Advice sought - exhaust cracking

[huntaero]

Hi Nev have a good look inside the exhaust tube fwd of the external crack, maybe the tube has cracked inside the heat muff.

[fairchild1934]

Annealing ferrous metals uses a very slow cooling -- and nonferrous often uses a heat and quench. Im not sure which category 300 series SS falls into---- I will try googleing on that topic----

[fairchild1934]

looking at the part-- my first thought on the possible movement was like a speaker cone. The doughnut ring attempting to pivot at the junction with the center pipe. Maybe being driven from the center pipe---- so the center pipe and the outer sleeve are out of phase with each other. But it could be twisting too I guess.

[Nev]

IMG_1554.jpg This picture shows what the internals of the exhaust look like. Disregard the finger pointing, it was after the #4 cylinder issue so the exhaust was in several pieces. However you can see the cone shaped baffle that normally resides inside the heat muff. The part that has now cracked two more times since then is to the left of my wristwatch where the pipe joins to the heat muff.

[fairchild1934]

Nev--- have a look at this post. Look at the longer paragraph 3/4 of the way down.
He mentions the issue of chromium depletion happening at the "toe" of the weld- which induces a crack to form. (because of the two alloys mixing at the boundary between weld and base metal. This is the "toe" area--- where the edge of the bead tapers off into the base material)

There is also mention of any carbon on the back side causes that carbon to migrate into the metal - making a local area with much higher carbon content-- which in turn makes that area drastically harder when it cools. That super hardness ensures a crack will form.

you can see on the pipe shown in the post--- the crack formed exactly in the area where the tube wall shows a "bubble" shaped bulge from being bent. This means the wall stretched more at that bubble so that spot was considerably thinner than anywhere else. That contributed greatly to the pipe deciding to crack there first. Prob way thin there. Prob hardened there from the stretching maybe.....

google says that the SS alloy can be annealed by heating to 1900 and quenching. However - this would prob have to be done in a furnace purged with argon. Not easy to find one that big with purge capacity. Tool and die guys use them but usually not big one (shoe box sized --)

Maybe just time for a new pipe ?

[Nev]

Nev--- have a look at this post

Do you mean in the post above with the photo ?