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From: Ernest Christley <echristley@att.net>
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Sent: Wed, December 22, 2010 2:35:58 PM
Subject: [FlyRotary] Re: 13B Turbo Manifold

Kelly Troyer wrote:
> Ernest,
>      Not sure what you mean by "Welds not cleaned up"...........All I see on these "TIG" welds is
> a slight discoloration at the edge of the weld bead caused by the arc heat..

The first link of a Google search on "stainless steel welding corrosion" was 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 harden") contain a small amount of carbon in solution in the austenite. Carbon is precipitated out at the grain boundaries, of the steel, in the temperature range of 1050° F. (565° C) to 1600° F. (870° C.). This is a typical temperature range during the welding of stainless steel.

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

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

They could have used a filler that made post treatment unnecessary, or they could have annealed it.  The link you gave 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.


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