To investigate the influence of vanadium, molybdenum, tungsten and silicon on the properties of the hot-working tool steel containing 0.35% carbon and 5% chromium, the authors measured the critical point, S-curves for the transformation of austenite, as quenched and as tempered hardness, retained austenite, dimensional changes, and mechanjcal properties at elevated temperature.
The results obtained were as follows:
(1) The critical point was raised with the vanadium, silicon and molybdenum content, respectively. The Ms-point was raised with the vanadium content, butlowered with the molybdenum additon.
(2) As the vanadium content increased, the temperature of the tip ofthe pearlite knee occured at a higher temperature and the pearlitic reaction was accelerated, moving the pearlite knee to the left, and displacing the beginning line for thebainite reaction to the left. And as the tungsten content increased, the beginning line for the pearlitic reaction was moved to the left and for the bainite reaction moved to the right.
(3) Full hardness was obtained by air-cooling from 1020°C to 1050°C, and as the vanadium content increased, higher austenitizing temperature was necessary for the obtaining the full hardness.
(4) The quantity of retained austenite after air-hardening from 1020°C measured by the magnetic method increased with vanadium addition, and the tungsten and silicon had little effect on the retention of austenite. This retained austenite decomposed at a temperature between 550 to 650°C and resulted in the volume expansion.
(5) During tempering, the hardness decreased slightly up to about 300°C and then increased. Maximum secondary hardness was exhibited at a temperature between 500 to 550°C. As the vanadium, molybdenum and silicon content increased, respectively, the room temperature hardness after tempering above 500°C increased.
(6) The volume ingrement resulting from air-hardening was less than oil-quenching from the ordinary temperature. As the vanadium content increased, the amounts of iength change after air-hardening from 1020°C decreased.
(7) As the vanadium and molybdenum content incresaed, the tensile strength at elevated temperature increased and impact strength slightly decreased. And both the tensile strength and impact strength increased with the silicon addition.