Heat causes the molecules to vibrate and spread out causing more room by expanding. This makes it easier to bend.
Text is such a poor way to communicate humour. You are probably joking, making reference to the the whole packing-molecules-closer-to-make-steel-denser-by-hammering thing, but it is hard to tell.
For the folk who didn't watch that discussion get hammered out, steel isn’t actually made of molecules. Atoms arranged into crystal structure grains, not molecules.
Bending is either elastic, or plastic. When clamped in a straightening fixture, that is elastic, if you took the blade out without heating it would return to it’s warped shape. The degree of counter bend applied by the fixture does not account for straightening even if the threshold for plastic deformation is lowered with the tempering heat following the quench. If it was just “easier to bend” there would still be hysteresis and the blade would still be warped after heating in a straight fixture, just less warped. Also, the charts I have seen show that yield strength doesn't show much change till temperatures of 500degC and we usually temper well below that, so I don't think "easier to bend" is the explanation.
If we are going to talk about straightening warp, might as well talk about why warp happens, like why grinding on one side of an annealed blade causes warp on quench, or what happens with uneven heating, or uneven cooling. I am interested. I have some ideas about why this happens, but I am no metallurgist and it has been a while since I did any serious reading on the subject (
Larrin
's book while excellent doesn't count by itself).
Regards
Chris
