why do you need to heat steel when hardening, molecules etc etc
Metalurgy is a fascinating science which has been used to mark the ages of civilization: the Bronze Age, the Iron Age, etc. Ancient sword makers heated steel until the polished surface showed a straw color (which we now know to be about 550°F, and due to surface oxides) and then plunged the blade into a tank of water. The blade was quickly removed, repolished, and observed closely for heat transfer from the core returning the surface to straw color. If so, it was quenched again. This technique left the surface hard without making the blade brittle. Today the steel crystal lattice is known to be either face centered cubic (iron atoms at the corners of a cube and iron atoms centered in the faces of the cube) or body centered cubic ( iron atoms at the corners of the cube and an iron atom at the geometric center of the cube). Most experienced machinists can estimate the temperature of steel from the color of its surface oxides.
Heat treatment deforms the crystal micro structure of the Steel. With heat (and cold) treatment the organisation or Diffusion of the micro particles can be manipulated. There is a higher concentration of particles after heat treatment, thus making the micro structure of the steel more dense, and the physical performance of the steel better. Stronger. Hope this helps ! Please like.
Gary is right, but maybe I can simplify it. It's very complex..... When you heat (hardenable) steel the carbon and iron form crystalline structures. Martensite and Austenite are most prominent, but there are others. Those are the hard crystals. When you quench it, those structures don't have time to dissolve as they would with slow cooling, so you freeze the crystalline structure in that state. Since that is too hard for most things, you temper it. Heating to a certain temp lets ~some~ of the crystals dissolve away. Annealing is heating and letting it cool slowly, so those structures will break up back into carbon and iron, leaving the steel soft. It's like a little atomic machine - crystals form, dissolve away, form again, with heat. It's waaaaay deeper than just that in detail, but that's the gist of it.
Halps is off base a little. The reason heat is involved in hardening of steels depends, a little bit, on how you are doing the hardening. If you are hardening by work hardening, you do not have to heat it at all. If you are barbarizing (case hardening), the temperature is raised to increase the diffusion rate of carbon into the steel, higher C content - higher hardness. If you are Thur hardening, you have to heat the steel to the austenitizing temperature (so the structure transforms to FCC crystals with higher solubility for C) and then quench it (cool it rapidly enough to freeze the C atoms in the martensite crystal structure). If you slow cool, the steel could end up softer than it started. All of these processes involve atomic level things. If you want to understand them on the atomic level, you need to study physical metallurgy of steels. It is a subject onto itself and is really quite fascinating if you are interested.
