[urq] RE: 2 generic questions... and a funny PS:

Fred Munro munrof at sympatico.ca
Sun Jul 25 08:46:18 EDT 2004

As yes, the fine art of hardening and tempering steel.

The case hardening Bob is referring to results from the formation of iron
carbide at the surface of the steel. This creates a very hard, very brittle
layer at the surface. If this is backed up by strong, ductile, un-carbided
steel inside the tool, you have a useful item. If you carbide the entire
tool it will likely shatter in use.

Most tool steels have carbon content in the steel. Heating the steel to
cherry red (~800 C) will cause the carbon to combine with the iron to form
iron carbides. If the steel cools slowly, the iron carbide breaks
dissociates into iron and carbon. If it is cooled quickly (quenched), the
iron carbide remains. In the case hardening process, extra carbon is added
to the surface by heating the part in a closed container that contains a
carbon source (powdered bone, wood, leather, charcoal, etc.). Over a few
hours the carbon generated from the organic material penetrates the steel,
forming iron carbides. The part is removed from the container and quenched.
Even quenching will result in a dull grey surface. Uneven quenching results
in a multi-coloured surface. This is where Bob's oil quench comes in -
quenching in oil can result in a bit of carbon adsorption but it will be
very small. Old time machinists used to keep cyanide powder in the shop -
dipping a red hot tool bit in the cyanide would nitride the surface, but
there were other hazards involved with the improper use of cyanide, death
being one of them.

Back to your punch.

Assuming it is good tool steel, try this:

1. clean and polish the surface
2. coat the surface with a soft soap (reduces scaling after quenching).
3. heat the punch to cherry red (the point where magnetic attraction
4. quench the punch in a clean brine solution (salt and water, brine
produces finer bubbles than water)
5. clean and polish the steel
6. draw the temper by heating it to a purple-blue colour. This should equate
to a Vickers hardness of 650-700 and is suitable for punches and chisels.

You have to draw the temper to make the punch useful. The hardening process
will have carbided the entire tool, making it brittle. "Drawing the temper"
is done by heating the tool and controlling the breakdown of the carbide.
The colour of polished steel changes with temperature, and can be used to
estimate the amount of carbide remaining.

If your tool doesn't have much carbon in the steel, you will have to add
carbon by case hardening. It would be cheaper to buy a new punch :o)


Fred Munro
'94 S4 (why is everything so hard....?)

-----Original Message-----
From: quattro-bounces+munrof=sympatico.ca at audifans.com
[mailto:quattro-bounces+munrof=sympatico.ca at audifans.com]On Behalf Of
Robert Myers
Sent: July 24, 2004 8:10 PM
To: Louis-Alain Richard; quattro at audifans.com; urq at audifans.com
Subject: Re: 2 generic questions... and a funny PS:

There is an old trick I've heard/read about but never tried.  Heat the
steel to nearly molten and then quench it in used motor oil.  Supposedly
(it sez here) some of the oil will be seriously degraded and the carbon
from the oil ends up in the outer layer of the steel forming a high carbon
content surface which is extremely hard.  The process is called case

Will it work for you?  I dunno but it might be worth a try.

At 07:45 PM 7/24/2004 -0400, Louis-Alain Richard wrote:

>1 ­ How do you re-temper the tip of a punch? I have some nice nail-pushers,
>punches, screwdrivers and various things that you blow with an hammer, but
>many are broken/worn/bent.
>  I also have a bench grinder (nice tool to regrind the tips) and a set of
>torches. I thought that heating the tip _white hot_ and then cooling it in
>water would be OK, but the tip is still too soft to my taste.
>So if there’s any good advice for re-commissioning a broken punch or
>screwdriver, I would like to know it.

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