I've always wondered about the mechanism at work when they torch arc an I=beam. Along what will end up the inside radius they heat triangular sections of the flanges, with the base of the triangle towards the edge of the flange ... I always thought it shrunk the metal enough at the triangle base to draw the beam into an arc. But, just researched it and it's exactly as you say .... the heated triangle expands but is constrained by the surrounding cool metal which forces it into a thicker cross section then when it cools it shrinks and draws the surrounding metal towards it, creating an arc along the beam. Volume of the heated triangle is the same but the thicker cross section changes the dimensions.
Very interesting, and something they didn't cover in school as we were more concerned with fatigue failures from repeated heating and cooling in power plants. So we learned mostly about expansion rates and their long term effects on welded joints and pipe fittings.
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