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Why we do not use metal

 
gardener
Posts: 3748
Location: latitude 47 N.W. montana zone 6A
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Good Morning all;
As many of you know my Dragon has been in the care facility for several months now.
One of the procedures was a door replacement.
Yesterday I disassembled the old door...
I was rather surprised  at what I found, I had no idea!
This door was built mainly with aluminum sheet and aluminum angle, pop riveted together.
1" thick CF board protected most, some of the angle was only protected by a thin strip of superwool.
At the top of the door , that superwool failed...
This is what I found....
Aluminum melts at over 1200 F ... this piece was all the way at the front of the burn chamber... the cool end of a batch box.
If I had temps that hot as far from the fire as you could get... how hot was it at the port???
My Skamal insulated firebrick were rated for 1750 F  they failed as well...
My guess is shop dragon was hitting 2000 F +
20200704_165257.jpg
1.5" aluminum angle
1.5" aluminum angle
20200704_165308.jpg
1.5" aluminum angle
1.5" aluminum angle
20200510_040536.jpg
Skamal insulated fire brick after 5 days test burn
Skamal insulated fire brick after 5 days test burn
 
gardener
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Location: Westbridge, BC, Canada
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Excellent information to be reminded of and evidence of what has been mentioned many times over the years about soft metal around the firebox in a properly functioning rocket stove.
Now with that said, I remember Matt Walker in several of his stove chats mentioning the use of a liner for his home cook stove core made out of RA 330 Stainless and how good its holding up after much use.
Very specific and expensive stuff, but just goes to show you that there are always exceptions to the rule if you know what your doing.
 
pollinator
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Location: Bendigo , Australia
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Have to ask, What is a dragon?
 
thomas rubino
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Hi John;   We call a properly functioning rocket stove a dragon.  
With a J tube rmh burning at top efficiently the sound you hear is a roar, hence the nickname The Dragon!
The piece of aluminum angle and the Skamal insulated fire brick are both from my shop rocket mass heater.   Named the shop dragon, she was originally an 8" J tube design.
This spring I converted it to a 7" batchbox design... Batchbox's burn so much hotter that my materials started failing. I am currently fabricating new parts that should hold up to the freakishly hot temperatures that a batchbox produces!

I'm sure your concerned , but have no fear  The Shop Dragon is rising from the ashes and will roar again!
 
Posts: 86
Location: Skokie, IL
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Hi Thomas,

Great info! I've often wondered about the doors. With no welding experience I would probably look to buy one.

What's your plan for rebuilding the door and firebox?
 
Posts: 27
Location: Billings, MT
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Metallurgist here (no really, it's on my business card and everything). Pure aluminum melts at >1200 F, but alloys start melting at a lower temp (same way that mixing salt and water makes a solution that melts at a lower temperature than either salt or ice).

The solidus temperature (where an alloy starts melting) for 6061 (which this probably is) is 1080 F. Not to diminish the performance of your stove by any means, just this piece of aluminum probably didn't hit >1200 F. Would be surprised if it even hit 1100 (molten aluminum gets REALLY runny). But yeah, aluminum is definitely not a great choice for a stove...
 
thomas rubino
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Hi all;
Nick, I think this angle indeed was sold as #6061, So your numbers would be spot on.
How hot it truly got, is less important than the insulation being not sufficient for the job.
The entire door assembly was 1/8" aluminum. 1" thick ceramic fiber board protected almost all of it with no problems.
The angle failure was caused by my poor door design with too thin an insulating strip.

Any metal exposed to the heat of a rocket is doomed to eventually fail.  Unless ... it can be insulated.
The secondary air stub at the base of the port, has been known to need replaced mid burn season.  (Under extreme duty)
Special expensive, high heat alloys are an exception.
Cast iron also can handle reasonable high temps as well without warping.

Nathan;  The firebox is rebuilt using 2700 F bricks .
The new door is currently under construction... along with six other projects.  
It will be steel this time rather than aluminum. It will be insulated with 1" cf board.
When I am happy with how the project is progressing.  I will create a new thread all about it.


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