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Date: Wed, 28 Feb 2007 10:56:43 -0500
From: Ernest Christley <echristley@nc.rr.com>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: engine mount 4130 vs 304 SS
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Russell Duffy wrote:

> Something Richard Sohn told me, and I soon after found to be VERY true. 
> The HAZ of stainless weldment will rust almost as easily as mild steel
> if not properly treated.  It must either be buffed to the original
> shine, passivated/pickled, or painted.  Since you won't have access to
> the inside of the tube, going stainless may not get you much.
>
>  
> Hi Ernest,
>  
> I can't say I've ever heard of, or thought about this problem.  I know 
> that the weld will carbonize (I think that's the correct term) if air 
> is allowed to contact the weld before it cools a certain amount, and 
> shielding gas can be used inside the tube.  Would this prevent the 
> rusting you're talking about?  
>  

I don't know.  I've just tried to clean up the welds I've made 8*).  
Richard, can you jump in here?

Lynn, it's actually not very hard to weld stainless.  If I can do it 
with my Harbor Freight torch, anyone can handle it.  You just use a 
fairly rich flame.  The test welds I ran broke just like my 4130 welds, 
right next to the weldment.

Thomas, I don't see the benefit of stainless being the protection of the 
inside of the tube.  The benefit would be not having to treat the 
OUTside.  Having sandblasted and painted a Dyke Delta airframe after 
fighting the Rust Wars* for several years while I was putting it 
together, I would sacrifice a few things not to suffer that pain again.  
I wouldn't substitute any old tube into an existing design, even though 
in a typical tube fuselage the tube diameter and wall thickness are 
chosen for many reasons other than absolute strength (construction 
practicality, buckling resistance, inventory simplification, etc).  A 
switch to stainless would require a clean sheet design, but I think it 
could possibly be easier to build and lighter when all is said and done.


*Rust Wars -- you're absolutely forbidden to sweat while welding a 4130 
frame, not matter how hot it is outside.  If sweat touches the airframe, 
it must be cleaned immediately with a damp rag followed by some sort of 
oil.  Each place you fail to treat in such a manner will start turning 
reddish brown by the next day.  I used to walk around regularly with a 
scotchbright pad and a can of WD40.

-- 
         ,|"|"|,              Ernest Christley       |
----===<{{(oQo)}}>===----    Dyke Delta Builder      |
        o|  d  |o        http://ernest.isa-geek.org  |