EFFECT OF NIOBIUM ON THE STRUCTURE AND PROPERTIES OF R9-TYPE HIGH-SPEED STEEL

D. M. LIKHOSHERSTOV

(Presented by Professor, Doctor A. N. Dobrovidov)

At present, the effect of niobium on the properties of high-speed steel has not been studied sufficiently fully, and the results of investigations are contradictory in character [1, 2, 3, 4, 5, 6].

The first investigation [1], carried out by Academician Gudtsov and other authors on steel EI 172 \((\mathrm{C}=1.0—1.15\%;\ \mathrm{Cr}=11—13\%,\ \mathrm{V}=2.0—2.6\%;\ \mathrm{Si}=1.0—1.6\%;\ \mathrm{Mn}=0.25\%)\) with additions of niobium from 0.1 to 2%, showed an insignificant increase in the economical cutting speed at a niobium content of 2%.

In a later investigation [2], carried out on steel of the EI 184 type \((\mathrm{C}=0.86—1.02\%;\ \mathrm{Si}=0.71—0.84\%;\ \mathrm{Mn}=0.18—0.28\%;\ \mathrm{W}=1.7—3.1\%;\ \mathrm{Cr}=8.1—8.9\%;\ \mathrm{Ni}=0.16—0.20\%;\ \mathrm{V}=2.5—2.94\%)\) with additions of niobium from 1.0% to 3%, a negative result was obtained. The cutting properties and forgeability of such steel with niobium decreased.

Brown and other authors [4], on forged R9 steel with 0.6% niobium, obtained an increase in heat resistance. The effect of niobium on the cutting properties was not given in this study. Chaadaeva [5] also obtained an increase in heat resistance on a medium-tungsten cobalt-containing forged high-speed steel \((\mathrm{C}=1.50\%;\ \mathrm{W}=11.45\%;\ \mathrm{V}=4.5\%;\ \mathrm{Cr}=4.10\%;\ \mathrm{Mo}=0.90\%;\ \mathrm{Co}=5.70\%;\ \mathrm{Nb}=0.97\%)\) in comparison with a similar steel without niobium. The cutting properties of these steels with and without niobium are 2–3 times higher in durability than those of R18 steel when machining the heat-resistant alloy EI 437. The effect of niobium in this steel was not analyzed, and it is apparently also obscured by the effect of cobalt.

Thus, there is no single opinion on the effect of niobium on the properties of forged high-speed steel in the literature considered above. Investigation of the effect of niobium on the properties of cast high-speed steel has not yet been covered. It is therefore advisable to carry out a more detailed study.

We cast 16 different high-speed steels based on R9 with variation in carbon content: 0.9, 1.2, and 1.4%, and with an addition of niobium from 0.05 to 5%.

Specimens measuring \(12 \times 12 \times 120\) were cast in a steel permanent mold from a temperature of 1440–1480°. Scrap forged R9 high-speed steel was used as the charge. For carburization, ...

R9 steel with a high carbon content. The ferroalloys were introduced by means of tin-plate cartridges in the following sequence: ferrochromium, ferrovanadium, ferroniobium. Before pouring, the metal was deoxidized with aluminum at a rate of 0.1% of the heat weight. Melting was carried out in a rammed quartz crucible in a high-frequency unit A3–43.

The purpose of this stage of the investigation was to study the effect of niobium on the properties of cast R9 high-speed steel and to determine its optimum carbon content.

Increasing the carbon content lowered the hardness in the as-cast state from 62 HRC for steel with 1.01% carbon (heat 7, Table 1) to 51.5 HRC in steel with 1.36% C (heat 03).

The introduction of niobium up to one percent into steels with 0.9% and 1.2% carbon caused a very slight decrease in hardness in the as-cast state. A noticeable decrease in hardness was found in steels with 0.9% carbon only when the niobium content exceeded one percent, and in steels with 1.2% carbon when the niobium content exceeded 3% (Table 1). With 4.54% niobium in steel with 0.85% carbon (heat 02), the hardness in the as-cast state decreased to 20 HRC. In this composition, almost complete wedging-out of the $\gamma$ region occurred, and a structure consisting of a ferrite–carbide mixture was obtained. Raising the carbon content from 0.85% to 1.18% (heat 6) prevented the wedging-out of the $\gamma$ region.

