Study of the Structure and Properties of Cast High-Speed Steels
Abstract Generated abstract
This study examines cast high-speed steels of RF-1 type composition with carbon contents from 0.5 to 1.3 percent, comparing specimens cast in metal chill molds and dry sand molds. Using metallographic observation, hardness measurements, magnetometric analysis during tempering, red-hardness testing, and annealing trials, the authors relate carbon content and cooling conditions to carbide eutectic morphology, retained austenite, hardness, red hardness, and annealability. Higher carbon contents increased retained austenite stability during tempering and raised red hardness, while low carbon contents promoted formation of a soft delta alpha phase and lower hardness. Sand casting produced coarser structures, lower red hardness, and better annealability than chill casting, suggesting that tempering schedules for cast high-speed steels may need higher temperatures or additional cycles compared with forged steels.
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STUDY OF THE STRUCTURE AND PROPERTIES OF CAST HIGH-SPEED STEELS
N. D. TYUTEVA and I. T. TIKHONOV
In this work, cast high-speed steels were studied which, in their contents of tungsten, chromium, and vanadium, are close to steel RF-1, with the carbon content varying from 0.5 to 1.3%. The steels were melted in a high-frequency induction furnace with a tube generator. From the melted steels, end-milling cutter knives were made. To determine the influence of the casting method on the structure and properties of the steels, they were cast in metal molds and in dry sand molds. The chemical composition of the steels, with indication of the casting method, is given in Table 1.
Table 1
| No. of heats | Casting method | C | W | Cr | V |
|---|---|---|---|---|---|
| 1 | in a chill mold | 0.5 | 18.0 | 3.0 | 0.8 |
| 2 | " | 0.7 | 18.0 | 2.3 | 1.0 |
| 3 | " | 0.8 | 18.6 | 3.8 | 1.3 |
| 4 | " | 1.0 | 15.0 | 3.9 | 1.6 |
| 5 | " | 1.15 | 18.0 | 3.8 | 0.7 |
| 6 | " | 1.2 | 17.0 | 3.5 | 1.5 |
| 7 | " | 1.3 | 17.0 | 4.0 | 1.3 |
| 8 | in sand | 0.5 | 19.0 | 3.4 | 0.9 |
| 9 | " | 0.6 | 18.5 | 3.7 | 1.0 |
| 10 | " | 0.8 | 17.0 | 3.8 | 1.3 |
| 11 | " | 1.0 | 19.0 | 3.9 | 1.2 |
| 12 | " | 1.3 | 22.0 | 3.4 | 1.3 |
The structure of cast high-speed steel depends not only on its chemical composition, but also on the cooling rate of the casting. The structure of high-carbon steels (1.0–1.3% C) cast in a chill mold has only two constituents—the white phase and a network of carbide eutectic. The structure characteristic of these steels, for heat No. 5 (1.15% C), is shown in Fig. 1. The structure of steel of heat No. 11, containing 1.0% C and cast in sand, as can be seen in Fig. 2, differs by a coarser structure of the carbide-eutectic network and by the presence, in the main metallic matrix, of products of austenite decomposition in the form of fine precipitates of secondary carbides.
When the carbon content in the steels is reduced to 0.7–0.8%, both in chill casting and in sand casting, regions of a troostite-sorbitic structure, called the dark phase, appear in the structure of the steels. At the same time, in the central regions of the dark phase one can observe the presence of small light fields of the $\delta(\alpha)$ phase, the amount of which rapidly increases with a further reduction in the carbon content in the steels.
Fig. 1
Fig. 2
Fig. 3
Fig. 4
The structure of cast steels of heat No. 3, cast in a chill mold, and heat No. 10, cast in sand, each containing 0.8% C, with regions of the dark phase, is shown in Figs. 3 and 4. The structures of steels with large amounts of the $\delta(\alpha)$ phase
melts Nos. 1 and 8 are shown in Figs. 5 and 6. The low carbon content in these melts (0.5%) led to the formation in the structure of considerable amounts of the soft constituent, the \(\delta(\alpha)\)-phase, which also resulted in a reduction in the hardness of the steel. After casting, the hardness of the steel of melt No. 1, cast in a chill mold, was \(45 R_c\); that of the steel of melt No. 8, cast in sand, was \(42 R_c\).
The structure of steels cast in sand molds, in comparison with the structure of steels cast in chill molds, is distinguished by a coarser grain size and by the presence in the carbide-eutectic network of so-called skeleton-like forms. The structure of a steel with a developed skeleton-like arrangement of the carbide eutectic is shown in Fig. 7.
