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[andris-berzins]

From http://www.dwarffortresswiki.net/index.php/Magma_proof  :

"According to the raw data files, magma's temperature is somewhere between 1300°C and 1400°C, which translates to 2300°F–2500°F .

Materials that resist magma are:

Metal:
Nickel — 2600°F (1427°C)
Nickel silver — 2600°F (1427°C)Verify
Iron — 2680°F (1471°C)
Steel — 2710°F (1488°C)
Platinum — 3000°F (1649°C)
Adamantine — 15000°F (8315°C)"

BUT structural steel and other metals are not fire safe construction materials. Structural steel at 400-500°C has lost its strenght almost completely. According to building codes when steel structure reaches 400-500°C it collapses. Not very realistic!  

[ikkonoishi]

Bauxite is also magma safe.

[TheSilverHammer]

The reason structural steel isn't safe to go above that temperature is because that it is simply not as strong as it was below those temperatures and if it is load bearing, then it stops doing it's fair share in supporting the building.  Then other steel supports need to pick up the slack, which is a problem if they are also very hot. It can cause a cascade failure, which is what happened in the world trade centers.

The steel doesn't melt, and in Dwarf Fortress a steel door / gate isn't load bearing, so even if it is only at 90% strength, it doesn't matter.

[andris-berzins]

Bridge over magma is load bearing structure.
All steels lose strength with increasing temperature. By 600°C, most structural steels have lost more than half their strength. At 700-800°C its strength approximately is 10% or less from strength at normal temperature. Not to mention plastic deformation. So any steel bridge over magma is imposible. Door or gate - ok if not under pressure from one side(from magma), but anyway they will deform and become unusable.

[0x517A5D]

Having studied and played with tool steel, I have to agree.  Even high-speed tool steel alloys lose their hardness at 1000°C.  This is because of a thermodynamic reaction which tends to change martensite crystals in the iron matrix into austenite crystals as the steel temperature rises.  (Proper treatment at lower temperatures, called quenching, drives the reaction the other way, hardening and toughening the steel.)  It really is unrealistic.  Even the superalloys don't go above 1100°C.

[andris-berzins]

Magma-proof materials!   Read this!
http://en.wikipedia.org/wiki/Refractory_metals  http://en.wikipedia.org/wiki/Refractory_ceramic

Best metal   for bridges and other stuff in/for magma  http://en.wikipedia.org/wiki/Tungsten
Melting point 3695 K, 3422 °C, 6192 °F

Physical properties of refractory metals, such as molybdenum, tantalum and tungsten, their strength, and high-temperature stability make them suitable material for hot metalworking applications and for vacuum furnace technology. Many special applications exploit these properties: for example, tungsten lamp filaments operate at temperatures up to 3073 K, and molybdenum furnace windings withstand to 2273 K.

[0x517A5D]

Yup, but I don't think Dwarfy McDwarfson could smelt tungsten.  If I recall, it is a very difficult process even when you get past the high heat required.

Now, carbon, interestingly, has an amazingly high melting point.  The problem is, it enters a self-sustaining reaction with certain components of the atmosphere at much lower temperatures.  In other words, it burns.

Ceramics are a more realistic proposal.  We just need to get kilns to do more than potash-to-pearlash.

[Grek]

quote:
It was first hypothesized to exist by Peter Woulfe in 1779 who examined wolframite and concluded that it must contain a new substance. In 1781 Carl Wilhelm Scheele ascertained that a new acid could be made from tungstenite. Scheele and Torbern Bergman suggested that it could be possible to obtain a new metal by reducing tungstic acid. In 1783 José and Fausto Elhuyar found an acid in wolframite that was identical to tungstic acid. In Spain later that year the brothers succeeded in isolating tungsten through reduction of this acid with charcoal. They are credited with the discovery of the element.[2][3]

Reducing acids into metals with charcoal sounds like something a alchemist would be doing.