On the Effect of Carbon Content and Casting Method on the Properties of Cast High-Speed Steels

N. D. Tyuteva and I. T. Tikhonov

Recently, considerable attention has been devoted in the domestic literature to the problem of cast cutting tools. However, the properties of cast high-speed steels, which differ significantly from the properties of forged steels, have not yet been sufficiently studied. This is a serious obstacle to the wide dissemination of cast tools.

In the present work, cast high-speed steels were investigated which, in composition, are analogous to steel EI-262, but with a varying carbon content within the range from 0.6 to 1.8%. Carbon is the most important component of high-speed steel. In forged high-speed steels, the carbon content is limited by the condition of their forgeability. In cast high-speed steels this limitation is absent, and therefore the study of the properties of cast steels as a function of carbon content is of special interest.

Production and Composition of the Steels

The steels studied were melted in a high-frequency induction furnace from scrap of steel EI-262. Loss of alloying elements due to burning was replenished by additions of the corresponding ferroalloys. The increase in carbon content was achieved by introducing specially prepared high-carbon “cast iron” based on steel EI-262. The melting was carried out under an acid slag formed during the melting of broken glass. Before pouring, the steels were deoxidized with aluminum. The composition of the heats is given in Table 1.

Table 1

From the steels melted, end-milling cutter blades were cast. Each heat was poured into metal rotating molds and into dry sand molds made from ordinary sand-clay materials.

Structure of the steels

Since the structure of cast high-speed steels has been analyzed in detail in published works [1], we shall confine ourselves to a brief description of the structures of our steels in the as-cast condition.

The structure of cast high-speed steel consists of primary dendritic formations and a network of carbide eutectic. The structure of cast high-speed steels depends on their carbon content, as well as on the casting method, and is highly varied. In addition to the carbide eutectic, the matrix structure of cast high-speed steel may consist of retained austenite, martensite, and products of low-carbon-alloyed austenite that decomposes above the temperature of the martensitic transformation. Owing to segregation inhomogeneity of cast steels, these constituents may be present simultaneously in different parts of the dendritic formations. In addition, when casting into sand, the matrix structure of high-speed steels is further complicated by the appearance of secondary carbides, which precipitate during cooling of the high-alloyed austenite from high temperatures.

When casting into a chill mold, a significant fraction of the primary dendritic formations in steels with low carbon content (heats Nos. 1 and 2) is made up of decomposition products of low-alloyed austenite. As the carbon content of the steels increases, a martensitic structure begins to predominate in the matrix (heats Nos. 4 and 5), which, with a further increase in carbon content, is replaced by a structure of retained austenite with individual rather coarse needles of martensite (heat No. 6). With an even higher carbon content in the steel, the martensite needles disappear, and only retained austenite remains in the matrix (heat No. 7). Secondary carbides are not observed in the matrix structure of high-speed steel when cast into a chill mold.

The amount of carbide eutectic in the structure of cast high-speed steel increases noticeably as its carbon content rises. At low carbon contents (heats Nos. 1 and 2), the carbide eutectic is present in the structure of the steels as a fine, discontinuous network. At high carbon contents, the carbide-eutectic network begins to occupy a significant fraction of the cast structure. In this case, when casting into a chill mold, owing to the high cooling rates, the carbide network retains its characteristic eutectic arrangement.

The structure of high-speed steels cast into sand molds is distinguished above all by the larger size of the primary dendritic formations in comparison with casting into a chill mold. Owing to coalescence of the carbides, the network loses its eutectic arrangement, and so-called “skeleton-like” forms are produced in it. The matrix structure of the steel when cast into sand is complicated, as noted above, by the precipitation of fine secondary carbides, which in high-carbon heats form a distinctive sorbite-like background. In the alternation of the structures predominating in the steel matrix as their carbon content increases, one can note a regularity analogous to that indicated above, only with a certain shift toward higher carbon contents in comparison with steels cast into a chill mold.

Magnetic Properties and Hardness of the Steels

The investigation of magnetic properties and hardness was carried out in the course of successive one-hour tempering treatments at temperatures of 100, 200, 300, 400, 500, 550, 575, 600, 625, 650, 675, and 700°C on specimens measuring \(2 \times 20 \times 30\) mm, prepared by grinding to an accuracy of \(\pm 0.02\) mm. In order to protect them from oxidation during tempering, the specimens were packed in sleeves and covered with chips. For plotting the graphs, average hardness values from 2–4 measurements were used.

