View Full Version : heat treat

07-25-2004, 08:48 AM
I have been looking for a way to heat treat tool steal parts (4140, A-6 D-2) . I would like to know if a pottery kiln would be suitable for this job. They seem to reach desired temp.and can be found at a more reasonable price than heat treat furnaces. What are the difference? Any help and suggestions would be appreciated. Thanks

07-25-2004, 09:54 AM
Yes they do work,but you will need to come up with a good temperature control.One local shop uses them to heat treat 40# chuncks of D-2 with no problems.

07-25-2004, 11:38 AM
Do the digital readout I see on some provide better control or they just buttons in place of knobs?

Paul F
07-25-2004, 01:10 PM
One potential problem with a pottery kiln instead of a purpose-built heat treating oven is a kiln is not built to control the amount of free oxygen inside...
With a kiln, you might have really excessive scale and decarburization.
You may be able to adjust the burners (assuming you're talking about a gas fired kiln rather than electric) to a reducing atmosphere to reduce the problem.

Also, an ACCURATE pyrometer or temperature control is really useful.
I've done a lot of heat treating in a rather primitive propane fired forge... but it takes a LOT more practice and a lot of attention to detail to get it right without an accurate temperature control.
It CAN be done! I've done it.. but it takes more than "heat it red and quench".

I hope the Kiln works out! It sounds like a great solution!

Paul F.

07-25-2004, 01:53 PM
I just made some pins for an air rifle that
had to be hard and tough. I used metric drill rod which I heated yellow hot with a
mapp gas and oxygen torch then quenched in
oil. That hardened them like glass so I
put them in my kitchens electric stove oven
set at 550 degrees in a pie pan for 45 minutes to temper them then quenched them in oil again.
The results were hard and tough pins, just
what I was looking for. The first time I
tried just hardened them without tempering
in the oven and they broke as if they were glass.

Paul F
07-25-2004, 02:50 PM

Yup... for simple parts, you can use a torch and get good results.
Try that with a complex part with close tolerances.. and you may end up with a pretzel, a cracked part, or part with hard and soft spots.

By the way, depending on what kind of drill rod you used (O1? W1? Something else?), Yellow hot was about 500 degrees hotter than you needed.
And you did right to temper the part in the oven for toughness.
When you heat treat a part, the hardening stage brings the part to it's maximum hardness.. then you temper it to decrease the hardness to a level where it's as tough as you need it to be.

Paul F.

07-25-2004, 03:12 PM
When I heat treat air hardening steels I use stainless steel wrap to keep it from scaleing.

07-25-2004, 06:58 PM
Severals years ago, Friend needed a controlled atmosphere for heat treating electrical plug pins by the thousands. An electric kiln did the job. First runs were no good, we tried shoving a pipe through the wall of the kiln and adding gas. Pretty leaky, wound up as suggested above, wrapping in stainless sheet, putting pipe in the sheet and wrapping tight. It worked OK.

I think a good coat of furnace mortar, painted on with elements removed, then re-install the elements would have made it tight enough to avoid the wrapping sheet.

You do not need pressure, but you need to keep the air out. We added another pipe, adjusted gas flow to just slowly make a bubble expand (kids bubble soap ). Adjusted the gas flow cold, furnace loaded. I suspect we could have used even less gas (no pressure at all) since the sheet kept the air from the parts. Last I heard the man was still in business.

I never knew those electrical plugs pins had so many steps needed to manufacture.

First runs he used the pottery "cones" to get his temps. worked OK for his small parts but the buyer wanted a temp/time curve followed so he purchased a controller.

He claimed ferrous metals annealing was easy, just bring up to temp and let it cool.

He also melted/cast aluminum from the kiln- quality was often poor, but so were his practices.

07-25-2004, 07:50 PM
Hello Paul,
I used to work in a steel mill,Armco Steel in Torrance, CA. I have seen them
bury castings in a sand pit about 20 feet deep.I do not have occasion to temper anything large.I learned to temper using a
forge.First heating then quenching a portion and rubbing the cool portion on some stone and watching the color crawl onto the cool portion then requenching when
it reached the color I was looking for.
This past project was a 3mm pin about
1 1/4" long. I tried just hardening them
but they broke like glass so I put them
in my electric kitchen oven at 550 for 45 minutes. As small as the pin was and using
a torch I may have got it a little too hot
but I was concerned with making sure that
it was no longer attracted to a magnet before I quenched it.I do agree with your assesment.The drill rod was electric furnace melt oil-hardening tool steel.
C.85-100, Mn1.00-1.40, Sl.50 Max, Cr.40-.60 W .40-60, V.30 Max Rockwell B approx.100
Rockwell C approx.20.