Join Date: Mar 2006
Location: CSA Consulate, Rm. 101, Glos. UK: p.s. - Real Dogs Have Feathering.
let's not get our terms mixed up.
annealing is heating the metal to the critical point then cooling it slowly to make it softer and more workable. sometimes the hot steel is covered in an insulate and allowed to cool overnight of for a couple of days...
hardening is done by heating to the critical temp, at which point the steel is no longer magnetic, then cooling it rapidly in oil or water. too quickly can cause excess stress and cracks. modern steels rarely are hardened by full immersion in cool water. hardening produces a steel that can be brittle, so this hardness is then tempered to reduce internal stresses producing a more shock resistant material.
tempering is done by reheating the hardened steel to well below the critical, holding it there for a controlled time to relieve the stresses then allowing it to cool in air. tempering reduced the hardness as well as the fragility. hard edges last longer but can flake off bits or even shatter like glass, a tad too soft is better than a tad too hard. you can bend a bent knife back to shape, but not one that has snapped.
specialty alloys can have variations in the heat treatment cycles including using liquified gases for sub-cooling to produce known properties and crystal formations. morden steels have fairly strict time and temperature regimes for annealing, hardening, and tempering that are digitally controlled with little room for deviation.
heck, asian smiths have been known to produce repeatable differential heat treatments, differential hardening, and tempering in one step by pouring boiling water from a teapot onto the edge of a weapon that has been heated to the correct color as judged by the master smith. this produces a softer less fragile spine graduating to a harder edge supported by the tougher spine. only takes a few decades of training and practice.
it's all a balancing game, you must be hard enough to hold an edge thru a reasonable amount of use before it must be resharpened, but not so hard it snaps or loses chunks of the edge. the blade must be also tough enogh to flex and return to shape without either getting permanent set, or snapping. again, like in japanese swords, a bend can be field corrected by a swordsman, a fracture cannot.
p.s. - don't take 'forged in fire' as an instruction manual in how to
produce or test, or use a good knife or sword.