Measurements of the magnetic properties were performed on an instrument whose schematic diagram is shown in Fig. 1. The core of coil 1, connected to an alternating-current supply, has an Ш-shaped form. At the location of the core slots there are two coils, 2 and 3, in which a current of opposite direction is induced. When a specimen is introduced into one of the coils, the current rectified by the cuprous-oxide rectifier 4 causes deflection of the pointer of galvanometer 5.

Fig. 1

The specimens were examined beyond their magnetic saturation. The voltage on primary coil 1 was monitored by voltmeter 6 and was regulated to a constant value by rheostat 7. Measurement of magnetic properties with an alternating-current instrument cannot claim high accuracy; therefore, the characteristic of the magnetic properties of the steels studied is given by us in instrument readings, without indicating the percentage amounts of the magnetic and nonmagnetic constituents.

The results of the investigations carried out on changes in hardness and magnetic properties during successive tempering treatments are given in Figs. 2–8. A generalization of these results is given in the graphs of Fig. 9, where the change in the properties of the steels studied is shown as a function of their carbon content.

Curves \(a\) of the graph (Fig. 9) show the change in the initial hardness of the steels as a function of their carbon content when cast in a chill mold and in sand. Both curves have a clearly expressed maximum. The decrease in the hardness of cast steels at low carbon contents is associated with the presence in their structure of regions with comparatively low hardness. The presence of these regions was noted by us in describing the structures. These regions are products of the decomposition of austenite weakly alloyed with carbon, which is obtained at low carbon contents in the steels in the central zones of dendritic formations and decomposes above the martensitic transformation temperatures. With increasing carbon content, the alloying of the austenite increases, and the hardness of cast steels rises. But, on the other hand, together with the increase in the alloying of austenite by carbon, the amount of retained austenite in the structure of cast steels increases. This, in turn, leads to a decrease in the hardness of cast steels at high carbon contents. The increase in the amount of retained austenite with increasing carbon content in the steels is also shown by the curves of changes in magnetic properties in graph \(e\) (Fig. 9).

Plate 1.

Vertical axis: Hardness, \(R_c\); Magnetic properties in relative values

Horizontal axis: Tempering temperature, °C

Legend:

— Cast in a chill mold
-- Cast in sand

Fig. 2

Plate 2.

Vertical axis: Hardness, \(R_c\); Magnetic properties in relative values

Horizontal axis: Tempering temperature, °C

Legend:

— Cast in a chill mold
-- Cast in sand

Fig. 3

Plate 3.

Hardness, \(R_c\)

Magnetic properties in fields on the order of, mOe

Legend:

— Casting in a chill mold
-- Casting in sand

Tempering temperature, °C

Fig. 4

Plate 4.

Hardness, \(R_c\)

Magnetic properties in fields on the order of, mOe

Legend:

— Casting in a chill mold
-- Casting in sand

Tempering temperature, °C

Fig.

Fig. 6

Fig. 7

Graph b (Fig. 9) gives the dependence of the maximum hardness attained in the temperature range 500–625°C during successive temperings on the carbon content of the steels. Graph c gives, correspondingly, the change in the tempering temperature that gives the maximum hardness.

Fig. 9.

Visible labels in the figure:

• Number of temperings
• Steel hardness, Rc
• Maximum hardness, Rc
• Tempering temperature giving the maximum hardness, °C
• Tempering temperature above which hardness falls to 60 Rc, °C
• Temperature interval of maximum residual austenite, °C
• Martensite content in specimens after tempering, mg
• Carbon content, %
• cast in a chill mold
• cast in sand

of temperings, on the carbon content in the steels. Graph c gives, respectively, the change in the tempering temperature that gives the maximum hardness.

Fig. 8.

