1. 1) M. Humenik: Amer. Machinist, March, 23 (1959), 125;
2. M. Humenik: The Iron Age, March 19 (1959), 101.
3. 2) P. Schwarzkopf and R. Kieffer: Refractory Hard Metals, Mac Millan Co., New York, (1953), 369;
4. M. Sugiyama and H. Suzuki: J. Japan Inst. Metals, 24 (1960), 618;
5. M. Sugiyama and H. Suzuki: J. Japan Inst. Metals, 24 (1960), 622;