Effect of Small Additions of Samarium on Certain Properties of Cast R18 High-Speed Steel
Abstract Generated abstract
This study examines whether small additions of samarium can improve the cast structure and heat treatment response of R18 high-speed steel, whose use in cast cutting tools is limited by reduced toughness. Experimental melts containing 0.005 to 0.3 percent samarium were produced in an induction furnace, cast into metal molds, and assessed by hardness measurements, microstructural analysis, and progressively increasing tempering. Samarium additions increased as-cast hardness, refined the grain structure, reduced dendritic features, and shifted peak tempering hardness toward lower temperatures, with evidence suggesting a reduction in retained austenite and improved heat resistance. The authors recommend 0.005 percent samarium for refining cast R18 steel and identify 560 degrees Celsius as an optimum tempering temperature.
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PROCEEDINGS
OF THE TOMSK POLYTECHNIC INSTITUTE NAMED AFTER S. M. KIROV,
HOLDER OF THE ORDER OF THE OCTOBER REVOLUTION AND THE ORDER OF THE RED BANNER OF LABOR
Vol. 263 1975
EFFECT OF SMALL SAMARIUM ADDITIONS ON SOME PROPERTIES OF CAST R18 HIGH-SPEED STEEL
L. B. SHTERN, V. A. KASHCHUK
(Presented by Professor, Doctor A. N. Dobrovidov)
The use of cast cutting tools is hampered by the reduced toughness of high-speed steel in the cast state [1]. It is known that improvement of the mechanical properties of cast steel can be achieved by modifying it [2]. It should be noted that the question of the effect of rare-earth metals on the structure and properties of tool steels has not yet been studied sufficiently.
In the present work, the effect of small additions of samarium on the cast structure, hardness, and behavior during progressively increasing tempering of R18 high-speed steel was investigated.
The samarium additions were: 0.005; 0.01; 0.03; 0.05; 0.1; 0.2; 0.3% by charge. Casting was carried out into a metal mold. The pouring temperature was 1480–1530°C. The weight of each melt was 1.7 kg. R18 high-speed steel and metallic samarium of grade SMM-3 were used as melting materials. Experimental melts were carried out in a high-frequency induction unit in a neutral crucible. After melting was completed, the steel was deoxidized with aluminum in an amount of 0.1%. Before pouring, samarium was introduced into the steel in the form of a specially prepared master alloy.
The hardness of R18 steel cast in a chill mold is 54–55 HRC. Additions of samarium increase the hardness value to 59–60 HRC.
Analysis of the microstructure of the experimental melts showed that samarium additions refine the grain of cast R18 steel, eliminating the dendritic structure characteristic of unmodified steel. The greatest modifying effect is observed when 0.005% Sm is added to the steel (Fig. 1, a, b). With a further increase in samarium content in the steel, the grain size increases, but remains smaller than that of cast unmodified R18 steel (Fig. 1, c).
Hardness measurements of castings of experimental steels during progressively increasing tempering showed that samarium additions increase the maximum hardness value of the steel, shifting the range in which maximum hardness exists toward lower temperatures compared with R18 steel (Fig. 2). Thus, for example, R18 steel with an addition of 0.3% Sm has a hardness of 65 HRC at a temperature of 560°C, whereas this steel without samarium additions has a maximum hardness
62 HRC at a temperature of 580°C. It should be noted that additions of samarium increase the heat resistance of cast R18 steel.
Analysis of the microstructure, hardness after casting, and changes in hardness under successively increasing tempering makes it possible to conclude that samarium reduces the amount of retained austenite in cast high-speed steel R18.
On the basis of the studies carried out, it is possible to recommend, for refining cast R18 steel, the introduction into it of 0.005% Sm. The optimum tempering temperature for this steel is 560°C.
The studies carried out make it possible
Fig. 1. Microstructure of steel ×120: a) cast R18; b) cast R18 with an addition of 0.005% Sm; c) cast R18 with an addition of 0.05% Sm
Fig. 2. Hardness of steels under successively increasing tempering
to suggest an improvement in the mechanical and cutting properties of cast high-speed steel R18 when it is modified with samarium.
REFERENCES
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V. T. Svishchenko. Modification of cast high-speed steel with boron. Izvestiya TPI, vol. 75, 1954.
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Properties and applications of rare-earth metals. Proceedings of the R.E.M. conference, November 1959, Chicago, M., Ill., 1960.