EFFECT OF NIOBIUM AND MANGANESE ON THE STRUCTURE AND PROPERTIES OF CAST HIGH-SPEED STEEL

D. M. LIKHOSHERSTOV

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

The effect of niobium on the structure and properties of cast high-speed steel was discussed by us earlier [1]. In the present article we consider the combined effect of niobium and manganese on the structure and properties of cast high-speed steel of the R9 type with an increased carbon content.

Studies on the effect of manganese on the structure and properties of forged high-speed steel are contradictory in character [2, 3, 4, 5, 6, 7].

Aristov believes that a manganese content in high-speed steel RF1 of more than 0.4% cannot be regarded as desirable, since it leads to a lowering of cutting properties [2].

Geller [6] points out that manganese lowers the temperature of the pearlitic and martensitic transformations and also increases the amount of retained austenite.

Minkevich [3] indicates that manganese carbides coagulate at low temperature and cannot contribute to an increase in red hardness.

In some studies [4], however, results were obtained that are opposite to those mentioned.

Brown et al. [4], with the addition of 2% manganese to EI 262, and to steel EI 262 with 4% W, 1.2% manganese, obtained an increase in red hardness and cutting properties. Steel EI 262 with 2.2% manganese, when tested by cutting by the facing method on steel with ultimate strength $\sigma_b = 65\ \text{kg}/\text{mm}^2$, gave a 40% increase in economical cutting speed in comparison with steel R18.

Thus, there is no single opinion on the effect of manganese on the properties of forged high-speed steel. For cast high-speed steels, the effect of manganese has not been discussed in the literature. Additional careful investigation is needed to clarify the effect of manganese for forged high-speed steel and to establish its effect on the properties of cast high-speed steel.

Manganese worsens annealability, and therefore its use for forged high-speed steel encounters serious difficulties. Cast tools can be manufactured with minimal machining (grinding), avoiding annealing. This makes it possible

broader use of manganese for cast high-speed steels.

In studying the effect of manganese on the structure and properties of high-speed steels, it is necessary to take into account the fact that, if manganese is introduced in large amounts, then, because of the low temperature of formation and coagulation of its carbides, it cannot increase heat resistance and cutting ability.

With such a formulation of the question, as Minkevich [3], Heller [6], and Meskin [7] indicate, manganese is a harmful element. But if one considers the effect of manganese on phase changes, on its ability to change the bonding forces of carbon with other elements, for example V, Ti, Nb, etc., which leads to an increase in the solubility of these elements during hardening, then it is quite possible for the cutting properties of certain compositions of high-speed steels to be improved, as was found in the above-mentioned investigations [4].

Kogan and Entin, studying the effect of elements that form sparingly soluble carbides on the decomposition of austenite in structural steels [8], came to the conclusion that, under the influence of manganese, the solubility of such stable carbides as VC, TiC, TaC, and NbC increases and, as a consequence, decomposition in the pearlitic region is delayed.

It is quite possible that manganese exerts an analogous effect in high-speed steel as well. In the studies [4], phase and carbide analyses were not carried out; therefore our remarks concerning the effect of manganese are tentative. Additional investigations are needed to clarify the supposition put forward, all the more so because in high-speed steel the effect of increasing the solubility of sparingly soluble carbides may be very strongly weakened because of the high alloying of high-speed steel in comparison with structural steel.

The effect of manganese and niobium was investigated by us on R9 steel with an increased carbon content (1.2%). This carbon content in R9 steel was adopted by us, on the basis of a preceding study [1], as optimal for steel with niobium. The niobium content was taken as equal to 2%, proceeding from the calculation that at such a content a considerable decrease in the heat resistance and cutting ability of R9 steel with 1.2% carbon is felt. The compositions of the steels investigated are given in Table 1.

Melting was carried out in a rammed quartz crucible in an AZ-43 high-frequency installation. Waste R9 steel was used as the charge. The carbon content in the steel was increased by means of high-carbon R9 steel. Ferroalloys were introduced after melting the charge in the following sequence: ferrochrome, ferromanganese, ferrovanadium, and ferroniobium. Before pouring, the metal was deoxidized with aluminum at the rate of 0.1% of the melt weight. The metal was poured at a temperature of 1440–1480°.

For the investigation, specimens of \(12 \times 12 \times 120\) were cast in a metal chill mold. After casting, the specimens underwent annealing, tempering, hardening, examination of the microstructure, measurement of hardness after various heat-treatment schedules, and determination of heat resistance and cutting ability. The results of the investigation are presented below.

