Newsgroups: rec.crafts.metalworking
From: John De Armond
Subject: Re: Heat treat ovens
Date: Sun, 27 Mar 94 22:16:17 GMT

russ@bbxrbk.basis.com (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

>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