At what temperature does steel start to warp or soften.
It depends upon the composition of steel. Carbon steel, which does not have many alloying elements, softens around 1200C while with alloying elements, this transition temperature either drops or raises depending upon what is added. Higher carbon makes this temperature drop. This is a very important question for forging since if a lower temperature is used, forging won't be easy and at higher than softening temperature, it is not easy to handle. The temperature at which steel begins to soften is, in general, much above what the metallurgists call AC1 or AC3 temperature.
Steel will only warp due to temperature under two conditions: 1. It is not heated evenly - in this case, the hotter parts will expand more than the cooler parts. 2. There are internal stresses in the steel (often from welding or cold-working - as an example, an I-beam or C-channel is usually hot worked, but some cold work can happen, so they can sometimes warp if you heat them ). This means that the steel won't warp due to temp if it's not 1 or 2. However, it could soften and fail if it has a load on it. As for softening, it really depends on what steel alloy you're looking at. Some steels (mainly the expensive alloys) don't really change until above 400 C (750 F), but mild steel really starts to weaken around 150-200 C (300-400 F) Rajashekhar has a point when it comes to forging. Steel isn't really soft enough to forge properly until above 1000C (1830 F), but if you're worried about using steel in a higher temp situation (like around a blast furnace or industrial oven), then make sure you choose a steel that has been rated to work at that temp.
