Low-Alloy Cast High-Speed Steels

N. A. Erofeev

(Presented by Professor Doctor A. N. Dobrovidov)

For the manufacture of cast cutting tools, steels R18 and R9, or steels close to them, are used chiefly [1, 2]. The manufacture of cast tools from cheaper grades of steel has been little studied and is considered only in work [3]. The present work gives the results of an investigation aimed at finding new compositions of low-alloy high-speed steels for cast cutting tools.

Selection of Steel Compositions and Their Melting

The elimination of forging in the manufacture of cast tools makes it possible to vary, within broad limits, the content of individual elements in the steel. For the investigation we cast 32 heats with different contents of carbon, vanadium, tungsten, and chromium (Table 1).

The first group includes tungsten-free chromium-vanadium steels, differing from one another in their carbon or vanadium content. The purpose of separating out this group was to search for a steel for manufacturing cast tools without tungsten.

The carbon content in forged high-speed steels is determined by the possibility of forging them and amounts to 0.7–0.9%. According to A. P. Gulyaev [4], increasing the carbon content from 0.7 to 1.3% in low-alloy forged high-speed steel continuously increases the cutting ability of the steel. I. S. Kvater [2] considers the optimum carbon content in cast steel to be 0.9–11%. According to the results of investigations by N. D. Tyuteva and I. T. Tikhonov [5], for steel of the R9 type the optimum carbon content is 1.0–1.1%.

The influence of vanadium on the properties of forged and cast high-speed steels is considered in works [6, 7, 8]. From the results of A. P. Gulyaev’s investigation [7] it follows that, when the vanadium content is increased to 2.5% in low-alloy forged high-speed steel, tool productivity rises sharply; with a further increase in vanadium content, tool productivity begins to fall. According to Rahstadt and Heller [9], vanadium improves the red hardness of steel when an increase in its content is accompanied by an increase in the carbon content.

I. S. Kvater [2] established that an increase in vanadium content ...

Table 1

NOTE: In the steel designations, the figures indicate the approximate content of carbon and alloying elements in weight percent in the following order: carbon, tungsten, chromium, and vanadium.

In steels of the fourth group, the fifth figure indicates the approximate silicon content.

from 2 to 4% in cast steel containing 1.0% carbon, 4% chromium, and 10% tungsten leads to a sharp increase in the durability of the cast tool.

The second group of steels includes low-tungsten chromium-vanadium steels. The purpose of distinguishing this group was to determine the effect of tungsten on the properties of cast low-alloy high-speed steel.

There is no single opinion regarding the optimum tungsten content in high-speed steel. According to A. P. Gulyaev [4], no more than 7–8% tungsten can dissolve in the solid solution of hardened high-speed steel; above this amount tungsten is used irrationally, since it goes into the formation of primary carbides, which take no part in increasing red hardness. I. S. Kvater [2] notes that cast high-speed steel with 18% tungsten is excessively alloyed and should be replaced by a more economically alloyed steel with 9–11% tungsten. Goodremont [10] investigated high-speed steels with 25–28% and 12–14% tungsten and established that the former do not give an increase in productivity compared with the latter.

The influence of chromium on the properties of wrought low-alloy high-speed steel is considered in the work of A. P. Gulyaev [4]. The best cutting properties were found at a chromium content equal to 4%. On this basis, the chromium content in both the first and second groups of steels was taken by us as 4–4.5%.

The third group of steels includes steels of the EI184 type with different carbon contents. Chromium and tungsten remained at the level of their contents in wrought EI184 steel. The vanadium content was taken as 2%.

The fourth group of steels comprises steels of the EI172 type. Their investigation pursued the same aim as that of the steels of the third group—to determine the possibility of manufacturing a cast tool from high-chromium steels by casting into a metal mold.

For melting all groups of the steels investigated, ShKh-15 steel scrap, low-carbon steel 10 to reduce the carbon content, chromium white cast iron to increase the carbon content, and ferroalloys were used.

The steels were melted in a high-frequency furnace. After melting was completed, the steel was deoxidized with aluminum and silicocalcium, calculated at 0.1% of each. Before pouring, a modifier—boron in the form of ferroboron—was introduced into the steel, calculated at 0.025% B.

