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To: Richnorth who wrote (82477)	2/24/2002 12:09:46 PM
From: E. Charters	Read Replies (1) \| Respond to of 116753

Oooo-kkk so it is hematite that is made at lower but magnetite that is made at higher? Well that is not precisely true. In fact, in a furnace, hematite is made from magnetite at elevated temperatures. In a fire oxygen is being used greedily by other abundant fuel, but produces CO2, so, in this case you have thermal baking with enough oxygen to convert steel to hematite, much like the roasting furnaces which rapidly convert magnetite to hematite at just above red heat. I know, I used to operate one. If you raise the temperature still further and conserve oxygen, you get iron again. It depends on the total environment. In a very smoky fire, you do not get the same effect as a pure flame in a burner, which has a very oxidizing part, as well as a reducing part. The conversion you get of the iron in a smoky fire is probably more likely to hematite, the FeO-Fe2O3 mix that adds to magnetite formula and iron carbide. If you did get magnetite in a fire, it would rapidly roast to hematite. If you burn steel in a wood fire, or apply a smoky acetylene flame to it, what happens to the surface? You get a red rusty scale almost immediately. That stuff is limonite-hematite, not magnetite. Hematite may be steel gray, shiny grey, deep red, or rusty. Magnetite us always black. If you burn steel well, the residue which flakes off is never magnetic. That is your clue to the iron species. Another thing the high temperatures do, is de-carburize the steel by converting C to CO2 and also hydrogenate the steel - that weakens it further. Finally of course, you get water spalling of the surface, by the water in the fuel flame, and high temperature red-yellow heat weakening too. The steel tempers in the flame. The surface skin, which is very vulnerable to scaling and oxidization, in these relatively oxygen poor environments paradoxically, is weakened by scaling and cracking, and really weakens the beam to bending stress. If you raise the heat high enough you get the steel burning as a fuel even in the presence of CO2 and water which is like a producer gas. C02 is actually a good oxidizer in certain conditions. I think long before this the beams have weakened through spalling, cracking and semi-melting to where they have failed. EC<:-}