Manganese, as is known, expands the \(\gamma\)-region and increases the stability of austenite during cooling. In the as-cast condition, the hardness of steel with an increased manganese content (0.68–2%) (melts 30, 50, 90, Table 1) is lower than in R9 steel with the usual manganese content (0.17%, melt 1) and in steel with niobium (melt 80).

The increased manganese content in the steel led to a decrease in hardness in the as-cast condition owing to a larger amount of retained austenite. It is also characteristic to note that a further increase

the manganese content from 0.68% (heat 30) to 2% does not substantially change the hardness value (Table 1).

Table 1

In steel with a manganese content of 0.68% (heat 30), the introduction of 1.72% niobium (heat 060) increases the hardness in the cast condition from 56.0 HRC to 62.5 HRC. In steel with the same niobium content but without the increased manganese content (heat 80), the hardness is approximately the same (62.0 HRC). Such a change in hardness indicates that, in this case, niobium reduces the stability of retained austenite.

The results given above for the change in hardness do not confirm Goudremont’s assertion [9] that niobium in high-speed steels increases the amount and stability of retained austenite.

Thus, increasing the manganese content in cast high-speed steel of the R9 type causes a decrease in hardness due to an increase in the amount of retained austenite. The introduction of niobium into such a high-speed steel with an increased manganese content, on the contrary, increases hardness by reducing the stability of retained austenite and makes it equal to the hardness of steel without increased manganese content and niobium additions; i.e., niobium in an amount of 1.72% in R9 steel compensates for the effect of manganese in an amount of 0.68%. In steel with a higher manganese content (2%, heat 90), an addition of 1.80%

niobium (heat 00) could not fully compensate for the influence of manganese in such an amount, and the hardness of this steel increased only partially, from 54.5 HRC (heat 90) to 58.0 HRC.

On tempering the steels under study, the influence of manganese and niobium is markedly analogous to that in the as-cast condition.

For R9 steel with the usual manganese content, and also with an addition of 1.62% niobium (heat 1.80, Table 1), the maximum hardness is reached after triple tempering at \(550 \pm 10^\circ\). In steels with an increased manganese content (heats 30, 50, 90), even quadruple tempering does not ensure the attainment of maximum hardness, especially in steel with 2% manganese. Increasing the manganese content raises the stability of retained austenite against tempering.

In steels with 0.68% manganese and 1.72% niobium (heat 060), triple to quadruple tempering is sufficient to obtain maximum hardness, i.e., as in the as-cast condition, niobium compensated for the influence of manganese in increasing the amount and stability of retained austenite.

In steel with 2% manganese and 1.80% niobium (heat 00), as in steel with manganese alone (heat 90), quadruple tempering is insufficient to obtain maximum hardness. In this case, niobium could not fully compensate for the influence of manganese.

On annealing under various schedules, beginning at 900° up to 1000° with a holding time of 2–3 hours (isothermal holding at 730–740° for 3–4 hours), manganese increases the hardness of the annealed structure at high contents of 1.5% and 2% (heats 50 and 90, Table 1). In steel with 0.68% manganese and niobium (heat 060), the hardness in the annealed condition differs only slightly from heats 1 and 80, but at 2% manganese (heat 00) the hardness reaches 41.5 HRC (Table 1).

Table 2

Change in hardness during tempering of cast steels

It is also characteristic that high-speed steels with manganese are very sensitive to the cooling rate after isothermal holding during annealing. Accelerated cooling causes an increase in hardness.

The high hardness in the annealed condition in cast high-speed steels with manganese and niobium limits their field of application because of the difficulty of machining. However, for tools of simple configuration, for which casting in a metal mold can be used, followed by tempering and final grinding without machining by cutting, the use of such steels is quite possible.

The hot hardness of the steels under study was determined by the method of A. P. Gulyaev [10]. At temperatures of 575, 600, 625, 650, and 675° a holding time of 4 hours was used, after which the hardness was measured. The temperature at which the hardness was taken as the value of hot hardness, at whi-

hardness decreased to 58 HRC. The results of measuring hot hardness are given in Table 1.

The change in hot hardness shows that manganese lowers the hot hardness of cast high-speed steel of the R9 type. An addition of 0.68% manganese lowers the hot hardness of this steel from 650 (heat 1) to 640 (heat 30), and with 2% manganese (heat 90) to 625°.

The addition of niobium to high-speed steels with manganese increases hot hardness: in steel with 0.68% manganese from 640 to 647° (heat 060), and in steel with 2% manganese from 625 to 637° (heat 00).