Fig. 1. Change in hardness during tempering of cast R9 high-speed steel with a carbon content of 0.9% and niobium from 0 to 5%.

A different picture of the influence of niobium is observed in steels with 1.4% carbon. In these steels, the introduction of niobium up to 5% caused a gradual increase in hardness from 51 HRC to 63 HRC (heats 03, 05, 06, 07, 08, Table 1).

The results of the change in hardness in the as-cast state did not confirm Goudremont’s assertion [3] regarding an increase in the amount and stability of retained austenite in high-speed steel upon the introduction of niobium.

Thus, in steels with 0.9 and 1.2% carbon, the introduction of niobium caused a decrease in hardness, while in steels with 1.4% carbon, on the contrary, it increased hardness. In the first case, the formation of niobium carbide apparently reduced the carbon content in solid solution, lowering its hardness; in the second case, the carbide formed, at a sufficient carbon content (1.4%) in the steel, reduced the stability of austenite during cooling, thereby increasing the hardness after casting.

After casting, the steels under investigation were tempered according to a threefold tempering schedule at a temperature of $550 \pm 10^\circ$ for one hour. After tempering, hardness was measured and the microstructure was analyzed.

In steels with a carbon content of 0.9%, the first and second tempering operations increased hardness (Fig. 1). With a further increase in the number of tempering operations, the hardness remains at the previous level only in steels without niobium and with a content of 0.37%. At a niobium content of 1.29% and more—

Table 1

its hardness after the second tempering decreases owing to softening of the martensite and the processes of carbide coagulation.

Approximately the same regularity in the change in hardness during tempering is also observed in steels with a higher carbon content, 1.2%. Tempering steels with 1.4% carbon caused a sharp increase in hardness, especially in steels with 2.31 and 4.09% niobium (Fig. 2).

Fig. 2. Change in hardness during tempering of cast R9 high-speed steel with a carbon content of 1.4% and niobium from 0 to 5%.

In the figure: vertical axis—Hardness, \(R_c\); horizontal axis—number of temperings (quenching, 1, 2, 3); tempering regime \(550^\circ\), 1 h. Curves: \(5.0\%\ \mathrm{Nb}\), \(3.0\%\ \mathrm{Nb}\), \(1.0\%\ \mathrm{Nb}\), \(0.3\%\ \mathrm{Nb}\), \(0\%\ \mathrm{Nb}\).

One of the main characteristics determining the serviceability of a cutting tool is its red hardness. We investigated the red hardness of cast steels by the method of A. P. Gulyaev [7]. Specimens of the steels studied were held for 4 hours at temperatures of 575, 600, 625, 650, and 675°. The temperature at which the hardness decreased to 58 HRC was taken as the measure of red hardness. The results of the hardness measurements are given in Table 1. The highest red hardness in the cast condition is possessed by steels with niobium contents of \(\sim 0.4\%\), 1% and carbon contents of 1.2% and 1.4%, i.e., steels of compositions 3, 4, 05 (Table 1). The red hardness of these steels is higher than that of the corresponding steels without niobium and of forged steel R18.

According to the results for red hardness, cutting ability, behavior during tempering, and technological considerations (carbon increases ...

...fluidity), the optimum carbon content for cast R9 steel with niobium should be chosen as 1.15–1.25%.

Phase analysis was carried out on steels with a carbon content of 1.2% in the annealed and hardened states. The electrolyte used was a solution of 75 g of potassium chloride and 5 g of citric acid per liter of water. The current density was selected within the range 0.02–0.04 A/cm² of the specimen surface. The duration of anodic dissolution of the specimens was 2–4 hours. The dissolution process was repeated several times.

The results of the phase analysis show that the amount of the carbide phase decreases sharply with increasing niobium content, both in the hardened and in the annealed states.

