Cast Cobalt High-Speed Steels

N. D. Tyutëva

In selecting a rational composition for high-speed steel, it is necessary to reconcile good cutting properties with the possibility of subjecting the steel to forging and machining. In doing so, one has to sacrifice cutting properties in favor of good workability. It is known, for example, that increasing the carbon content in high-speed steel has a very favorable effect on tool life, but worsens forgeability and increases hardness after annealing. The introduction of certain other elements into steel has the same effect. The technology for manufacturing a cast tool is much simpler and cheaper than the technology for manufacturing a forged one, namely:

“With the technology now established, the process of manufacturing a tool from forged high-speed steel is very lengthy and labor-intensive.

The elements of this process are:

a) melting the steel and casting it into ingots;
b) annealing the ingots;
c) scarfing the ingots;
d) forging the ingots into blanks;
e) annealing the forged blank;
f) machining the tool;
g) hardening and tempering the tool;
h) grinding and sharpening the tool.

The above list of technological operations indicates the complexity of the process of manufacturing a tool from a forged blank, requiring the participation of a number of shops and the use of scarce equipment and units. At the same time, it must be borne in mind that the manufacture of a tool from a forged blank is accompanied by a large quantity of high-speed-steel waste. For example, when an ingot is forged into a blank, waste amounts to no less than 35–40% of the ingot weight. If, however, steel losses are counted from the ingot to the finished tool, they reach 70–75% of the initial ingot weight. Thus, in tool production a large quantity of high-speed-steel waste is formed in the form of crop ends during forging; chips during machining; blanks rejected during mechanical and heat treatment of the tool; and also in the form of broken and worn tools. The use of these wastes, especially chips, is small, and the latter at most plants lies as dead stock. In addition, producing the required tool geometry when manufacturing it from a forged blank is associated with complex machining performed on various machine tools. Reducing the workload of machine tools in tool manufacture will make it possible to save a considerable amount of labor and funds.

To assess the significance of this problem, it is enough to point out that, according to data of the Second All-Union Conference of Toolmakers, the stock of sta-

of toolmaking production is determined at 20% of the total machine-tool stock of the machine-building industry. From the foregoing it follows that the question of a fundamental rationalization of toolmaking production is of great urgency and requires the earliest possible solution.

A number of the questions indicated above find their solution in the manufacture of cutting tools by casting them in a mold. In this case certain operations, inevitable when a forged blank is used, “are completely eliminated” [1].

The process of manufacturing a cast tool according to the technology we adopted consists of the following:

a) melting the steel and pouring,
b) heat treatment—tempering,
c) grinding, sharpening.

“In comparison with the process of manufacturing a tool from a forged blank, here the operations of annealing ingots, scarfing them before forging, forging the ingots into blanks are eliminated; the amount of machining is reduced. Thus, scarce equipment is relieved, labor costs are reduced, losses of high-speed steel are decreased owing to a considerable reduction of machining allowances, and the entire technological cycle of tool manufacture is shortened” [1].

All these considerations led us to the idea of manufacturing tools by casting.

In manufacturing tools by casting, it becomes possible to vary arbitrarily the chemical composition of the cast steel, increasing the carbon content or introducing such additional alloying elements as cobalt, boron, molybdenum, or increasing the amount of tungsten.

In the first heats we tried to increase the carbon content by introducing graphite into the crucible in various amounts. The powdered graphite was packed in an envelope made of sheet iron and introduced into the crucible in this form. It was not possible to alloy the steel with carbon by this method. Chemical analysis of the heats did not reveal an increase in the carbon content in heats alloyed with graphite.

Fig. 1

The second method of alloying with carbon was proposed by Professor A. N. Dobrovidov. According to this method, the carbon content is increased by adding to the charge a cast iron specially melted for this purpose.

Manufacture of synthetic cast iron: waste high-speed steel was placed in the crucible alternating with electrode scrap. After melting, the steel was stirred with a carbon electrode for 20 minutes. As a result, very brittle white cast irons with a carbon content from 3.8 to 5.0% were obtained. The microstructure of the synthetic cast iron is shown in Fig. 1.

Alloying with cobalt was carried out by adding 5, 10, 15% cobalt to the charge.

Table 1

Chemical composition of the heats, in percent

Effect of cobalt on the properties of high-speed steel

On the basis of studies investigating the effect of cobalt on the properties of high-speed steel, the following may be stated: cobalt raises the tempering temperature of high-speed steel; for example, after tempering at 700° in steel with 15.65% cobalt, as Academician Gudtsov [2] indicates, the hardness values correspond to \(R_c = 61\text{–}62\), as against \(R_c = 54\) for steel without cobalt. Cobalt lowers the stability of austenite in steels with a cobalt content of more than 9%. This is explained by the fact that “cobalt greatly increases the rate of migration of carbon atoms in the crystal lattices of austenite” [2]. Cobalt causes a process of dispersion hardening of the \(\alpha\)-phase, which proceeds especially intensively in ferrite alloyed with cobalt.

The increase in red hardness of high-speed steel under the influence of cobalt is well known. The increase in red hardness is the more effective, the higher the quenching temperature. Cobalt forms solid substitutional solutions with iron.

“Measurement of the lattice period of the \(\alpha\)-phase showed that, as the cobalt content increases, the lattice period decreases (from 2.8665 to 2.8650 angstroms), which indicates replacement of iron atoms by cobalt atoms” [2].

