Annealing, Hardening & Tempering Steel

The heat treatment of steels is a very large topic, too vast to cover fully here. But a small sub-set of the information available is presented here, as applied to home workshop apllications. If more information is required, use your Internet seach engine, or look up a good reference book. The Steel Knowledge Base is a good place to start. Different temperatures, alloy composition, cooling rate etc all have an effect during heat treatment.

For steel to be heat treated, it MUST contain carbon. Cast iron and most mild steel (too little carbon content) parts cannot be hardened. It is only carbon steels (silver steel, tool steel, spring steel etc) with >0.5% carbon which can be heat treated. The various terms used are annealing, hardening and tempering.

These heat treatment processes work by taking advantage of, and using, the different internal crystaline states of steel at different temperatures, and forcing that state to remain when cool. The general term is heat treatment.


Annealing

The process of softening the steel. A piece of steel is softened or annealed prior to being machined further, and relievies any stresses.

To anneal a piece of steel, it should fist be heated to approx 820°C [1500°F] (bright cherry red), and then allowed to cool s-l-o-w-l-y.

 

Hardening

Hardens the steel. Involves heating the steel and cooling rapidly. The steel will be hard, and sometimes brittle as well.

To harden, heat the part to approx 820°C [1500°F] (bright cherry red), and then quench (cool) in oil or cold water, moving the part about to ensure even cooling. The steel part will be very hard, but also brittle and may be subject to shattering. Oil quenches are slower than water.

When cold, brighten one broad surface with a piece of emery cloth in preparation for tempe

 

Tempering

Removes the brittleness that occurs during hardening. Involves partial re-heating the steel and using contolled cooling rates. Slow cooling softens the steel, fast cooling makes it hard. Tempering controls the steel toughness while maintaining hardness, and springiness returns. The steel will still be hard, a bit more flexible but not brittle.

The ideal temperature for tempering steel with 0.6% to 0.7% carbon is around 380°C [700°F] (dark blue-grey). Quenching in oil gives better results for tempering than using water.

Different applications may require different degrees of tempering.

Colours for differing degrees of tempering
[Lower temperature = harder and more brittle]
Colour Temp (°C) Typical use
Pale or light straw 230° Lathe tools
Dark straw 245° Taps, dies, milling cutters, etc
(cooled in oil)
Purple 285° Center punch, stone drills
Dark blue 300° Cold chisels
Light blue 350° Springs
Dark blue-grey 380° General use

Most oils can be used for quenching. Old lubricating or engine oil works just fine.