High-speed steels with low carbide inhomogeneity

Conclusions

• 1. Two concentrations of tungsten are characteristic for high-speed steel containing no more than 1.7–1.9% V and possessing good grindability.

• a) at 7–8% W — high red hardness and secondary hardness values and a lower carbide inhomogeneity are assured; but sensitivity to overheating is preserved;

• b) at 12–13% W — the fine grain is maintained, but the steel displays a somewhat higher carbide inhomogeneity than the steel with 7–8% W.

• 2. The sensitivity of 7–8% W steel to overheating may be reduced by addition of 0.1–0.2% Ti, or 0.01–0.02%N, or 0.7–1% Ni. The above additions increase the strength and diminish the amount of residual austenite.

• 3. In order to reduce carbide inhomogeneity it is possible to decrease the contents of chromium in steel to 2.8–3%. This causes the strength to increase and the quantity of residual austenite to diminish. A drop in chromium contents to below 2.2–2.5% prevents the dissolution of complex tungsten carbides and reduces considerably the secondary hardness.

• 4. For tools with a thin cutting edge (tape, reamers, broaches, and so on) it is recommended to use steel R12 containing 0.8–9% C, 12–13% W, 2.8–3% Cr, 1.5–1.9% V, and 0.3–0.6% Ni (quenching from 1235–1255°C in oil or in hot media and triple tempering at 560–570°C).

• 5. For instruments designed for machining of hard (up to HB 200) materials under moderate cutting conditions, and which must have a low carbide inhomogeneity, it is recommended to use steel R7T (0.75–0.83% C, 7.6–8.2% W, 2.7–3.2% Cr, 1.2–1.5%V, 0.1–0.2% Ti, 0.4–0.8% Ni), or steel R7 into which nitrogen should be introduced instead of titanium (quenching from 1200–1220°C in oil or in hot media, tempering at 540–550°C, three times 1 hour each).