“Chemical analysis of the carbide precipitate of tempered steel revealed the presence of cobalt in the carbide. In the presence of cobalt there occurs a process of depletion of the carbide in tungsten and enrichment of the solid solution (austenite) in tungsten.

Course of the process:

\[ \mathrm{Fe_3W_4C} \rightarrow \mathrm{Fe_6W_2C} \]

It may be assumed that part of the iron atoms in the carbide will be replaced by cobalt atoms” [2].

Consequently, cobalt increases the solubility of carbides in austenite. Academician Gudtsov and Engineer Gelfand [2] draw the following conclusions from their work:

1. Cobalt promotes a more favorable redistribution of tungsten in the steel, reducing its concentration in the excess carbide and increasing it in the $\alpha$-phase, thereby increasing the alloying of the martensite with tungsten. This is probably connected with the considerable increase in the solubility of carbides in austenite.

The greater alloying of martensite with tungsten is evidently the reason for the increased red hardness of cobalt high-speed steel.

1. The observed decrease, in the presence of cobalt, of the lattice parameter of the $\alpha$-phase (martensite) should make the precipitation of carbides from the $\alpha$-solution more difficult. Considering, however, that the change in the parameter is relatively small, this fact should be assigned secondary importance.

2. The increase in red hardness of cobalt steel can also be partly explained by the fact that, in the presence of cobalt, the process of dispersion hardening of the $\alpha$-phase, leading to an increase in hardness, undergoes greater development.

Heat Treatment of End-Mill Cutters

The heat treatment of the cast cutters consisted only of tempering. It is assumed that the cutters were quenched during cooling in the chill mold. The results of changes in Rockwell hardness after three one-hour tempers at different temperatures are given in Table 2.

Table 2

Rockwell hardness ($R_c$) of end-mill cutters after three one-hour tempers

Note: After the first temper the cutters were cooled in air; after the second and third, they were cooled together with the furnace.

The hardness of the end-mill cutters after three one-hour tempers:

I temper at $580^\circ$, cooling in air,
II temper at $600^\circ$, cooling in air,
III temper at $620^\circ$, cooling together with the furnace.

To increase toughness, the cutters were given a higher temper (see Table 4).

After tempering at $t = 645^\circ$, the cutters of heat No. 5 gave the maximum durability, which is explained by the increased toughness of cobalt steel when tempered at $t = 645^\circ$. When cutters of the same heat were tested after tempering at $t = 580^\circ$, the cutters were characterized by increased brittleness.

Table 3

Hardness values after
III tempering at \(t = 620^\circ\)

Table 4

Rockwell hardness of face-milling cutter blades after fivefold tempering for one hour

Results of blade tests

Eight different grades of cobalt cast steels were subjected to testing. The endurance of the cast blades was compared with the endurance of forged high-speed steel (RF-1) with hardness \(R_c = 65\).

The test of face-milling cutter blades was carried out on ShKh-15 steel with hardness \(H_b = 200\), at a cutting depth of \(3\ \text{mm}\) and a feed of \(0.274\ \text{mm/rev}\).

The geometry of the cutting elements was as follows: angles in the longitudinal plane (parallel to the axis of the cutter):

front angle \(\gamma_1 = +10^\circ\),
back angle \(\alpha_1 = +13^\circ\).

Angles in the transverse plane (perpendicular to the axis of the cutter):

front angle \(\gamma_2 = +10^\circ\),
back angle \(\alpha_2 = +10^\circ\).

The tests gave the following ratio of endurance at identical cutting speeds (see Table 4).

Fig. 2

Fig. 3

Fig. 4

Fig. 5

Thus, the testing of the cast tool showed that it is possible to produce a cast tool not only not inferior in durability to a forged one,

Fig. 6

but significantly superior to it (see the test graph in Fig. 6, summary table of tests 6 and 5).

Table 5

Analysis of the microstructure

The microstructure after casting (cooling after casting in metal molds replaces the hardening operation) consists of primary carbides in the form of a ledeburitic network of very fine structure. In Fig. 2 is shown the structure of steel of melt No. 8 immediately after casting. The microstructure after tempering: a ledeburitic eutectic of sufficiently fine structure and secondary carbides (see Figs. 3, 4, 5).

Table 6

SUMMARY TABLE
of tests of face-milling-cutter blades of several heats.

Machined material—ShKh-15 steel. Depth of cut 3 mm. Number of revolutions \(n = 48\) rev/min.

Blade wear per one pass (over 8 minutes), columns 1–11.

Conclusions

1. In the course of cutting tests, a heat-treatment (tempering) regime was selected. The optimum tempering temperature is 640–650°.

2. The tests established that, after heat treatment, the specimens should have a hardness no higher than 65–66 Rockwell (C scale).

3. The highest durability was shown by knives of melt No. 5 (0.76% C, 14.68% W, 3.28% Cr, 1.0% V, 15% Co).

4. The chemical composition of melt No. 5 may be considered optimal for cobalt cast steel.

Literature

1. Revis I., Kvater I., et al.—Cast Tool, Mashgiz, 1945.

2. Gudtsov N. T., Gelfandt K. M.—The effect of cobalt on the properties of high-speed steels, Publishing House of the USSR Academy of Sciences, issue 1, 1947.