides, vanadium carbide is formed, since vanadium has a greater chemical affinity for carbon than for chromium. The almost identical carbide-forming ability of tungsten and vanadium did not cause any noticeable redistribution of tungsten between the phases of the steel when vanadium was introduced into it. The noted redistributions of chromium are confirmed by a decrease in the amount of primary carbides, which is associated with the formation of a vanadium carbide that is lighter in weight and richer in carbon instead of the heavier, carbon-poor carbides of chromium and iron.

The total amount of carbides and the amount of secondary carbides after triple tempering at 560° C decrease when vanadium is introduced into the steel (Table 2). This also indicates an increase in the degree of depletion of the solid solution in carbon.

Thus, the experimental data show that, with an increase in the vanadium content in the steel, carbon and chromium are redistributed between the phases of the steel—the solid solution is depleted in carbon and enriched in chromium. The same effect on the composition of the quenched solid solution is produced by the introduction of vanadium into the forged steels R9 and R18 [6].

The invariability of the alloying of the solid solution with tungsten and the slight increase in the concentration of chromium in it upon the addition of vanadium to the steel provide grounds for using Fig. 4 to select the ratios between vanadium and carbon and to explain the influence of these components on the properties of the steel independently of the presence or absence of tungsten and chromium in it.

Tungsten in High-Speed Steel

Fig. 5 illustrates the effect of the tungsten content in steel on its distribution between phases, on hardness, the amount of carbides, and its heat resistance after quenching.

An increase in the tungsten content in steel leads to a continuous increase in its amount both in the solid solution and in the carbides. The curve corresponding to the fraction of tungsten (\(W_p\)) in the solid solution is located above the curve corresponding to the fraction of tungsten (\(W_k\)) in the carbide phase. The position and course of these curves indicate

Fig. 5. Dependence of hardness, heat resistance, amount of carbides, and the distribution of tungsten between the phases of quenched steel on its tungsten content. Steels: 1.2–2–4–2.5; 1.2–6–4–2.5; 1.2–8–4–2.5

a “favorable” distribution of tungsten between the phases of quenched steel. For all three steels \(W_p : W_k > 1\). However, as the tungsten content in the steel increases, this ratio approaches unity. For example, for steel 1.2—2—2—2.5, \(W_p : W_k = 2\), while for steel 1.2—8—4—2.5, \(W_p : W_k = 1.2\). With further addition of tungsten to the steel, an equal distribution (\(W_p : W_k = 1\)) of it between the phases of quenched steel is obtained. This corresponds to a tungsten content in the steel of approximately 12–13%, obtained by extending the curves \(W_p\) and \(W_k\) to their intersection. Exactly the same dependence of the distribution of tungsten between the phases of quenched steel on its tungsten content is obtained from the data of work [16]. The compositions of the steels investigated in work [16], the composition of the phases with respect to tungsten, and its distribution between the solid solution and the carbides are given in Table 4. However, the intersection of the curves \(W_p\) and \(W_k\) for these steels is obtained at 15–16% tungsten in the steel. The two examples presented clearly confirm the correctness of Fig. 3 and show that the amount of tungsten necessary for its equal distribution between the solid solution and the carbides, for the same reasons as for vanadium, is inversely proportional to the carbon content in the steel.

With tungsten contents in high-speed steel of more than 12–13% (at 1.2% C) and more than 15–16% (at 0.8% C), an unfavorable distribution of it among the phases of the hardened steel will be obtained \((W_{\mathrm{p}} : W_{\mathrm{k}} < 1)\). With such amounts of tungsten in the steel, most of it will go into the formation of primary carbides, which do not increase the heat resistance of the steel and intensify its carbide heterogeneity. The solid solution will thereby be depleted in carbon. With a considerable content in the steel

Table 4

of tungsten, as also of vanadium, and at unchanged carbon content, a poorly hardening or completely non-hardening steel will be obtained. In order to preserve the high heat resistance of such steels, it is necessary, in parallel with increasing the amount of tungsten, to increase the carbon content in the steel. But in such steels heat resistance will be ensured by saturation of the solid solution with carbon, and not by tungsten, which in considerable amounts will pass into carbides. The noted regularity in the influence of the tungsten content in high-speed steel on its properties and on the distribution of carbon and tungsten between the steel phases in the hardened state explains the content of these elements in steel R14F4. But the composition of this steel was selected on the basis of experimental data on heat resistance and endurance tests of the tool in cutting, and not on knowledge of the regularities of distribution of alloying elements and carbon among the steel phases.