The combined addition Nb + Mn has a more favorable effect on hot hardness than the addition of manganese alone. If, with the addition of manganese to R9-type steel (heat 1, Table 1), the hot hardness decreases, then with the introduction of Nb + Mn it rises again, remaining, however, below the hot-hardness level of steel without additions (heat 1), but above the level of steel with the addition of niobium alone (heat 80).

Apparently, the simultaneous addition of 0.68% manganese and 1.72% niobium increased the alloying of the solid solution as compared with steel containing 1.62% niobium (heat 80) and with steel containing 0.68% manganese (heat 30). Owing to the increased alloying of the solid solution, the hot hardness correspondingly rose from 642 (heat 80) to 647° (heat 060).

The cutting performance of the steel compositions studied was determined on undercutting tools with a cross section of \(9 \times 9\) (\(\alpha = 10^\circ\), \(\gamma = 18^\circ\), \(\varphi = 60^\circ\), \(\varphi_1 = 15^\circ\)) in longitudinal turning of shafts made of carbon steel with hardness \(HB = 111\ \text{kg/mm}^2\) and stainless steel with hardness \(HB = 167\text{–}174\ \text{kg/mm}^2\). After casting, the tools for testing underwent triple tempering at \(550^\circ\)—1 hour. The hardness varied within the range 62–64 HRC. The cutting conditions were selected so that in one case the tools worked more for wear, and in the other more for heating of the cutting edge. In the latter case, cutting performance was determined mainly by the hot hardness of the steel.

On carbon steel, turning was carried out at \(V = 136\ \text{m/min}\), \(S = 0.26\ \text{mm/rev}\), \(t = 2\ \text{mm}\), and on stainless steel at \(V = 70\ \text{m/min}\) with the same feed and depth of cut. In both cases, the cutting test was carried out without cooling.

When turning carbon steel, failure of the tool occurred mainly because of wear of the cutting edge, whereas when turning stainless steel, failure of the tool occurred as a result of loss of hot hardness. Tool life was taken as the time of tool operation until complete loss of cutting ability. The test was carried out on a 1A62 lathe, which made it possible to maintain a constant cutting speed by means of stepless regulation of the electric-motor speed. The test results are given in Table 1.

An addition of manganese to cast R9-type steel in an amount from 0.68% to 2% increases cutting tool life.

The combined addition Nb + Mn (heats 060 and 00) increased cutting tool life to an even greater extent than the addition of manganese alone.

In both cases, when turning carbon steel, the increase in tool life is more significant than when turning stainless steel. Apparently, the reduced hot hardness of cast steels with manganese also accounted for the obtaining of such results.

Conclusions

1. An addition of manganese to cast high-speed steel of the R9 type in an amount of 0.68% to 2% lowers hardness in the as-cast state, requires an increase in the number of temperings to obtain optimum hardness, increases the hardness of steels in the annealed state, and lowers hot-

resistance (Table 1). All these phenomena are due to the ability of manganese to expand the γ-region and to increase the stability of retained austenite, and to lower the temperature at which pearlitic and martensitic transformations begin.

1. The introduction of 1.72–1.80% niobium into cast high-speed steels containing manganese reduces the negative influence of manganese listed above (item 1), and at a manganese content of 0.68% reduces it almost to nothing.

2. An addition of manganese in amounts from 0.68% to 2% to cast high-speed steel, as well as a combined addition of manganese (0.68% and 2%) and niobium (1.72–1.8%), increases cutting life. Moreover, the combined addition increases life to a greater extent than manganese alone.

LITERATURE

1. D. M. Likhosherstov. The influence of niobium on the structure and properties of high-speed steel of the R9 type. Izv. TPI, vol. 114, 1963.

2. N. P. Aristov. The influence of nickel, manganese, and copper impurities on the technological and cutting properties of high-speed steel. Stal’, Nos. 7–8, 1941.

3. N. A. Minkevich. Low-alloy high-speed steels. Metallurgizdat, 1944.

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

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

6. Yu. A. Geller. Tool Steels. Metallurgizdat, 1961.

7. V. S. Mes’kin. Fundamentals of Steel Alloying. Metallurgizdat, 1959.

8. L. I. Kogan and R. I. Entin. On the influence of elements forming hard-to-dissolve carbides on the decomposition of austenite. DAN SSSR, vol. XCIV, No. 4, 1954.

9. E. Houdremont. Special Steels, vol. II, 1960.