Tungsten melts at 3422 C. Why isn't every hot part made of it?
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TL;DR: A melting point is a number for the periodic table, not a design rule. Tungsten is very dense, hard to make and shape, brittle at room temperature, and oxidises in air when hot. Where it works, the environment is controlled and the mass is either a feature or affordable. The situation: tungsten has the highest melting point of any metal, 3422 C or 3695 K in the sources in our archive. So why is not every rocket nozzle, furnace and re-entry part made of it? The numbers Density is 19.25 g/cm3, more than four times Ti-6Al-4V (4.43 g/cm3) and about two and a half times steel. To make that concrete: a 10 mm cube of tungsten weighs about 19 g, the same cube of Ti-6Al-4V about 4.4 g. Elastic modulus is 411 GPa, about twice steel's. Thermal conductivity is 174 W/m.K, thermal expansion is very low at about 4.5 millionths per kelvin, and Vickers hardness of the pure metal is listed at 3.43 GPa. Why not everywhere? - Weight. In anything that flies, density is a tax on every kilogram of launch mass. A tungsten part has to justify being four times denser than titanium. - Making and shaping it. With that melting point you cannot cast it the usual way. It is made by sintering powder and is hard to machine, so parts are expensive and shapes are limited. It is also known to be brittle at room temperature, so you design around it. - Oxidation. Tungsten oxidises in air at high temperature (general knowledge), which is why it is so often used in vacuum, in inert gas, or under a coating. A material that melts at 3422 C is only useful at 2000 C if the atmosphere lets it survive. Where it does win Places where mass is a feature or the temperature is extreme and the environment is controlled: radiation shielding and ballast, electron-beam and plasma-facing parts, and nozzle throat inserts in some rocket designs. That last one is from general knowledge, not from our data. My take: the real design question is "what does it cost me in weight, in manufacturing and in atmosphere to keep it at that temperature?" For most hot parts the answer is that a coated alloy, a carbon composite or a ceramic is the better compromise. Which hot-section jobs do you think tungsten is genuinely the right answer for, and where is it more of a showpiece than a solution?