The experimental studies of such refractory metals as tungsten, molybdenum and tantalum have so far been often performed to measure their maximum strength to hold out with their integral hardness against the heat to which they are constantly exposed. In view of the fact that c. 1000°C has been the maximum temperature hitherto used in the experiments, while higher temperature limit is in prospect for the capacity of these refractory metals, a new hardness tester has been designed for temperature as high as 1600°C, and it has been used in the present experiment for the hardness test of tungsten, molybdenum, tantalum and tungsten-molybdenum alloys.
The investigations have been carried out with the following results.
(1) Tungsten and molybdenum, when work hardened, will gain in temperature strength to certain degrees below their recrystallization temperature.
(2) In tungsten, molybdenum and tantalum the temperature required for their transition from the ductile to the brittle is considered to be in correlation with the bend point which represents the rate of hardness integrity of these metals respectively against heat, on the curve of the testing temperature.
(3) Molybdenum indicated the bend point at Th
≈0.4, where its remarkable softening began, while tungsten showed no clear bend point below Th
(4) Tungsten-molybdenum alloys, though they ranged between tungsten and molybdenum in maintaining their integral hardness in room temperature, showed even higher rata of hardness integrity against heat than tungsten in certain range of high temperature.
(5) The rate of hardness integrity of tungsten, molybdenum and tantalum against heat obtained by the present investigators were found within the range of the scattered data obtained by other investigators.
(6) Molybdenum, cast or annealed, appears to have rate of hardness integrity against heat in proportion to its tensile strength in high temperature.