spalling of SS in a high temp kiln 1

[stilettobob]

I was approached by a friend with this question and thought this forum could help me out. "i make glass stuff in the kiln. the process is to fuse up different pieces
into a flat blank (multiple firings at 1500F) on a mullite or colloidal
silica board. the last firing is to shape it. for bowl shaped things, i
sometimes use found steel objects (kitchen bowls, woks, serving trays, etc).
glass starts moving around 1100, and fully forms the shapes at around
1200-1250F. the steel forms have a wash (alumina/kaolin clay mixture) to
prevent the glass from sticking. this wash is mixed with water and sprayed
or brushed on steel when it's around 300-400, so it will actually stick. the
steel is very smooth, as one doesn't want to impart a pattern on the glass
object.

we've found that the steel will spall, turn dark, and actually start to rust
at around 400F. when taken to 1200F or so, flakes of black metal will come
off the form, sometimes getting into the glass. sometimes we can create a
reduction atmosphere in the kiln by inserting something into it at temp to
catch fire and use the o2, but it's very hard in an electric kiln to create
an oxidizing atmosphere. i was wondering if there's any way to prevent this
rusting/spalling of the forms?"

 

[mcguire]

It sounds like the stainless behaving normally. Is it really stainless and which grade? I don't expect such behavior at those temperatures.

 

[stilettobob]

I can't imagine it's anything quality as it's used in cheap kitchen bowls. Do you think it would help to purge the kiln with in inert gas (Argon)?

 

[mcguire]

No, you want oxidizing conditions. I misspoke above. I meant to say that it didn't seem to be behaving normally.
There could be something about the wash that's screwing up the protective oxide stainless normally forms. Eliminate any sulfur or chloride-bearing components and get as close to a pure alumina wash as you can.

 

[kenvlach]

I highly recommend boron nitride for your mold coating. Both aerosol & brushable forms. Highly inert to ~2000 C. GE makes & uses:
"Glass Making
BN's high temperature lubricating properties make it ideal in glass making where it helps minimize surface defects, improve mold/die life and release, and reduce clean-up time. Most glass formulations will not stick to BN."

http://www.advceramics.com/acc/applications/release_agents/

ZYP Coatings also makes BN and other high-temp coatings. Download their catalog for release coatings for glassmaking (1.44 MB). Have used their zirconia product for protecting steel & graphite molds from liquid aluminum alloys. Better than alumina.

http://www.zypcoatings.com/

Solid graphite is pretty useful for shaping glass, and graphite paint is good for painting molds. (Less expensive than BN, too.) Might react with your iron/steel/stainless steel pipe at oven temperature, though, or make the glass slip off. Glass won’t stick to BN coating, either.

macguire is right about maintaining oxidizing conditions [Fe+2 (black) reacts with glass (even pure silica), Fe+3 is much better and Cr+3 best] and avoiding sulfates and chlorides. I agree that the behavior is not normal for stainless steel, and doubt there’s sulfate or chloride in your alumina + clay (hasn’t the stuff been leached for eons?), so my guess is that your pipe is steel. I suggest that you buy 316 stainless steel tubing or Schedule 10 pipe. My Carpenter Technology book shows a ‘safe scaling temperature’ of 1600 F. Don’t use any coating. You will be pleased that the thermal conductivity is much less than steel Sch. 40 pipe (maybe 1/5). If not available in your area, there’s McMaster-Carr (

http://www.mcmaster.com).

They also have the aerosol BN mold release (part no. 10515K35, $37.13 for 13 oz. can).