Thus, when increasing the carbon content in high-speed steels R18 and R9, etc., it is necessary to reduce the amount of tungsten, which will lead to its rational use, to a reduction in carbide heterogeneity, and to a reduction in the brittleness of the cutting edge of the tool. Thus, for example, in work [6] a steel of composition 1.02% C, 25.0% W, 4.17% Cr, and 1.08% V was investigated. In determining the distribution of tungsten between the phases of the steel hardened from 1300°, it was established that the solid solution contains only 7.75% tungsten. The remaining tungsten, in the amount of 17.25%, remained in excess carbides. The alloying level of the solid solution of this steel remained at the level of the alloying of the solid solution of steel R18, while twice as much tungsten went into the formation of primary carbides. The latter will undoubtedly intensify the carbide heterogeneity of the steel and reduce its strength. As a consequence of what has been said, the cutting properties of tools made of R18 steel are not improved by adding tungsten to it [18, 20].

The investigation carried out shows that the most economical high-speed steel with respect to the use of tungsten, at 0.8% C in it and with optimal properties, is steel R16, not R18; the alloying of its solid solution with tungsten is also approximately 8%. At equal heat resistance of steels R18 and R16, the latter will have

lower carbide heterogeneity and, consequently, higher strength in the heat-treated state compared with the first. The P12 grade steel developed by Yu. A. Geller and A. S. Kremnev [16] is not sufficiently alloyed with tungsten. In the solid solution of this steel after quenching there is 7% W. Increasing the tungsten content in the steel to 16% (P16 steel) will increase the concentration of tungsten in the solid solution to 8%, which will improve the heat resistance and cutting properties of tools made from this steel.

On the basis of our data and the literature data [6, 7, 15, 16], the same dependence as for vanadium is obtained for the distribution of tungsten between the phases of quenched steel and the content of carbon and tungsten in it. This dependence is shown in Fig. 6, in which the dashed straight line corresponds to an approximately equal distribution \((W_p : W_k = 1)\) of tungsten between the solid solution and the carbides, while the solid straight line corresponds to the optimal ratio between the tungsten and carbon contents in tungsten steel for obtaining high hardness [17]. These straight lines divide the entire area into four regions, differing in the ratios \(W_p : W_k\), and, consequently, in the properties that the steel acquires at these ratios.

Fig. 6. Distribution of tungsten between the phases of quenched high-speed steel as a function of the tungsten and carbon contents in it

At the bottom of the first region are ratios \(W_p : W_k > 1\), at which a solid solution low-alloyed with tungsten is obtained. Therefore the steel will have low heat resistance. Such an amount of tungsten is contained in low-alloy tool steels, for example KhVG, KhV5, etc. With a high carbon content and the same amounts of tungsten in this region, the brittleness of the steel increases because of the large amount of carbides. At ratios \(W_p : W_k > 1\), corresponding to a low carbon content, in the second region, also

steels with low heat resistance are obtained, but for another reason—the insufficient saturation of the solid solution with carbon. An example of such a steel is steel 3Kh2V8 for hot-heading dies.

In the third and fourth regions are steels for which the ratios \(W_p : W_k < 1\). Their high heat resistance, despite the unfavorable distribution of tungsten between the phases of the hardened steel, is ensured by sufficient saturation of the solid solution with carbon. Both the concentration of carbon in the solid solution and its role in increasing heat resistance grow with increasing carbon content in the steel. But at a high carbon content in a high-speed steel, the tool will have low cutting durability because of high brittleness.

Thus, the most heat-resistant high-speed steels with respect to tungsten and carbon content are located near the solid straight line, and with respect to rational utilization of tungsten—on the dashed straight line or close to it.

The degree of depletion of the solid solution in carbon when tungsten and vanadium are introduced into steel is different, since the carbide-forming ability and the carbon content in the carbides of these elements are different. For example, carbide \(\mathrm{Fe_3W_3C}\) contains 11 times less carbon than vanadium carbide \(VC\). The indicated difference between tungsten and vanadium and their different solubility in austenite at a given carbon content in it explain the different amount of these elements required for equal distribution of them between the phases of the steel.

