Re: Header Wrap and Stress Corrosion Cracking

[bearbvd@domain.elided (Greg Hermann)]

>Dave J wrote:
>
>"One more thing about header wrapping tape - I have heard many people caution
>that the extra retained heat stresses the metal and can cause it to crack.  I
>believe this is mainly a problem on mild steel not stainless steel, but it is
>possible that I have these two backwards.  Does anyone know for sure?...."
>
>FWIW, we typically only worry about stress corrosion cracking on insulated
>stainless steel (300 series) equipment that experiences temperatures between
>55-120C.
>
>bob brady

Typically, so far as I know, intergranular stress corrosion cracking is a
problem which goes with most 300 series stsinless steels at elevated
temperatures. I think with carbon steels at the sort of temps involved, the
whole part goes away!

Type 321 and type 347 SS are immune to the problem--they are what is used
for things like tubular exhaust manifolds upstream of turbochargers (when
things are done RIGHT) and for exhaust manifolds on aero engines (where
weight and reliability  are truly serious concerns). The SAE Handbook lists
these two as being meant for exhaust manifold applications. And--insulating
a mainfold made of either of them, even upstream of a turbocharger
turbine--will not elevate their temperature to a high enough level to cause
corrosion or cracking problems.

BUT--watch out for thermal expansion--there will be enough thermal
expansion/contraction that real allowances for it, including expansion
joints, will be necessary.

Regards, Greg

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