Fig. 5
The small size of the cast tools ensures the hardening of cast high-speed steel both when cast in metal molds and in sand molds. The behavior of the steels during tempering after casting was investigated by magnetometric analysis, the results of which are given in Table 2. The magnetometric analysis was carried out on an alternating-current instrument, and therefore the amounts of retained austenite in the steels given in Table 2 are approximate. The results of the magnetometric analysis make it possible to note the rapid increase, with increasing carbon content in the steels, in the amount of retained austenite and the increase in its stability during tempering.
Fig. 6
Fig. 7
Table 2
| Melt No. | Casting method | C content, % | Amount of retained austenite, %, after casting | Amount of retained austenite, %, after tempering: 1st, 560° | Amount of retained austenite, %, after tempering: 2nd, 560° | Amount of retained austenite, %, after tempering: 3rd, 560° | Amount of retained austenite, %, after tempering: 4th, 600° | Amount of retained austenite, %, after tempering: 5th, 620° |
|---|---|---|---|---|---|---|---|---|
| 1 | in a chill mold | 0.5 | 5.5 | 5 | 4 | 4 | — | — |
| 3 | ” | 0.8 | 28 | 14 | 9 | 8 | 3 | — |
| 5 | ” | 1.15 | 84.5 | 80 | 69 | 62.5 | 34 | 8.5 |
| 7 | ” | 1.3 | 88.5 | 84.5 | 81 | 74 | 42.5 | 20 |
| 9 | in sand | 0.6 | 7.5 | 4.5 | 3 | 3 | 2 | — |
| 10 | ” | 0.8 | 21.5 | 16.5 | 7.5 | 3.5 | 3 | — |
| 11 | ” | 1.0 | 46 | 41 | 39 | 32 | 9 | — |
| 12 | ” | 1.3 | 83 | 78.5 | 75 | 65 | 40 | 20.5 |
As Table 2 shows, when the carbon content in the steels is above 1%, normal triple tempering at 560°C leads only to the beginning of decomposition of retained austenite (melts Nos. 5 and 11), and at a carbon content of 1.3%, even after five tempers at 620°C, about 20% retained austenite still remained in the structure of the steels (melts Nos. 7 and 12). Correspondingly, the hardness of the high-carbon steels also increased considerably more slowly during tempering.
It should also be noted that, because of segregation heterogeneity, which leads to the formation in the structure of cast steels of regions of highly alloyed austenite in areas adjacent to the carbide eutectic, even at low carbon contents in the steels, after triple tempering a certain amount of retained austenite remained in them (melts Nos. 1, 3, 9, and 10). Therefore, when tempering cast high-speed steels, it will apparently be advisable to use higher tempering temperatures than those adopted for forged steels, or to increase the number of tempering operations.
The red-hardness of the steels studied was determined by the Heller method. The results of the red-hardness determination are given in Table 3. These results show a considerably higher red-hardness
Table 3
| Melt No. | Casting method | C content, % | Red-hardness, °C |
|---|---|---|---|
| 2 | in a chill mold | 0.7 | 590 |
| 3 | ” | 0.8 | 600 |
| 4 | ” | 1.0 | 640 |
| 6 | ” | 1.2 | 650 |
| 10 | in sand | 0.8 | 580 |
| 11 | ” | 1.0 | 600 |
| 12 | ” | 1.3 | 620 |
the toughness of steels cast in a chill mold, compared with steels cast in sand, and the increase in the red hardness of cast steels with an increase in their carbon content.
A cast tool made of high-speed steel, as indicated, is quenched during the cooling of the casting and thus does not require heat treatment other than tempering. However, in a number of cases a cast tool may require additional machining, for which annealing of the cast steels will be necessary. From this standpoint, testing cast steels for annealability is of some interest. Measurement of the hardness of the steels under study after annealing carried out in the manner usual for high-speed steels showed that, in steels with a carbon content of about 0.8%, when cast in sand molds, hardness values are obtained after annealing that correspond to the hardness of annealed forged high-speed steels, namely 20–23 \(R_c\). With an increase in the carbon content of cast steels, the hardness after annealing rises to values of 32–35 \(R_c\). Steels cast in metal molds are characterized by appreciably poorer annealability. The lower red hardness and better annealability of steels cast in sand molds can be explained by the presence in their structure of precipitates of secondary carbides, which facilitate the processes of their subsequent coagulation.
The investigations carried out showed that the structure and properties of cast high-speed steels vary substantially depending on the casting method and especially on the carbon content of the steels. In this connection, the study of the cutting properties of cast steels will be the subject of further investigations.