The points on the straight lines of equal distribution of tungsten and vanadium between the phases of hardened steel (Figs. 4 and 6) determine the maximum solubility of these components in the solid solution as a function of the carbon content in high-speed steel. This solubility is equal to half the content of tungsten or vanadium in the steel, determined by the position of the points on the lines of equal distribution of these components. The course of these straight lines shows that the maximum solubility of tungsten and vanadium in austenite decreases as the carbon content in the steel increases, and consequently so does its concentration in the solid solution. Moreover, the value of the maximum solubility of tungsten and vanadium in the austenite of high-speed steel is close to their maximum solubility [7] in the austenite of ternary systems iron–carbon–alloying element \((W, V)\). In regions with ratios \(W_p : W_k > 1\) or \(V_p : V_k > 1\) there are relationships between the content of tungsten and carbon or vanadium and carbon in high-speed steel for which the solid solution in the hardened state is undersaturated with tungsten and vanadium. Above the lines of equal distribution of these components are relationships between the content of tungsten and carbon or vanadium and carbon in the steel at which the austenite is saturated with tungsten and vanadium.

From the slope of the straight lines of equal distribution one can determine the relationships between the content of vanadium and carbon or tungsten and carbon in high-speed steel for preserving the ratios \(W_p : W_k = 1\) or \(V_p : V_k = 1\). It follows from Fig. 4 that when the vanadium content in the steel is increased or decreased by 1%, the carbon content must be increased or decreased by 0.2%. This same ratio was selected experimentally in order to preserve high heat resistance in steels R18 and R9 if vanadium and carbon are introduced into them above the average content [7]. Fig. 6 shows that when tungsten in high-speed steel is increased or decreased by 1%, the carbon content must be changed by 0.1%, respectively. The difference in the amount of carbon that must be introduced into steels when the content of tungsten or vanadium in them is increased by 1% is explained by the different carbide-

forming ability of these components and their solubility in the austenite of high-speed steel.

The alloying of the solid solution with vanadium and chromium decreases when tungsten is added to the steel (Table 2). The observed redistribution of these components between the carbide phase and the solid solution should be explained by their considerable dissolution in the carbide $\mathrm{Fe}_3\mathrm{W}_3\mathrm{C}$, the amount of which increases continuously when tungsten is introduced into the steel.

The continuous increase in the hardness of the steel (Table 1) after quenching, when tungsten is introduced into it, is due to a decrease in the amount of retained austenite as a result of an increase in the temperature at which the martensitic transformation begins; this can be caused only by depletion of the austenite in carbon. The increase in this temperature due to depletion of the austenite in carbon is more effective than its decrease due to dissolution of tungsten in the austenite. The decrease in the carbon content in the austenite explains the continuous increase in the amount of primary carbides when tungsten is added to the steel. The total amount of carbides and the amount of secondary carbides after tempering also increase continuously (Table 3).

Chromium in High-Speed Steel

To study the effect of chromium on the alloying of phases, steels with a high chromium content were cast: $1.0—4—8—2$, $1.0—4—12—2$, and $1.2—4—8—2$, which differ from one another in carbon content, and from the other steels in chromium content. The redistribution of chromium between the phases of quenched steel is indicated by the change in the ratio $\mathrm{Cr}_p:\mathrm{Cr}_k$. For example, for steel with 1.2% C, 4% W, 4% Cr, and 1.5% V, and for steels $1.0—4—8—2$, $1.2—4—8—2$, and $1.0—4—12—2$, it is equal to 6.0; 7.0; 4.0 and 5.8, respectively. Consequently, the more chromium there is in the steel, the more of it passes into the carbides, and at some chromium content in the steel an equal ($\mathrm{Cr}_p:\mathrm{Cr}_k = 1$) distribution of it between the carbides and the solid solution will be obtained. This amount of chromium also depends on the carbon content in the steel. The lower the carbon content in the steel, the more chromium must be introduced into it for an equal distribution of it between the carbides and the solid solution.

The change in the ratios $\mathrm{W}_p:\mathrm{W}_k$, $\mathrm{V}_p:\mathrm{V}_k$, and $\mathrm{Cr}_p:\mathrm{Cr}_k$ with an increase in the content in the steel of one of these elements indicates that equal distribution of chromium between the phases of quenched steel will occur at a considerably higher content of chromium than of tungsten and vanadium. This amount of the alloying element is inversely proportional to its carbide-forming ability and to the carbon content in the steel.