The mold was filled with steel directly from the crucible at a temperature of 1500–1540°, controlled by an optical pyrometer. The weight of each heat was 1 kg. Casting was carried out on a centrifugal casting machine at 400–500 rpm into a composite metal mold. At the same time, 6 wedge-shaped plates with ribs, measuring 40×25×10 mm, were cast. The cast plates were cooled together with the chill mold to room temperature, after which they were separated from the runner. The small cross-section of the plates promoted the combination of hardening with casting. Their heat treatment was limited only to tempering.

Microstructure of the Steels Investigated

The cutting and mechanical properties of a cast tool whose hardening is combined with casting are determined by the structure formed during crystallization. Under identical crystallization conditions, a change in composition affects the cast structure of the steel.

In studying the structure of steels of the first and second groups, it was established that increasing the vanadium content to 2.5–3.0% leads, in the structure of hardened steel, to the appearance of cryptocrystalline martensite instead of acicular martensite, and to a decrease in the amount and stability of retained austenite. This is evident when comparing the hardness of the hardened steels 1.2—4—4—1.5 and 1.2—4—4—4, or of the steels 1.2—2—4—1.5 and 1.2—2—4—4.5. The hardnesses of steels 1.2—4—4—1.5 and 1.2—2—4—1.5 are respectively equal to 56 and 55 \(R_c\), while the steels with the higher vanadium content have a hardness of 63 \(R_c\).

An increase in vanadium content to 4—4.5% leads to the formation of a dark constituent in the structure. The appearance of this constituent is a sign of heterogeneity in the structure of the steel and of deterioration of its properties. The composition and heat treatment of the steel must be selected so as to exclude this constituent from the steel structure. Increasing the vanadium content above 3.0% also adversely affects the eutectic structure: the fine-network structure of the eutectic is replaced by a coarse, bushy structure resembling carbide liquation in forged steel.

Tungsten, when its content in steel is increased, affects the structure of hardened steel more weakly than vanadium. This can be explained by the fact that depletion of the solid solution in carbon with increasing tungsten content in the steel occurs more slowly than with increasing vanadium content, since the tungsten carbides that form contain less carbon than vanadium carbides. For example, the carbide \(\mathrm{Fe}_3\mathrm{W}_3\mathrm{C}\) contains 11 times less carbon than the carbide \(\mathrm{VC}\). The less intense effect of tungsten on the structure of the steels studied, as compared with vanadium, is also seen from the following example. Increasing the tungsten content in the steel fourfold (steels 1.2—2—4—2.5 and 1.2—8—4—2.5) increased the hardness by 3 \(R_c\) units, whereas increasing the vanadium content in the steel twofold (steels 1.2—4—4—1.5 and 1.2—4—4—3) increased the hardness from 56 to 61 \(R_c\) units.

Raising the tungsten content from 2 to 8% does not cause qualitative changes in the structure, but increases the amount of martensite and decreases the stability of retained austenite. The eutectic of steels containing tungsten forms continuous regions bordering the primary grains. Its amount increases as the tungsten content in the steel is raised.

Carbon, when its content in steel is increased, increases the amount of retained austenite, makes it more stable against tempering, and causes a change in the eutectic structure from a fine-network to a coarse structure. When the carbon content in steel is raised above 1.1%, fine-acicular martensite is also replaced by coarse-acicular martensite. The effect of carbon on the structure of steel may be illustrated by the example of steels 1.3—4—4—3 and 1.0—4—4—3. The structures of these steels after triple tempering at \(560^\circ\) differ from one another. In the first steel there is more retained austenite, the martensite needles are coarser, and the eutectic has a coarser structure. The difference in the structure of these steels also determined the difference in their durability. The lower durability of steel 1.3—4—4—3 in comparison with steel 1.0—4—4—3 may be attributed to the coarse eutectic and coarse-acicular martensite.