Visible labels in the figure:

• Heat 7
• Maximum hardness after tempering, Rc
• Tempering temperature, °C
• Cast in a chill mold
• Cast in sand

During tempering at temperatures above 500°C, in cast high-speed steels, owing to the heterogeneity of their structure caused by segregation, processes may proceed simultaneously that lead both to an increase and to a decrease in its hardness. Whereas in some regions of the cast structure the transformation of retained auste-

...of austenite into martensite, accompanied by an increase in hardness; in other, less alloyed regions, processes may occur involving the decomposition of already formed martensite and the coarsening of carbides, leading to a decrease in hardness. At first, as the carbon content in the steels increases, the role of the transformation of retained austenite into martensite during tempering increases, and the value of the maximum hardness rises. But, on the other hand, an increase in the degree of alloying of the austenite, as shown by the course of the curves in graph v, requires a higher tempering temperature for its transformation into martensite. At the same time, the rate of decomposition of martensite in the less alloyed regions increases, and, after the maximum has been reached, the curve of the highest hardness obtained during successive tempering begins gradually to decline with a further increase in the carbon content of the steels.

One of the most important characteristics of high-speed steels as a material for the manufacture of cutting tools is red hardness. In our experiments, the red hardness of the steels studied can be characterized by the value of the temperature at which the curve of hardness change at the end of successive tempering operations (Figs. 2–8) decreases to a value of \(60R_c\). The change in red hardness as a function of the carbon content in the steels is illustrated by graph g (Fig. 9). With an increase in carbon content to approximately 1.0–1.1%, a considerable increase in red hardness is observed. With a further increase in carbon content, red hardness continues to rise, but much more slowly. This course of the red-hardness curves is apparently connected with the fact that, at carbon contents above 1.0–1.1%, the possibilities of the alloying elements with a high carbide-forming capacity are exhausted, and in the carbide-forming process the role of elements forming less stable carbides increases. Graph g (Fig. 9) also makes it possible to note the considerably higher red hardness of steels cast in a chill mold as compared with steels cast in sand. The lower red hardness of steels cast in sand may be explained by the fact that, under conditions of slow cooling of the casting, secondary carbides precipitate from the austenite. This facilitates the process of carbide coarsening during tempering and leads to a more rapid decrease in hardness. Apparently, the lower value of the maximum hardness in the process of successive tempering of steels cast in sand, in graph b, can be explained in the same way.

Graph d (Fig. 9) shows the change in the values of the temperatures at the beginning and end of the transformation of retained austenite during successive tempering, as a function of the carbon content in the steels. Graph e gives the change in the magnetic properties of the steels in the as-cast state and after the final tempering at a temperature of 700°C. The decrease in magnetic properties with increasing carbon content in the steels after the final tempering, which leads practically to complete decomposition of retained austenite in all the steels, is associated with an increase in the amount of carbide phase in them.

Conclusions

1. The study of the structures of cast steels confirms the considerable advantages of chill casting. Steels cast in a chill mold have a fine-grained structure, and in the structure of the carbide eutectic, even at higher carbon contents, the formation of “skeleton-like” forms is not observed. At the same time, steels cast in a chill mold possess higher red hardness and higher hardness after tempering. Thus, chill casting, in addition to the usual advantages in the manufacture of cast cutting tools, also possesses special specific advantages.

1. With an appropriate selection of the carbon content in steels, it is possible to obtain a cast tool with sufficiently high hardness without additional heat treatment, as Penskii and Razuvaeva [2] also indicated in their work. However, since in this case the structure of cast high-speed steel contains a considerable amount of retained austenite (Figs. 5 and 6), refusal to carry out tempering operations should be considered inadvisable.

2. As the carbon content in cast high-speed steels increases, their red hardness increases. At the same time, a significant increase in red hardness is observed only up to certain limits of carbon content. Taking into account that, with increasing carbon content, the brittleness of cast high-speed steels also increases, it is advisable to raise the carbon content only within those limits that give a significant increase in red hardness. On the basis of these considerations, for cast high-speed steels of grade EI-262, a carbon content of 1.0–1.1% may be recommended.

REFERENCES

1. T. A. Lebedev and I. A. Revis.—Structure and properties of cast high-speed-steel tools, Mashgiz, 1949.

2. A. P. Penskii and Z. L. Razuvaeva.—A cast tool requiring no heat treatment, Mashgiz, 1946.