Heat Treating Steel

Steps

Heat the steel evenly past the point where a magnet stops sticking — about 770 °C, a bright cherry red — and hold a minute per 5 mm.

Quench: plunge point-first into warm oil (water for plain low-alloy, but it cracks more) and agitate.

Test with a file: it should skate off without biting.

Clean to bright metal so you can see the colours.

Temper in an oven at 200 °C for an hour (a knife), or watch the oxide colours run to straw or bronze with a torch.

Cool in air.

The Science

Iron's crystal structure changes above about 730 °C, and dissolves carbon into it. Quench fast enough and the carbon is trapped in a distorted lattice — martensite, glass-hard and brittle. Temper at 150–350 °C and some of that strain relaxes: the trade of hardness for toughness, tuned by temperature and time along an Arrhenius curve. Cool slowly instead and the carbon comes out as soft pearlite — annealing.

How It Fails

Cracks in the quench from a sharp corner, water on a steel that wanted oil, or waiting too long to temper. Soft spots from uneven heating or a slow quench. Warp from quenching a long blade flat.

Summary

Changing a steel's hardness and toughness by heating and cooling on purpose — annealing to soften, quenching to harden, tempering to take the brittleness back out.