The principal structural feature of high-chromium steels of the ÉI184 and ÉI172 types in the hardened condition is the presence of a considerable amount of retained austenite. However, the stability of auste-

Table 2

Distribution of alloying elements among the phases of low-alloy cast high-speed steel after quenching (casting)

resistance to tempering in these steels is different. The large amount of retained austenite and its low stability during tempering in steels of the EI172 type is explained by the high alloying of the austenite with chromium, which has a high diffusion rate and forms carbides that coagulate readily.

The retained austenite of steels of the EI184 type is distinguished by increased resistance to tempering in comparison with the austenite of steels of the first and second groups; this should be explained by the high concentration of carbon in the austenite of the former steels. The latter is confirmed by comparing the chemical composition of the carbides of steels \((1.0—1.2)—4—8—2\) and \((1.0 \div 1.2)—4—4—3\). With equal tungsten content and a vanadium difference of one percent in these steels, the tungsten and vanadium content in the carbides of the former steels is 2–2.5 times lower than in the latter (Table 2). The lower content in the carbide of strong carbide-forming elements, which give carbon-rich carbides, will to a lesser extent deplete the solid solution of carbon.

It is necessary to note the presence of sharply expressed intracrystalline segregation in steels of the EI184 type. This can be judged from the location of the retained austenite at the periphery and of martensite in the middle of the grains. Martensite is formed from the less alloyed solid solution, which crystallized earlier than the solid solution at the periphery of the grains. The structural inhomogeneity of steels of this type determined their reduced durability in comparison with steels of the second group.

Behavior during tempering and red hardness of low-alloy cast high-speed steels

To study the behavior of low-alloy cast high-speed steels during tempering, sequential tempering was carried out in the temperature range from 100 to 650° and multiple tempering at temperatures of 560 and 600°. The holding time during tempering was one hour.

The sequential-tempering curves of the steels investigated (Fig. 1)

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

in their character do not differ from the curves obtained in sequential tempering of forged or cast high-speed steel

of ordinary composition. All the curves are characterized by a decrease in hardness in the temperature range 300–400°. At temperatures above 400° an intensive increase in the hardness of the steel begins, which depends on the composition of the steel. The greatest increase in secondary hardness occurs during tempering within the range 500–600°. After the maximum secondary hardness is reached, the curve of hardness change declines. The predominant processes here are those of the decomposition of martensite into a ferrite–carbide mixture. The intensity of the decrease in hardness depends on the rate of diffusion and the concentration of alloying elements and carbon in the alpha solid solution, on the retarding effect of these elements on the precipitation of carbon from it, and also on the properties, degree of dispersion, and capacity for coagulation of the carbide phase. A sharp drop in hardness occurs in all steels without tungsten and with a low vanadium content. Such behavior of these steels during tempering should be explained by the lower stability of martensite and the greater tendency toward coagulation of chromium carbides, in comparison with the martensite and carbides of steels alloyed with vanadium and tungsten.

When comparing the data obtained during successive tempering of the steels investigated, one can determine the influence of alloying elements and carbon on the maximum value of the secondary hardness, its temperature, and the red hardness of the steel.

There are several methods for determining red hardness [4, 11]. In our case, the characteristic of red hardness is the temperature at which the curve of hardness change at the end of successive tempering (Fig. 1) falls to a value of 60 units on the \(R_c\) scale. Red hardness may also be characterized by the hardness after sixfold tempering at 560 or 600°.

The value of the indicated characteristics of the steel is determined by the alloying of the solid solution, which depends on the composition of the steel and the quenching temperature.

The influence of increasing the content of alloying elements and carbon in steel on secondary hardness, on the temperature of this hardness, and on the red hardness of certain steels is shown in Figs. 2–4.

Fig. 2. Influence of carbon content on the maximum value of the 2nd hardness, the temperature of this hardness, and on the red hardness of steel.

From the curves in Fig. 2 it is seen that increasing the carbon content in steel leads to a continuous increase in the indicated characteristics, although the content of alloying elements in the solid solution, both in these steels and in other steels differing only in carbon content, decreases because of the increase in the amount of carbides (Table 2).

The increase in red hardness and secondary hardness in such steels can be explained by an increase in the concentration of carbon in austenite. When carbon is added to steel, the chemical composition of the crystallizing phases changes—the solid solution is depleted in alloying elements and enriched in carbon.

