While industrial heat treatment of steel
is a subject complex enough to fill several books, the average DIY
buff can expand his skills with some basic knowledge
This node attempts to explain the basics in layman
Heat treatment is threefold:
What you need:
- Annealing - The process of softening metal to make it more malleable, so it can be shaped with a minimum of effort. Steel gets brittle when it is shaped by hammering, so it is necessary to re-anneal it from time to time, to prevent cracks from forming when worked cold.
- Hardening - Steel can be either hard and brittle, or soft and malleable, with any gradation in between. The better the quality of the steel, the more hard it can be made without making it too brittle.
- Tempering - This is the process of reducing hardness to the desired level, and relieving internal stresses caused by hardening.
- A heating device - Oxyacetylene or Propane torch, LPG or kerosene stove, wood fire, or in a pinch, even an engine will do (Ever seen an engine running with the silencer off? As they say in Hell, 'There's plenty of fire for everyone')
- Tongs or other holding apparatus. A vise-grip wrench is useful for heavy stuff.
- A container of lime, or other insulating stuff - sand will do nicely if its clean and dry (don't think catbox!)
- A container of water or tempering oil.
- Good ventilation.
To anneal, just heat the hell
out of the steel (careful not to melt
it if you use an Oxyacetylene torch!) and dunk it into the container of lime. Allow it to cool fully (It takes a couple of hours because the lime or sand holds the heat in). Now the steel will be soft enough to cut and file
without much effort
Once you are done shaping your masterpiece
, its time to harden it. For most applications, you would like to temper only part of it (e.g. for a custom ground chisel
, only the tip) . So heat it white hot
again and plunge
it into the water or tempering oil (You might want to be wearing gloves
!) . The quantity of water should be large enough to cool the object rapidly. The steel will now be hard and brittle, and the hardness can be gauge
d by running a file on it.
Now for tempering, grind
a part of the surface
till its shiny
. Then indirectly heat the steel from the other side, while observing the shiny part. It will go through the following sequence of
- Light yellow
- Dark yellow
- Dark blue
The color is a handy way to gauge the temperature
, and you stop when the desired temperature (and color) is reached. The more you heat it, the more it softens, so depending on the application, you can achieve the desired level of hardness. For most cutting edge
s light yellow
is enough. Narrow axeblade
s need to be a little softer (better a bent edge than a broken one!) . Sometimes for aesthetic
reasons you might go all the way to blue
, but it is quite difficult to get an even color
ing and the resulting temper is too soft for most purpose
s. The exact hardness will depend on the quality of the steel, but you can start getting a feel for these things
after a while.
Heavy metal is cool!!