Effect of Small Boron Additions on the Properties of Cast High-Speed Steel
Abstract Generated abstract
This study examines how small boron additions affect the structure and mechanical properties of cast R18 high-speed steel tools produced by centrifugal casting in metal molds. Experimental heats containing 0 to 0.1 percent boron were cast, tempered, and evaluated through hardness, bending, impact toughness, and metallographic observations, with triple tempering at 580 degrees Celsius identified as the preferred treatment. Boron modification produced a fine-grained structure with thin eutectic interlayers, while the main mechanical improvements appeared after tempering rather than in the as-cast state. The best impact toughness was observed at 0.007 percent boron, whereas higher boron contents tended to reduce strength and toughness.
Full Text
EFFECT OF SMALL ADDITIONS OF BORON ON THE PROPERTIES OF CAST HIGH-SPEED STEEL TOOLS
N. D. Tyuteva, Yu. A. Evtoshkin
(Presented by Doctor of Technical Sciences, Prof. A. N. Dobrovidov)
The question of increasing the strength and toughness of cast high-speed steel is of great importance for the further development and introduction into production of cast tools. However, up to the present time the mechanical properties of cast high-speed steel have been studied insufficiently, especially the change in mechanical properties under the influence of small additions of boron.
The purpose of the present work was to study the influence of boron on the properties of tools made of high-speed steel cast in metal molds.
The material of the investigation was cast high-speed steel of grade R18 with additions of boron within the concentration range from 0.007 to 0.1% and cast steel of the same grade without boron.
Melting of Experimental Steels
All steels were melted in a high-frequency furnace with a crucible capacity of 2 kg.
Casting was carried out into metal molds (see Fig. 1) by the centrifugal method. For this purpose, a single-spindle centrifugal casting machine with a vertical axis of rotation and a speed of 500–600 revolutions per minute was used.
Table 1
| Heat No. | carbon | tungsten | chromium | vanadium | Boron content, %, by synthesis |
|---|---|---|---|---|---|
| 1 | 0.82 | 18.0 | 3.8 | 0.9 | — |
| 2 | 0.79 | 18.6 | 3.1 | 0.6 | 0.007 |
| 3 | 0.83 | 17.1 | 4.0 | 0.8 | 0.015 |
| 4 | 0.78 | 18.2 | 3.6 | 1.2 | 0.03 |
| 5 | 0.84 | 17.4 | 3.0 | 0.7 | 0.1 |
Boron was introduced into the crucible after deoxidation in the form of ferroboron powder 70–90 sec before pouring the steel into the molds. The steel was deoxidized with aluminum, calculated for an addition of 0.1% aluminum.
During remelting of the steel, carbon burnoff was compensated by introducing, in the calculation, 0.2% of it. For this purpose, to 2 kg of the steel being remelted we added 80 g of an alloy consisting of carburized R18 steel. By remelting R18 steel with a large amount of electrode breakage, we were able to obtain an R18-based alloy with 5% carbon. The addition of such an alloy, without reducing the main quantity of alloying elements in the steel, significantly increases the carbon content in the melt.
Fig. 1.
The temperature of the steel before pouring was maintained within the range 1450–1480°C.
For mechanical tests, specimens measuring 10X10X60 mm were cast.
Heat Treatment of the Specimens
Investigation of the as-cast and tempered steel, as well as of the hardened and tempered steel, shows that the hardened and tempered steel has poorer mechanical properties owing to the presence in it of a naphthalene-like fracture. As is known, a naphthalene-like fracture occurs during secondary hardening without an intermediate annealing operation. In addition, studies of hardness and microstructure show that hardening of specimens made of high-speed steel with boron additions takes place during cooling after casting into metallic molds. Therefore the heat treatment of the specimens consisted only of the tempering operation. As can be seen from the graph (Fig. 2), the tempering temperature of cast high-speed steel with a boron addition should be taken as equal to
580–600°. The increase in the tempering temperature of cast steel in comparison with forged steel is explained by the greater alloying of the austenite, since the hardening of cast steel takes place from higher (optimal) temperatures in the casting process. Triple tempering at 580° gives a considerable increase in hardness, especially in steel with an addition of 0.007% boron (Fig. 3).
