2 generic questions... and a funny PS:

[Robert Myers]

Thanks, Fred, for bringing back some Freshman Chemistry from about 50 years 
ago.  It's been a while.  ;-)


At 08:46 AM 7/25/2004 -0400, Fred Munro wrote:

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

--snip--


____

Lies - the real Weapons of Mass Destruction!

Bob
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