How To: Heat Treating at Home

I made up some new pushrods, pushrod ends and locknuts on the lathe the other day.  These parts will wear and rub with every revolution of the motor, so they need to be heat treated to become harder than standard mild steel.  One simple method is to case harden parts using a torch, with a water quench.  Using Kasenit, a better case hardening can easily be attained, say  .010” thick on the parts.  This is a suitable method for mild steels and low carbon steels.  But a different approach would be needed for through-hardening of parts, tool steels for example, or chrome-moly parts like axles or cams.  Case hardening leaves the center of the part relatively soft and ductile (able to flex a bit) while the outer skin is hard, to resist wear.

In the old days, blacksmiths would use various compounds to heat treat parts.  Common items were ground up animal bones, or very used motor oil (with lots of black carbon in it).  Adding carbon to the part allows the chemical composition to alter enough to take on good hardness properties via the heating and quenching procedures.  Of course there are other procedures involved in heat treatment such as annealing, tempering, etc, but this article will not address those.

It is pretty simple to use Kasenit, the only things needed are an oxy-acetylene torch (or a dedicated heat treatment oven) and a bucket of water: 

 

Application instructions for Low Alloy or Low Carbon Steel:
Heat the work uniformly to a bright red (1650 – 1700 degrees F), remove any scale with a wire brush, dip, roll or sprinkle the Kasenit powder on the component. The powder will melt and adhere to the surface, forming a shell around the work. Reheat to 1650 – 1700 degrees F, hold at this temperature for a few minutes and quench in to clean cold water. This will give the component a completely hard case of uniform character and depth.

 

A rosebud tip works better than a smaller welding tip.  The multiple orifices of the rosebud put out a lot of heat without a large velocity.  The part needs to get red hot, but don’t let the metal melt or to start to puddle. 

Wear your welding goggles.  Otherwise you might hurt your vision.  At a minimum, you’ll see black spots after looking at the flame.  Also, do this work outside if you can.

 

Hold the part in the flame until it is red hot throughout.  Maybe 30 seconds or so, depending on the size of the part.  Then dunk the part into the Kasenit powder and make sure to coat all the surface completely.  The powder will clump and stick to the part, making an ugly shell of carbon-type powder all over. 

 

 

Now re-heat the part and with the clumped powder until it is again red hot.  Then quickly dunk it into cold, clean water.  The clumps will drop off.  Be sure to swirl the part in the water.  You don’t want to hold it still in the water and let the water boil around the part.   That would not cool the part uniformly, as the air bubbles get hot and do not help to cool the metal.  This seems like a very esoteric thing, but it is well documented in heat treating reference books.

 

 

After the part comes out of the water, clean off the gunk, then check the hardness with an old file.  A file will not cut hardened steel.  Just push the file across the part surface and look for any cutting action.  With practice, you’ll be able to see and hear the file sliding and not cutting.   The file will slide across, just as if the part were glass.

 

The part will be discolored from the heat and the Kasenit compound.  I just put some rouge on the buffing wheel and polish the part with a few quick spins.  Shown below are two pushrod ends, one before polishing and one after.  The lower part is ready for further polishing then cleaning and nickel plating.

If the part is not hard enough, just do the process a second or third time.  Heat to red, dip in Kasenit, heat to red, dip in water.  More heating actions make the part harder, a heat soak lasting more minutes makes the depth of the hardness deeper.

Case hardening should not alter the dimensions of the part very much.  But it may warp parts, especially if they had an odd or non-symmetric shape before heat treating.