Crack in exhaust pipe

[Bluey]

About 60 hours flight time ago, a crack in an exhaust pipe on my airborne microlight xt 912 was detected. The pipe was promptly repaired by an exhaust specialist. On Saturday after a flight I noticed a hairline crack in the heat affected zone parallel to the weld. I am wondering whether any of the exhaust repairs such as exhaust tape can be used to limit the damage and extend the life of the existing pipe before it needs to be replaced? It is only a short u shaped tube whose replacement cost is $600. It attaches to the exhaust port of a rotax 912 80 up.

 

 

[Guest Maj Millard]

It is common for cracking to appear next to a weld on exhausts. There is a weakness right next to the weld bead. Has the person who did the prev repair used the correct rod for the metal being welded ?.....Generally it's 305 stainless on rotax systems. Is the pipe mounted such that unnecessary vibration or stress is putting strain on the pipe or weld ?...........................................................................................Maj....

 

 

[Bluey]

I don't know if they used the correct weld for this exhaust except to say that they had worked on helicopter exhausts in the past. They certainly seemed to know what they were doing. Will using exhaust wrap help with this sort of problem? As far as vibration is concerned, the engine seems pretty smooth to me. I did a carnie sync just 60 hrs ago and plan to do this every 100 hours. No other pipe has ever showed any other sign of cracking.

 

 

[fly_tornado]

its possible that carbon from the exhaust has probably contaminated the weld making it brittle.

 

 

[Guest Maj Millard]

That's possible Tornado, or it just could be an old, or faulty pipe. Generally cracks on exhaust pipes want to keep going Bluey, so covering it in wrap may just hide the horrible truth until it's too late. I have on a few occasions found exhaust pipes cracked radially more than 3/4 around...The main engine-mount tube in the photos attached were the result of a radially cracked exhaust pipe on a Storch....Heating up a chromemoly engine mount tube when it is stressed is not a real flash idea !............................................Maj...

 

 

 

 

[Riley]

Considering that it's cracked twice in the same area I'd reckon the Major may have tipped a possible cause. Undue mis-alignment at the 'flex' joints or rust-frozen joints in the exhaust hook up which allows a constant force or pressure can ultimately cause a stress fracture due to engine vibration or heat/cooling cycles. Worth checking to ensure that the entire exhaust system is free and floating on the connection springs.

 

 

[ossie]

Okay, so how does one fix a crack in the window....??

 

 

 

[Bluey]

The original crack may well have been caused by a misaligned exhaust as one of the exhaust lugs showed extensive wear as did the spring that was attached to it. Needless to say, I check that part of the exhaust very carefully before each flight for any signs of wear.

 

I just spoke to the repco dealer that sold me the exhaust wrap and he assures me that wrapping a cracking exhaust is a commom remedy to extend the life of the part. I will definitely get more advice before giving this a go but he was quite adamant about it.

 

 

[dazza 38]

Okay, so how does one fix a crack in the window....??[ATTACH=full]13521[/ATTACH]

Plug the hole and fill it with putty.lol

 

 

[RickH]

Hey Guys definitely DO NOT wrap the crack as mentioned above you can be almost certain that this is a result of Heat affected Zone cracking, if this a genuine Rotax exhaust it is probably manufactured from 4130 or similar and these metals are susceptable to work hardening, annealing the whole thing may be a solution but I strongly recommend against wraping.

 

RickH

 

 

[ossie]

Plug the hole and fill it with putty.lol

may have a job finding someone to plug that hole..

 

hang on...!! here's a thought...cabin crew, yep, cabin crew.... they'll have someone............

 

 

[David F]

Heat affected zone cracking usually occours when insufficient filler metal has been deposited in a fast hot weld especially on stainless steel.

 

Dave

 

 

[RickH]

Hey Dave I suggest you do a whole lot more research on HAZ cracking, I would be glad to instruct you further but the subject is just to involved to go into on this forum.

 

 

[David F]

Hey Rick

 

Dont need no instruction from you or anybody else.I just posted a note on what I have noticed as a common weld defect on stainless steel joints based on 40 years experience in the trade.

 

 

[RickH]

Hey Dave, No offence meant, Just that all to often on these forums people post well intentioned but inaccurate material(sometimes dangerous), example the idea of wrapping the crack as suggested by the "reputable Repco Rep".

 

RickH

 

 

[RickH]

For anyone really wishing to truely understand the issues of heat affected zone cracking and its various causes I suggest you visit www.deltaschoolloftrades.com they have a very good publication on the subject, there are also several other sites out there that have some very good info.

 

RickH