Electrical resistivity and thermal conductivity of tool steels at operating temperatures
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In the annealed and heat treated condition steel R18 has the lowest thermal conductivity.
Reduction of the tungsten content (R9) increases the thermal conductivity at low temperatures.
Replacing part of the tungsten with molybdenum substantially increases the thermal conductivity at low temperatures.
The lower the thermal conductivity at low temperatures, the higher the temperature coefficient of thermal conductivity, which holds true for austenitic, highly alloyed, and carbon steels (in the latter the temperature coefficient of thermal conductivity is negative).
During heating the thermal conductivity of the high-speed steels investigated (except for those with cobalt) becomes almost identical. At 800–900°C, i.e., near the critical points, steels R18, R9, and R6M3 have approximately the same thermal conductivity. In the heat treated condition the thermal conductivity of these steels is already approximately the same at 550–600°C.
The addition of cobalt to high-speed steels slightly increases the thermal conductivity at room temperature and strongly increases the temperature coefficient of thermal conductivity. At operating temperatures of 500–600°C the thermal conductivity of heat treated steel R9K10 is 30% above that of R18 at the same temperatures, which is responsible (along with the higher red hardness) for the higher cutting properties of cobalt high-speed steels.
KeywordsThermal Conductivity Cobalt Tungsten Molybdenum Electrical Resistivity
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