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Date: Wed, 22 Dec 2010 15:35:58 -0500
From: Ernest Christley <echristley@att.net>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: 13B Turbo Manifold
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Kelly Troyer wrote:
> Ernest,
> =20
>     Not sure what you mean by "Welds not cleaned up"...........All I se=
e=20
> on these "TIG" welds is
> a slight discoloration at the edge of the weld bead caused by the arc=20
> heat..

The first link of a Google search on "stainless steel welding corrosion" =
was=20
http://www.mcnallyinstitute.com/04-html/4-1.html

 From that site:

INTERGRANULAR CORROSION

All austenitic stainless steels (the 300 series, the types that "work har=
den") contain a small amount of carbon in=20
solution in the austenite. Carbon is precipitated out at the grain bounda=
ries, of the steel, in the temperature range of=20
1050=B0 F. (565=B0 C) to 1600=B0 F. (870=B0 C.). This is a typical temper=
ature range during the welding of stainless steel.

This carbon combines with the chrome in the stainless steel to form chrom=
ium carbide, starving the adjacent areas of the=20
chrome they need for corrosion protection. In the presence of some strong=
 corrosives an electrochemical action is=20
initiated between the chrome rich and chrome poor areas with the areas lo=
w in chrome becoming attacked. The grain=20
boundaries are then dissolved and become non existent. There are three wa=
ys to combat this:

     * Anneal the stainless after it has been heated in this sensitive ra=
nge. This means bringing it up to the proper=20
annealing temperature and then quickly cooling it down through the sensit=
ive temperature range to prevent the carbides=20
from forming.
     * When possible use low carbon content stainless if you intend to do=
 any welding on it. A carbon content of less=20
than 0.3% will not precipitate into a continuous film of chrome carbide a=
t the grain boundaries. 316L is as good example=20
of a low carbon stainless steel.
     * Alloy the metal with a strong carbide former. The best is columbiu=
m, but sometimes titanium is used. The carbon=20
will now form columbium carbide rather than going after the chrome to for=
m chrome carbide. The material is now said to=20
be "stabilized"

They could have used a filler that made post treatment unnecessary, or th=
ey could have annealed it.  The link you gave=20
says it is made of T304.  Depending on which T304, the carbon ranges from=
 .03 to .08%, so it may not be an issue at all.