Heat-resistance of WRe20 alloys
Heat-resistance of WRe20 is a function of wolframium and rhenium. A melting point of wolframium is 3410 °C; a temperature of rimming action equals to that one which is on the Sun’s surface (6690 °C). Wolframium alloys are in a lead through all other alloys because of their refractory qualities and hardness. Rhenium comes short of its properties in relation to wolframium because its melting point is 3170 °C, a temperature of rimming action is 5870 °C. Rhenium outstrips other metals in heat resistance, a high lasting-quality, and corrosive resistance.
Special features of alloying
Tungsten-rhenium wire that is alloyed with 5% of rhenium rises a plasticity of alloy, increasing a temperature off recrystallizing. It is very important in usage for positive thermocouple electrodes. If tungsten-rhenium wire is used in a form of negative electrode, so that it is necessary to have 35% of rhenium in composition in order to make a thermal emf as efficient as possible.
Tungsten-rhenium wire is stable to helium, dry nitrogen, hydrogen, nitrogen. An alloying with silica masteralloys increases a stability two-three times as much at a thermal cycle up to 2000 °C. Detectors WRe20 can be exploited in a vacuum environment. In order to provide more effective work in a vacuum environment at temperatures of 1800 °C and more, it is necessary to use electrodes of a big diameter. WRe20 is successfully used in a form of a differential thermocouple. These can be two equal thermocouples, connected opposite one another. These thermocouples are capable of providing a measuring of temperature drop between two junction points: conventional and effective.
A reference value of thermal emf is reducing to grade 24,59 mV at a cooldown to 1600 °C. This index equals not more than 23,31 mV at a temperature 1500 °C. A level of a graded thermal emf is 21,97 mV at a temperature 1400 °C.
One of the main demerits of these alloys is a bad reproducibility of thermoelectromotive force, for which reason there is a need to group thermocouples by category А-1,2,3. The first category A-1 gives a possibility to use alloy at a max. temperature of 2200 °C; application mode of A-2 and A-3 is limited to 1800 °C. It is strictly forbidden to use a wire of different production runs because each of them has its «own» index of thermal emf. As far as rhenium and wolframium are rare metals, they are very expensive. Although a price for tungsten rhenium wire is much more than a price for thermocouples that consist of other elements, but still they are cheaper than alternative monitoring instruments for thermal cycles in similar industrial conditions.
Tungsten-rhenium wire is used in a manufacture of a converter and measuring installation. It is also used in HWR (heavy water reactor) for measuring the highest temperatures in metal industry (hot-metals). A measurement error is very small (±0,01°С) up to 2500 °C. It makes equipment for measuring HWR the most accurate in such an aggressive environment.
A standard reference table of tungsten-rhenium thermocouples at a 0 °C temperature of exposed ends.
|Temperature of a tip °С||Thermoef mV, for temperature, °С|
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