The effect of vanadium, when its content in steel is increased, on the same characteristics of steels is seen from Fig. 3. When the vanadium content is increased to 3%, the red hardness and the maximum ...

value of the secondary hardness. A further increase in the vanadium content leads to a decrease in red hardness and in the temperature corresponding to the maximum value of secondary hardness. This effect of vanadium is explained by the depletion of the solid solution in carbon during the formation of vanadium carbide rich in carbon. This is confirmed by data from phase analysis of steels differing in vanadium content. For example, the higher vanadium content (29.6%) in the carbide precipitate of steel 1.2—4—4—4, compared with its content (17.9%) in the precipitate of steel 1.2—4—4—3, considerably depleted the solid solution of the first steel in carbon. This led to the formation of a less stable metallic matrix in steel 1.2—4—4—4, which softens more rapidly during subsequent heating in tempering or during cutting.

Fig. 3. Effect of vanadium content on the maximum value of secondary hardness, its temperature, and on the red hardness of steel.

The alloying of the solid solution with vanadium in the steels cited differs very little, while the alloying of the carbides of steel 1.2—4—4—4 with vanadium is approximately twice as great as that of carbides 1.2—4—4—3 (Table 2). The increase in red hardness and other characteristics with an increase in the vanadium content to 3.0% is explained by the increased alloying of the solid solution with vanadium. For example, the concentration (0.94%) of vanadium in the solid solution of steel 1.2—4—4—1.5 is approximately twice as low (1.78%) as in steel 1.2—4—4—3. Steels with a higher concentration of vanadium in the solid

Fig. 4. Effect of tungsten content on the magnitude of maximum secondary hardness, its temperature, and on the red hardness of steel.

solution are less prone to softening during tempering, both because of the greater alloying of the martensite and because of the formation of a larger quantity of finely dispersed, difficult-to-coagulate carbides. A further increase in the vanadium content (steel 1.2—4—4—4) to 4.2 did not cause an increase in the saturation of the solid solution with vanadium (1.78%), since it went into the formation of carbides. Comparing the phase-analysis data for steels 1.2—4—4—(1.5—3—4), one can

one may conclude that, for saturation of the crystallizing solid solution of these steels, about 1.8–2.0% vanadium of its total content in the steel is required. The remaining part of the vanadium goes into the formation of primary carbides, which do not increase the red hardness of the steel, while the solid solution is thereby greatly depleted in carbon. It follows from the foregoing that the optimum amount of vanadium in the steels studied, with 0.9–1.1% carbon, is 2.5–3.0%.

An increase in the tungsten content within the limits considered (up to 8%) in cast low-alloy steels leads to a continuous increase in the maximum value of the secondary hardness and red hardness; this is associated with an increase in the alloying of the solid solution with tungsten, accompanied by a simultaneous increase in its content in the steel (Table 2).

As a result of increasing the alloying of the solid solution with tungsten and of the retarded precipitation of its carbides from the solid solution during heating, there is an increase in red hardness, in the maximum value of the secondary hardness, and in the temperature of this hardness (Fig. 4).

An increase in the chromium content in the steels under consideration affects the red hardness and the maximum value of secondary hardness differently than an increase in the steel of tungsten, vanadium, and carbon.

The negative effect of an increased chromium content was most sharply manifested in steels of the EI172 type. These steels showed a comparatively high secondary hardness, but at low tempering temperatures. The maximum value of the secondary hardness of steel 1.0—0—12—3—2, 63 units on \(R_c\), was obtained at a temperature of 525°, while for steel 1.3—0—12—3—2, 66 on \(R_c\), it was obtained at 550°. Characteristic of these steels is the sharp drop in secondary hardness after its maximum value has been reached. For example, in steel 1.3—0—12—3—2 the hardness, from a maximum value of 66 on \(R_c\) at 550°, falls to 55 at a tempering temperature of 575°. The reason for such low stability of the solid solution alloyed with chromium is the high rate of chromium diffusion already at low tempering temperatures (400–500°). The magnitude of the secondary hardness is affected—

Fig. 5. Influence of repeated tempering at 560° on the hardness of cast steel.