If the ratios $\mathrm{W}_p:\mathrm{W}_k$ and $\mathrm{V}_p:\mathrm{V}_k$ are compared for steels $1.0—2—8—2$, $1.0—4—12—2$, and $1.2—4—8—2$, as well as for other steels, it is evident that the alloying of the solid solution with tungsten and vanadium increases when chromium is introduced into the steel and decreases when the carbon content is increased (Table 2). The latter is confirmed by the data of work [16], in which new high-speed steels with different chromium contents were studied. In determining the distribution of tungsten between the phases of quenched steels investigated in work [16], it was found that the solid solution of steel with 6–7% W and 4% chromium contains 5.5% tungsten, while the solid solution of steel with the same tungsten content but with 2% chromium contains only 4.5% tungsten. In the solid solution of steels with 11–13% W and 4 and 3% chromium, 6.8 and 5.5% tungsten, respectively, is dissolved.

Proceeding from our experimental data on the effect of additions

chromium to the steel on increasing the alloying of the solid solution by tungsten and vanadium, one can give another explanation of this fact in comparison with the explanation given in [16]. Since the steels studied in the present work were quenched from the highest possible temperature owing to the combination of quenching of the specimens with their casting, it does not seem possible [16] to explain the increase in the alloying of austenite by tungsten and vanadium upon addition of chromium to the steel by a decrease in the thermal stability of the carbides because of a weakening of the interatomic bonding forces in them. This can be attributed only to an increase in the solubility of tungsten and vanadium in austenite with a simultaneous increase in the concentration of chromium in it. This should be regarded as the primary cause of such a phenomenon.

With an increased chromium content in high-speed steel, the concentration of tungsten and vanadium in the alloyed ferrite of annealed high-speed steel increases. The latter is confirmed by the alloying of the ferrite of annealed steel 1.0—4—12—2, which contains 1.77% tungsten, 0.97% vanadium, and 5.10% chromium, which is much higher than the alloying of the solid solution by these elements in other steels in the tempered state (Table 3). The concentration of tungsten and vanadium also increases in the ferrite of forged high-speed steels with an increase in their chromium content [16]. This initial, higher concentration of tungsten and vanadium in the solid solution, as compared with their lower content in the alloyed ferrite at a low chromium content in the steel, determines a greater initial concentration of tungsten and vanadium in austenite, which additionally increases owing to the dissolution of secondary carbides during heating for quenching of high-speed steels.

The greater stability of retained austenite and the larger amount of it in high-chromium steels [19, 16], and, consequently, the decrease in the hardness of the quenched steel upon introduction of chromium into it (Table 1), are explained by the increased alloying of the solid solution with tungsten, vanadium, and chromium and by its smaller depletion in carbon as a result of the formation of chromium carbides poorer in carbon.

With an increase in the chromium content in high-speed steel, its red hardness decreases [16, 21]. The reason for this is the precipitation during tempering of a large amount of carbides enriched in chromium and most prone to coagulation. The latter causes intense softening of martensite and reduces the red hardness of the steel. For example, during tempering of high-chromium steels 1.0—4—8—2 and 1.0—4—12—2, the chromium content in the carbide precipitate increased by 2.3 and 3%, respectively, compared with its content in the carbide precipitate of these steels in the quenched state. In all steels with 4% chromium and the same content of the other elements, after tempering the chromium in the carbide precipitate increased by no more than 1.5% (Tables 2, 3).

Summing up the influence of the content of carbon, vanadium, tungsten, and chromium on the properties of high-speed steel, one may speak of a limiting content of each of these components at which the best mechanical, technological, and service-resistance properties are obtained. Schematically, such a dependence can be represented by a curve with a maximum (Fig. 7), the position of which depends on the nature and content of each component and on the quantitative ratio of all components in the steel. As can be seen from Fig. 7, when the content of each of these elements is greater or less than the optimum amount, determined by the position of the points on the straight line of equal distribution (Figs. 4, 6), one or several properties of the steels decrease. When the carbon content in the steel is reduced below its optimum content, for example, 0.7—0.8% for steel R18 or 1.2% for steel R14F4, the red hardness decreases because of depletion of the solid solution in carbon, although the alloying ...

the alloying of the solid solution increases. When the carbon content is raised above the optimum, the heat resistance of the steel increases due to saturation of the solid solution with carbon, but at the same time the tool life in cutting decreases because of the high brittleness caused by the large amount of carbides and their nonuniform distribution. When the vanadium, tungsten, and chromium content in the steel is below or above the optimum corresponding to the given carbon content, its heat resistance decreases. The reason for this in the first case is insufficient alloying of the solid solution by one of these elements or by all of them simultaneously, and in the second case it is depletion of the solid solution in carbon and, in addition, an increase in the brittleness of the steel due to an increase in the carbide phase.

