Does anybody have any experience-based advice on prolonging the life
of an electric kiln? I.e., will a slow and gentle ramp-up, say, 500
F. per hour, to 1800 F. or 2000 F. make it-- the heating elements
and the refractory-- last longer than bringing it up to such temps
as quickly as possible?
I just got a small Paragon PMC kiln that I intend to use in
heat-treating alloy tool steel such as 440C stainless-- heating a
piece for 5 minutes or so at 1800 F. for the hardening phase and
then tempering 4 hours at 400 F. would be typical-- and want the
kiln to last as long as possible. Seems as if thermal shock would be
hard on it.
Many thanks!
John Neary
Tesuque, NM, USA
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John in NM was wondering about thermal shock of fast heating/cooling
cycles on a kiln , so I’ll offer up my experience. I’ve used my
small Cress (C 100 E, about 8.5" cube inside) for the last 20 years
heat treating the pancake dies I make. I probably do a batch every 10
days on average, and I never considered being nice to it by using
slow heat/cool cycles. It’s on full to heat up, and when the dies are
all out after quenching, it’s off with the door open while dies are
wiped and washed, and then back on low for tempering. It was a used
kiln when I got it, in good shape. Now it has crumbling spots where
the bricks have deteriorated (but my guess is that that’s more from
physical shock of opening and closing the door thousands of times),
and I’ve replaced the element several times, and other parts once.
That’s probably about 400 loads (but according to another estimate of
how many dies I’ve made in those 20 years, it could be 600 or 700
loads). That’s a life that doesn’t need to worry about getting
prolonged, in my (sometimes) humble opinion. I’ve seen kilns used
for wax burnout that get trashed in a few short years, so heat
treating steel is pretty nice to kilns in comparison, and the (soft)
refractory material they’re made from is good at taking thermal
shock, so I wouldn’t worry about cycling slowly. Speaking of which,
it’s hot for dies right this minute…
We have been using a muffle kiln and have had to replace a lot of
elements rather quickly. I was told by the mfgr. and the shop we were
getting the new elements from, that it was the burning of the wax
that was etching the elements and that if we could remove the wax or
at least drain it rather than burn it, the elements would last MUCH
longer.
If I was programming my Paragon SC2 to do what you say, I’d bring it
up to the 1800F as quickly as possible and after the short hold time,
have it cool to temper. If you need a quick cool to 400, I’d be
cracking open the door of the kiln to drop the internal temp faster.
Everyone who owns a kiln seems to push the limits of the equipment
sometimes, but Paragon kilns seem to (so far) be up to the task.
(usual disclaimer - just a very satisfied Paragon customer).
The elements depend on an oxide coating for protection. Every time
you burn out wax you start off oxidizing then run reducing as the
wax burns then finish up oxidizing. This causes the elements so be
destroyed a little in every cycle. it will go faster each time it I
goes through a burnout cycle. This is a reason for steam de waxing
to melt out the wax at a low temperature outside the kiln. You need
high temperature steam to make it work best. Industrially they use
100 psig (340F) steam and the melt out cycle is very short. Not
practical for small scale work. Their are too many safety issues.
You should do a melt out first over water as best you can and try to
keep wax out of your high temperature kiln. A 15 psg pressure cooker
will only give you 250 F . You really need to get a bit hotter…
Doing a melt out maybe 400 F (no higher) over a pan to catch the wax
should help a lot.
jesse
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