Fig. 2.
On the basis of the indicated results, the optimal tempering regime adopted was triple heating at 580° for one hour each time, followed by cooling in air. After tempering, the specimens were subjected to grinding.
Fig. 3.
High-speed steel cast in metal molds is distinguished by the presence of large internal stresses; therefore, immediately after casting, the specimens must be subjected to tempering. For the same reason, cast high-speed steel is less susceptible to the formation of grinding cracks if it has been subjected to triple tempering.
Structure of the Steel
The quality of a cast tool, its durability and mechanical strength are determined by the structure of the steel used. As is known, high-speed steel cast in sand has low impact toughness because of the presence in its structure of a brittle carbide eutectic, which is arranged in the form of interlayers. When high-speed steel is cast in metal molds, owing to the high cooling rate, the carbide eutectic has the form of a fine broken network. The steel is obtained fine-grained, which is further promoted by the introduction of boron as a modifier. After casting, the structure of the steel consists of martensite, retained austenite, and ledeburitic eutectic. During tempering at \(580^\circ\), internal stresses are relieved and retained austenite transforms into martensite.
Mechanical Tests
Of particular value in mechanical tests are bending and impact tests, i.e., the kind of loading that the tool undergoes in service. It is known that this type of cast instru-
Table 2
| Heat No. | Boron content, % | Type of heat treatment | Bending strength, kg/mm² | Impact toughness, kgm/cm² | Rockwell hardness |
|---|---|---|---|---|---|
| 1 | 0 | after casting | 56—61 | 0 | 60—63 |
| 1 | 0 | after tempering | 64—100 | 0—0.3 | 63—64 |
| 2 | 0.007 | after casting | 48—52 | 0.2—0.4 | 61—62 |
| 2 | 0.007 | after tempering | 84—112 | 0.5—0.7 | 65—66 |
| 3 | 0.015 | after casting | 50—58 | 0.1—0.2 | 62—64 |
| 3 | 0.015 | after tempering | 100—118 | 0.3 | 64—65 |
| 4 | 0.03 | after casting | 49—56 | 0.1—0.4 | 61—62 |
| 4 | 0.03 | after tempering | 70—95 | 0.2—0.5 | 64 |
| 5 | 0.1 | after casting | 50—54 | 0 | 60—62 |
| 5 | 0.1 | after tempering | 76—93 | 0.2 | 64—65 |
tools, such as parting-off cutters, while having good cutting properties and red-hardness, fail because of breakage under the action of certain bending and impact loads. Therefore, in the present work, primary attention was devoted to increasing the strength and impact toughness of cast high-speed steel.
For bending tests, eight specimens were taken from each heat; of these, four were tested in the as-cast condition and four after tempering.
The experiments were carried out on a Gagarin press using a fixture for testing small specimens. The tests were accompanied by automatic recording of diagrams. The diagrams show that fracture of both as-cast and tempered specimens occurs without plastic deformation.
Impact-toughness tests were performed on 12 specimens from each heat; of these, 6 specimens were tested in the condition after casting and 6 after tempering. The tests were carried out on a pendulum impact machine with a variable energy reserve. This made it possible to select the fracture energy so that the obtained values of impact toughness were greater than the possible error and scatter of the results. The specimens were made in accordance with GOST 1524-42. The notch in the specimens was made after heat treatment, which made it possible to avoid the appearance of casting and thermal cracks in the notch.
The results of the mechanical tests are given in Table 2.
Conclusions
The structure of boron-modified cast high-speed steel has a fine grain with thin interlayers of eutectic.
The introduction of boron does not produce a significant increase in the strength properties of cast, heat-untreated steel. However, specimens subjected to triple tempering at \(580^\circ\) showed, in testing, higher and more stable values of the bending strength limit.
Impact toughness reaches its maximum value with the addition of \(0.007\%\) boron. A further increase in boron leads to a decrease in the strength and toughness of the steel.