Fig. 7. Dependence of the properties of high-speed steel on the content of carbon, vanadium, tungsten, and chromium in it (schematic)

With a simultaneous increase in the content of carbon, tungsten, and vanadium in the steel, its heat resistance increases, but the tool life in cutting will decrease because of brittleness.

For steels differing in the content of one alloying element, the ratios of the alloying of the solid solutions by these elements before tempering are close to these ratios after tempering (Tables 2, 3, 5). For example, for steels 1.2—4—4—1.5; 1.2—4—4—3 and 1.2—4—4—4, differing in vanadium content, these ratios for the hardened and tempered states are equal to 1 : 2 : 2 and 1 : 1.6 : 2, respectively; for steels 1.2—2—4—2.5; 1.2—6—4—2.5 and 1.2—8—4—2.5 (with different tungsten content), they are 1 : 2.6 : 3.2 and 1 : 2.2 : 3.0; and for steels 1.2—4—4—1.5 and 1.2—4—8—2, differing in chromium content, these ratios are 1 : 1.7 and 1 : 2.

In all the steels investigated, during tempering the solid solution is depleted in chromium by 30–40%, in tungsten by 50–60%, and in vanadium by 65–70%, i.e., the chromium content in the solid solution decreases by approximately 1/3, the tungsten content by 1/2, and the vanadium content by 2/3 (Table 5). The approximate constancy of these quantities for one element and their difference for different elements indicate that the fraction of the alloying element that passes from the solid solution into carbides during tempering does not depend on the initial concentration of this element in the solid solution and is directly proportional to its carbide-forming ability.

After triple tempering at 560°, as before it, the solid solutions of the steels investigated are alloyed to a greater extent with chromium and, to a lesser extent—

tungsten and vanadium. During tempering, the “favorable” distribution of tungsten and vanadium between the solid solution and the carbides is replaced by an “unfavorable” one. The distribution of chromium between the same constituents remains “favorable,” but close to an equal distribution \((\mathrm{Cr}_{\mathrm{r}} : \mathrm{Cr}_{\mathrm{k}} = 1)\).

Table 5

Decrease in the content of alloying elements
in the solid solution after triple tempering at \(560^\circ\)

*) In the denominator—the decrease as a percentage of the initial concentration in the solid solution.

Conclusions

1. With an increase in the carbon content in high-speed steel, there is a continuous depletion of the solid solution in tungsten, vanadium, and chromium, both after quenching and after triple tempering at \(560^\circ\). The carbon concentration in the solid solution thereby increases.

2. The distribution of a carbide-forming alloying element between the phases of quenched high-speed steel may be “favorable” \((M_{\mathrm{r}} : M_{\mathrm{k}} > 1)\) or “unfavorable” \((M_{\mathrm{r}} : M_{\mathrm{k}} < 1)\). In the region of “favorable” distribution of the alloying element there is an optimum content of it, at which the steel has the best properties. The amount of alloying element required for its equal distribution between the solid solution and the carbides is inversely proportional to the carbide-forming ability of the element and to the carbon content in the steel.

3. With an increase in the chromium content in the steel, the concentration of tungsten and vanadium in the quenched solid solution increases. Regardless of the carbon content in high-speed steel, chromium is distributed between its phases only “favorably.” During the redistribution of alloying elements between the solid solution and the carbide phase in the tempering process, the carbide phase is enriched more intensively with chromium than with tungsten and vanadium.

1. The distribution of tungsten and vanadium between the phases of quenched steel depends on its carbon content. The most “favorable” distribution of these elements between the phases of the steels studied is obtained at a carbon content of up to 1.1–1.2%.

2. The relative alloying level of the solid solutions of steels that differ in the content of only one alloying element is the same before tempering as the relative alloying level of the solid solutions after tempering. In the course of triple tempering at 560°, the solid solution is depleted by approximately \(1/3\) in chromium, by \(1/2\) in tungsten, and by \(2/3\) in vanadium. The fraction of the alloying element that passes from the solid solution into carbides during tempering does not depend on the initial concentration of this element in the solid solution and is directly proportional to its carbide-forming ability.

3. On the basis of the experimental data of the present work and literature data, it has been established that the most heat-resistant and economical steel in terms of tungsten use, in comparison with steel P18, is a high-speed steel with 16% tungsten (P16).

4. The influence of the contents of carbon, tungsten, vanadium, and chromium in high-speed steel on the distribution of these same components between the phases of the steel in the quenched state has been established. Using the regularities obtained, one can choose the best ratios between the contents of carbon, tungsten, vanadium, and chromium in high-speed steel, at which higher properties are obtained.

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