Fig. 6. Influence of repeated tempering at 560° on the hardness of cast steel.

is affected not only by the temperature and duration of tempering, but also by its repetition. The behavior of the steels under investigation during repeated tempering was studied at two temperatures—560 and 600°. Tempering at 560° made it possible to determine the optimum number of tempers for obtaining maximum hardness of tools made of this steel. Tempering at 600° served to clarify the behavior of the steels under investigation under high-temperature heating. The duration of each tempering was one hour. The change in hardness of some steels during repeated tempering at a temperature of 560° is shown in Figs. 5–7. Analysis of the results obtained confirms the conclusions drawn earlier concerning the effect of changes in the content of various elements in the steel on its stability during heating. Thus, the resistance of the martensite of the steel to softening in the course of repeated tempering increases with increasing carbon content in the steel.

Fig. 7. Effect of repeated tempering at 560° on the hardness of cast steel.

A very favorable effect is produced by increasing the tungsten content in the steel—the more tungsten the steel contains, the higher the hardness after any number of tempers. Increasing the vanadium content has a positive effect only up to 2.5–3.0%.

Cutting Properties of the Steels

The data obtained on the red hardness of cast low-alloy high-speed steels made it possible to assume that cutting tools made from some of the steels would have sufficiently high durability. Eleven steels were tested by cutting under plant conditions. The purpose of the test was to obtain a comparative assessment of the cutting properties of low-alloy cast high-speed steels of different compositions. Since quenching was combined with casting, the heat treatment of cutters made from these steels was reduced only to tempering according to the optimum schedule (Table 3).

The cast and tempered plates were mechanically fastened in specially manufactured holders, after which they were sharpened. The cutting conditions were chosen so as to obtain one-hour durability of cutters made of forged steel R9. For a comparative assessment of the cutting properties of the steels under investigation, cutters made of forged R9 steel after heat treatment according to the standard schedule were tested under the same conditions.

In the cutting tests, blanks for rollers made of steel 40 with a Brinell hardness of 180 were used as the work material. The tests were carried out on a lathe in longitudinal turning without cooling. The depth of cut was \(t = 2\) mm, the feed \(S = 0.3\) mm/rev. All cutters were tested at a cutting speed of 30 m/min, corresponding to 60-minute durability of cutters made of forged R9 steel. Complete breakdown of the cutters served as the criterion of blunting. The results obtained for the durability of the steels are given in Table 3.

Table 3

The data on the cutting properties of the steels tested make it possible to draw the following conclusions.

With an increase in the carbon content in cast low-alloy high-speed steels, the life of the cutters decreases, although the red hardness increases. The optimal carbon content in the steels studied, from the standpoint of maximum life, should be considered 0.9–1.1%. The reduction in cutting properties with a further increase in the carbon content of the steel is explained by the increased brittleness of the cutters, caused by the coarse acicular martensite and the coarse structure of the eutectic.

An increase in the tungsten content in the steels studied continuously increases the life of the cutters by increasing the alloying of the solid solution with tungsten.

Cutters made of cast steel of composition 0.9–1.1% carbon, 2.5–3.0% vanadium, 4–4.5% chromium, and not less than 3.5–4.0% tungsten have a life equal to that of cutters made of forged steel R9.

The optimal vanadium content in the steels studied should be considered 2.5–3.0%. Increasing the vanadium content in the steel to 4–4.5%, in accordance with the reduction in red hardness, reduces the life of cutters made of such steels.

Cutters made of cast steels of the EI184 type have low life in comparison with cutters made of experimental steels (1.0–1.2)—4—4—3, which contain 4% less chromium and 1% more vanadium than the former. Therefore the optimal chromium content in cast low-alloy high-speed steels should be considered 4–4.5%.

Thus, when casting small-section plates in a metal mold, whose hardening occurs during the casting process, it is possible to select the composition of a low-alloy steel that is not inferior in life to cutters made of forged steel R9.

According to the results of life tests, such a steel is the steel of the following composition: 0.9–1.1% carbon, 3.5–4.0% tungsten, 2.5–3.0% vanadium, and 4–4.5% chromium.