X-ray structural and chemical analyses of the carbide precipitates have not yet been carried out; therefore it is difficult to make any final judgment about the influence of niobium. However, the decrease in the amount of the carbide phase indicates that, with the introduction of niobium, a carbide with a small specific volume—NbC, or one similar to it—is formed. The specific volume of niobium carbide is approximately 4.5 times smaller than that of the complex carbide Fe₃W₃C, which constitutes the main part of the carbide phase of R9 steel. As a strong carbide-forming element, niobium, by taking up carbon, reduces the possibility of formation of other carbides—in this case, the carbide Fe₃W₃C. This leads to a decrease in the total volume of the carbide phase.

The cutting properties of high-speed steels with niobium were investigated on right-hand turning tools with a cross section of 10×10×100, cast in a metallic chill mold. After casting, the tool blanks underwent triple tempering according to the regime 550±10° for one hour, followed by grinding and sharpening of the cutting part ($\alpha = 10^\circ$, $\gamma = 15^\circ$, $\varphi = 50^\circ$, $\varphi_1 = 15^\circ$). The hardness of the tools after sharpening was within the range 62–64 HRC. The tools were tested to complete wear on a smooth shaft made of steel with $H_B = 111$, $\sigma_b = 40$–42 kg/mm², at a cutting speed of 136 m/min, $S = 0.26$ mm/rev, and $t = 2$ mm, without cooling, on a 1A62 lathe. The lathe was equipped with a motor with smooth stepless speed control, which made it possible to maintain precisely a constant cutting speed as the diameter of the shaft being tested changed. The test results are given in Table 1.

The highest durability in cutting tests was shown by steels with a carbon content of 1.2% and 0.9% at 0.3% and 1% niobium (heats 3, 4, 10, 01). Tools made from these steels showed a service life 2.5–3 times higher than tools made from R18 steel. Tools made from steels with 1.4% carbon showed reduced durability because of increased brittleness.

Conclusions

1. Niobium in R9-type steel with 0.9%, 1.2%, and 1.4% carbon, in an amount of up to one percent, has a positive effect on hot hardness and cutting properties. A further increase in the niobium content lowers the hot hardness; therefore, introducing it in amounts greater than one percent is not advisable.

2. The positive influence of niobium on R9 steel in small amounts (up to 1%) and its negative influence at higher contents (3–5%) is explained by the strong carbide-forming action of niobium. At a low niobium content in the steel, it apparently is present partly in solid solution, increasing its degree of alloying. At a significant niobium content, niobium forms a large number of sparingly soluble carbides, which take little part in increasing the alloying of the solid solution.

1. In R9-type steel with an addition of niobium, it is necessary to increase the carbon content from 0.9% to 1.2%. At this content, the maximum heat resistance of the steel and the best cutting properties are obtained (Table 1).

2. The best compositions among the steels studied are those with 1.2% carbon and 0.44, 1.09% niobium (items 3, 4 in Table 1). Their cutting properties are 2.5 times higher than those of forged R18 steel.

References

1. N. T. Gudtsov, A. N. Bekova, S. A. Kazeev, and A. V. Polyakov. Effect of certain special elements on the properties of a chromium–silicon–vanadium substitute for high-speed steel. Metallurg, No. 1, 1939.

2. N. T. Gudtsov and L. D. Maisakova. Izv. AN SSSR, OTN, No. 12, 1947.

3. Gudremon. Special Steels. Vol. 2, 1960.

4. A. P. Braun, G. L. Kuruklis, M. T. Durdo. Modification of high-speed steel. Mashgiz, 1956.

5. M. S. Shalaeva. Medium- and low-tungsten high-speed steels with cobalt. Metallovedenie i obrabotka metallov, No. 3, 1958.

6. M. S. Polyak. High-speed hardfacing alloys. Stanki i instrument, No. 1, 1957.

7. A. P. Gulyaev, M. P. Fadyushina. Red hardness of high-speed steel. In: Methods and Practice of Metallographic Investigation of Tool Steel, Mashgiz, 1961.