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
laymans terms.
Heat treatment is threefold:
- 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.
What you need:
- 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.
The process:
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
gauged by running a file on it.
Now for tempering,
grind and
polish 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 colors:
- Light yellow
- Straw
- Dark yellow
- Brown
- Maroon
- Purple
- Violet
- 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 edges light
yellow is enough. Narrow
axeblades 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 coloring and the resulting temper is too soft for most
purposes. 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!!