Conclusions

1. In cast low-alloy high-speed steels, when the content of alloying elements and carbon is varied, the nature of the change in hardness, and in the amount and stability of retained austenite, is analogous to their change in wrought steel.

2. Carbon considerably increases the red hardness of steel, but increases the amount and stability of retained austenite and raises the brittleness of the steel. An increase in the carbon content of steel impoverishes the solid solution in alloying elements, enriches it with carbon, and increases the amount of carbides. In cast low-alloy steels it is advisable to raise the carbon content only to 1.1%.

3. Increasing the vanadium content in the steels studied leads to qualitative changes in the structure, which, when its content is increased to 3.0%, are expressed in the replacement of ordinary martensite by cryptocrystalline martensite, and when the content is increased to 4–4.5%, in the appearance in the structure of a dark constituent and in the replacement of the network structure of the eutectic by a clustered one. Vanadium increases the red hardness and cutting durability of cast low-alloy high-speed steels containing 0.9–1.1% carbon, but only when its content is up to 3.0%. In this case vanadium has a favorable effect on the steel, reducing the amount and stability of retained austenite. With a further increase in the vanadium content in the steel, the red hardness and cutting durability decrease because of strong impoverishment of the solid solution in carbon.

Vanadium can be used to improve the cutting properties of steel only in combination with other alloying elements—tungsten and chromium—since low-alloy cast high-speed steels without tungsten and with an increased vanadium content do not possess sufficiently high red hardness, and in cutting tests showed low cutting properties.

1. An increase in the tungsten content of steel continuously raises the red hardness of the steel and reduces the amount and stability of retained austenite by impoverishing the solid solution in carbon. At the same time, impoverishment of the solid solution in carbon occurs less intensely than when the vanadium content of the steel is increased. The minimum tungsten content in low-alloy cast high-speed steel for producing from it a sufficiently serviceable tool is 3.5–4.0%.

2. Increasing the chromium content above 4–4.5% in cast low-alloy high-speed steel does not give positive results, since chromium lowers the red hardness of the steel and increases the amount and stability of retained austenite.

3. Low-alloy cast high-speed steel of composition 0.9–1.1% carbon, 4–4.5% chromium, 3.5–4.0% tungsten, and 2.5–3.0% vanadium is equivalent in cutting properties to wrought high-speed steel of the R9 type; and with a higher tungsten content (6–8%) it possesses higher cutting properties. Heat treatment of a tool of small section made of such steel, when cast in a metallic mold, is reduced to triple tempering at 560°. Steel of the indicated composition is recommended for the manufacture of cast tools by casting into a metallic mold.

References

1. Lebedev T. A., Revis I. A. Structure and Properties of Cast Tools Made of High-Speed Steel, Mashgiz, 1949.

2. Kvater I. S. Cast High-Speed Steels, Mashgiz, 1952.

3. Tushinskii L. I. Investigation of Cast Low- and Medium-Alloy High-Speed Steels, Author’s Abstract of a Dissertation, Leningrad, 1953.

4. Gulyaev A. P. Theory of high-speed steel, Machines and Tools, Nos. 2–3, 1946.

5. Tyuteva N. D., Tikhonov I. T. On the question of the influence of carbon content and casting method on the properties of cast high-speed steels, Proceedings of the Tomsk Polytechnic Institute, vol. 68, issue 1, 1951.

6. Kvater I. S. Influence of vanadium on the properties of high-speed steel, Bulletin of Mechanical Engineering, No. 11, 1949.

7. Gulyaev A. P. Vanadium in low-alloy high-speed steels, Steel, No. 4, 1941.

8. Goryunov I. I. Casting of tools in metal molds, Cast and Surfaced Tools, Collection of Articles, Mashgiz, 1951.

9. Rakhstadt A. G. and Geller Yu. A. Structure and properties of high-speed steel with an increased vanadium content. Collection “Structure and Strength of Metals,” Mashgiz, 1953.

10. Gudremon E. The Science of Special Steels, ONTI—NKTP, 1937.

11. Geller Yu. A. Method for determining the red hardness of high-speed steel, Factory Laboratory, No. 3, 1947.