Hi Gregor, Ian,
I had a really good colour chart in a metalworking text back in
school days but that link's the nearest I've found to it.
What you're trying to do is snap freeze the steel in a particular
state, rendering it ridiculously glass-hard, then re-heat it just
enough to partially reverse the process so it's still hard enough to
be useful without being as brittle as glass. That's my condensed
layman's view of the process.
A useful technique I first learnt from my school metalwork teacher
was to heat the end couple of inches of a screwdriver to cherry red,
quench the end inch or so, give the tip area a quick clean with
emery and then watch the oxide colours as the heat flows into the
tip from the rest of the shaft. Quench to taste.
If that sounds too rushed a similar technique is to quench harden
the whole end. Clean up in slow time, then wave the flame gently at
the shaft an inch or two back from the tip and watch the oxides at
the tip as the heat flows down the shaft as above. Again, quench to
taste.
BTW, "taste" varies with application. A tile scoring scriber can be
pretty hard. If a cold chisel were that hard it may chip in use.
These methods ensure the tip is as hard as you need while the
adjacent shaft is not left glass hard. If you try to do the
tempering process from the tip end you'll likely leave a brittle
glass-hard zone just above the tip.
Grab an old screwdriver and have a play. It all sounds far more
complex than it is until you do it. It can be. Professionally,
programable temperature controlled ovens are used for repeatable
results on large jobs like car leaf springs. But a propane or butane
torch can do pretty well in the backyard shop using nothing more
than the colours and a bucket of water. Just as long as you start
with a tool steel. Mild steel doesn't have the right range of carbon
content (unless you case harden it but that's another subject).
John
--- In 7x12minilathe@..., "steam4ian" <fosterscons@...>
wrote:
G'day Gregor & John.
I seem to have good results heating to dull cherry red and
quenching.
Then cleaning the surface back to bright. Tempering is done by
heating the body of the tool until the it is straw generally and
mauve going towards purple at the tip and then quenching the tip
only. do'nt over heat the tip or it wil be brittle
It has worked marvels on a cheap cold chisel and some spring steel
rod I bought from a model shop.
The colour charts in John's link are most helpful. The whole
process
sounds much more complicated than it realy is, try it. The spring
steel from the model shop is a cheap material for experimentation;
it
is used for aircraft undercarts.
You haven't given the size of the torch. I have used the wok
burner
on the kitchen stove!
One good turn deserves another.
Regards,
Ian
