[Stoves] black sheet corrosion retort kiln bottom plate

[Crispin Pemberton-Pigott]

Dear Chris

I suggest you consider trying SS 409 which is typically about 1.5 times the price of mild steel. It can be thinner so if 3mm MS was working in terms of strength, 2mm SS 409 will work as well. It is much stronger.  The patent has expired so it is available from multiple sources.

Being an SS, the heat transfer rate will be lower, something close to ½ that of MS. Being thinner will compensate for some of that. Assume you will lose 1/3 of the heat transferred from the fire under the end of the chamber, so it will take a larger external fire to get the same pyrolysing effect in side.   It has an operating temperature of more than 700 C which really suits your application.   It will go brown when heated, unlike high chrome stainless steel.  I suggest getting hot-rolled sheets, not cold-rolled because of the albedo of the CR.

This material (also known as 3CR12) is difficult to cut with an angle grinder and cutting disk, when thick. I suggest you have it guillotined instead. It is very abrasion resistant. If you are forced to cut it with a grinder, don’t let it get red hot. Cut by moving the disk contact point back and forth on the cut-line.  Using a thinner disk will help (a bit).  If it gets red hot it will harden considerably and it is already abrasion resistant.

It cuts quite well with a saw but make sure there is adequate pressure to get the teeth well into the material.  In general stainless has to have a deep tool penetration or the surface hardens.

If you are not sure what the material offered is, look for a small % of titanium, about 1/3 to 1/2 of a %.  It is a low chrome (12%), titanium-stabilised stainless steel.

It can be bend in a press brake but it will not bend back.  The elongation is about 16% (only) so plan for a larger than normal radius such as 2T or 2.5T instead of 1.5. You can weld it with any stainless steel arc rod, all of which are made from better material.  The area around the weld will harden considerably.  It spot-welds very well.

If you punch holes it will need close to 75% of the depth sheared, higher than a standard MS of 33%.  This provides a very smooth hole wall but at the expense of a much higher than normal pressure requirement per mm2 cut  Increasing the die hole clearance from 1/10thT to 1/6th or 1/5thT will probably reduce the punching force needed, if you don’t mind the bottom burr.

It will laser cut well, but is a bit messy plasma cutting.  The edge will need to be cleaned with a grinder, and the cut edge will be hard.  Nigel PP warns that above some temperature the chrome atoms will let go of the oxidation sites and condense into droplets within the material. After that there is going to be some irreversible corrosion, though I think you will be so happy you won’t care.

This material is available down to 0.3mm which is used in a double layer for catalytic converter housings.

You can buy hot-rolled sections like angle iron made of the same material.

Good luck with the demonstration.  You might want to use it in another thickness on top as well.

Regards
Crispin
From: Stoves <stoves-bounces at lists.bioenergylists.org> On Behalf Of dr.adam
Sent: Wednesday, September 18, 2019 2:53 AM
To: Discussion of biomass cooking stoves <stoves at lists.bioenergylists.org>
Subject: Re: [Stoves] black sheet corrosion retort kiln bottom plate

Dear Teddy
HELP !
You might be familiar with metal sheet dealers in Nairobi?
Near Mombasa/ Kilifi  the company komaza.com (new face of forestry) built one of my retort-kilns.
I guess they used poor quality black sheet 8"x4", 3mm for the bottom plate and they complained that already after 4 batches operation they had these horrible holes near the fire chamber.

You have any idea which kind of more heat resistant metal plates (boiler sheet?) might be available in Kenya, any name of a dealer? Kenya with its tea producing industry and use of steam boilers, should have such sheets available?
Thanks in advance.
We might see us on 5th Nov. in Nairobi Conference  :-)
Chris ADAM

[Inline image]
I have even doubt they used a 3mm sheet (they paid 7500KSH for it)?


On Monday, 16 September 2019, 13:03:33 CEST, Cookswell Jikos <cookswelljikos at gmail.com<mailto:cookswelljikos at gmail.com>> wrote:


A very interesting short video on a very clever company that's started up in Kenya

https://www.youtube.com/watch?v=EaJNe0im-eM<https://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DEaJNe0im-eM&data=02%7C01%7C%7C2fcaf5b88820466dca5408d73c051dd5%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C637043864984174515&sdata=%2FwJ4Ta7ItgWQCIKDbITy%2F7AGr1xrkiSLZJ1B6eZjyfE%3D&reserved=0>


Any suggestions for better sawdust burning stoves sub 10$ that could be locally made?

Best,

Teddy


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