What a quench actually does

Everything before the quench is shaping. The quench is where a piece of shaped steel becomes a knife or becomes scrap.

Heat the blade past its critical temperature — the point where the crystal structure changes and it stops being magnetic — soak it just long enough for that change to go all the way through, and then cool it fast enough that the structure does not have time to revert.

Too slow and you get a soft blade that will not hold an edge. Too fast, or uneven, and it cracks or warps. The window on a simple carbon steel is not enormous.

Then it goes into the tempering oven, twice, to trade a few points of hardness back for the toughness that keeps an edge from chipping out the first time it hits a bone.

Every blade gets tested after this. A file should skate off the edge and bite at the spine. A blade that fails goes in the scrap bin, and there is no version of this shop where it does not.