Subject: Re: Heat treat ovens

From: [email-masked]
Date: 25 Mar 94 20:19:28 PST

In article <[email-masked]>, 
[email-masked] (Russ Kepler) writes:

> Now that I've got my 4130 problems sorted I've get a follow-on
> problem: I need to heat treat my 4130.  For this I've been looking at
> a couple of different ovens.  Paragon, in Texas, seems to have a nice
> little line of burn-out and heat treat ovens.  The smallest of their
> programmable ovens could work well, but is a bit pricey at $700 or so.

Why buy one?   They are quite simple to build.   Buy a
copy of Dave Gingery's book on the "Li'l Bertha" electric
furnace and make your own!   This book is available from 
Lindsay Publications (see the FAQ).

Basically, you start with a five gallon metal paint bucket.
Then you make a collapsable cylindrical form for the center.
Wrap the outside of this form with a piece of plastic tubing
the same diameter as your heating element, insert the form
into the bucket, and fill the space around it with castable
refractory.   When it is set, remove the center form, fill
the groove with the heating element, attach the wires, and
you are in business.   (OK, I skipped some of the details,
but you get the idea.)

The heat control is the type used on electric stoves with
continuously variable heat.   They simply control the on/off
timing ratio from zero to 100 percent, so you will have to
calibrate the furnace.

Dave shows several variations on the design so you can make
one for your exact needs.   If you don't like the results,
you are only out about $50 instead of ten times that.
You now know enough so that your second one will be perfect.

--  Alan E. Frisbie               [email-masked]
--  Flying Disk Systems, Inc.
--  4759 Round Top Drive          (213) 256-2575 (voice)
--  Los Angeles, CA 90065         (213) 258-3585 (FAX)

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From: [email-masked] (John De Armond)
Date: Sun, 27 Mar 94 22:16:17 GMT

>Who have I missed?  I'm looking for a 300degf...2000degF oven with a
>9-14" capacity, 120V at no more than 20A.  There's got to be a few
>more makers out there...

You need to look for ceramic kilns.  They work wonderfully for heat
treating and because there is such a large amount of churn in the
ceramics hobby, such kilns are almost always available used and
very cheap.  My wife has a wall full of 'em that she uses with her
art glass (and my heat treating :-).  An example is a Paragon
15" dia X 20" tall round kiln that we paid $100 for.  IT came 
complete with a "kiln sitter".  The kiln sitter is a thermostat
that uses the melting of ceramic temperature cones to trip.  The
cone implements a crude time-temperature integral device but was 
pretty useless for our glass work.  IT might work fine for tempering.

Digital temperature controllers have gotten so cheap that it's not
worth the effort to try and build one.  For example, Omega 
Engineering sells the CN9000A series 1/16th DIN self-tuning PID 
controller for about $135.  This is what I use on our kilns.  I 
chose the contact closure, duty cycle option in conjunction with a surplus
mercury displacement relay ($20) to control the kiln.  The duty cycle
option varies the on to off ratio over a specified period to regulate
the temperature within the proportional band.  This little controller
brings the kiln up fast and holds it to within a degree of its digital
setpoint.  Omega has an 800 number and will sell on open account to
almost anyone right on the spot.  Plus their ~40 lbs of hardcover
catalogs they'll send you for the asking is an education in instrumentation
and thermal technology.

I kinda doubt you'll find a kiln as large as you want that will run on
120 volts.  The aforementioned kiln which runs on 240 volts at about 20
amps, (4800 watts) takes almost an hour to reach 2000 deg F.  The
largest 120 volt one I've ever seen was about a cubic foot or so.

One thing you should be aware of about bare element kilns.  The atmosphere
inside the kiln must remain strongly oxidizing (or inert with something
like argon) or else the element life is VERY short.  A reducing 
atmosphere will kill the elements in hours (just ask me how I know!!)
If you plan on doing case hardening or similar things that produce
reducing atmospheres, you need to plan on protecting the elements or
else going with the much more expensive shielded element furnaces.
The easiest way to protect the elements is simply to purge the atmosphere
with argon.  It requires a sufficient purge that no carbon monoxide can
reach the elements.  If it does, it reduces the protective oxide layer
on the element and that leads to failure.

If you want to buy a new kiln, an outfit here in Atlanta called Olympic
makes very nice kilns much cheaper than Paragon, the price leader
in the field.  For example, a friend recently got a 3 ft X 2 ft oval
kiln for under $500.

John
-- 
John De Armond, WD4OQC, Marietta, GA    [email-masked] 
Performance Engineering Magazine.  Email to me published at my sole discretion
Suppose Clinton were choking on food and you were the only one present.
Would you: a) pack in more food, b) duct tape his mouth shut, c) handcuff 
his hands behind his back, d) all of the above.

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From: [email-masked] ()
Date: 31 Mar 1994 08:08:55 GMT

Thanks for the informative post on kilns and kiln
technology! I grew up in a family of potters, and was
aware of all the pieces-parts, but had no idea that the
prices have dropped SO MUCH!  Cool! or maybe HOT DAMN
would be more appropriate 8-)

  Any way, a bit of trivia that will probably be of
interest to other metal workers out there is the fact
that ceramic kilns are designed, and built, to heat up
and cool down rather slowly.  Heating clay up to quickly,
or unevenly, tends to to thermally stress the piece and
can cause it to warp, crack, or explode (later  by water
vapor expansion during initial stages of firing). 
Sometimes a kiln can be rewired or additional elements
added to increase its rise time...

  Also, WRT internal kiln environment, there are elements
manufactured that cast the element into a shield which
makes it nearly impervious to the atmosphere.  However
these are EXPENSIVE! 

  EBo --

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From: [email-masked] ()
Date: 31 Mar 1994 08:11:00 GMT

... also, could you provide phone numbers and addresses
for Omega, Paragon, Olympic, etc.?

  EBo --