Fractured exhaust at mainifold..suggestions please

[Sinbad1]

Finally dropped the old girl into the water yesterday and on start up discovered that the exhaust elbow which attaches to the manifold was blowing. On inspection it has a small crack all around, but behind, the weld which attaches to the mild steel flange which in turn bolts to the manifold.

This is a water cooled exhaust system and is manufactured out of a considerable piece of stainless piping in the shape of an upside down U before dropping down into a water muffler.

I have removed it and left it with the stainless welder but feel that there must be a simpler and better system for this part of the exhaust system.

Any ideas appreciated.

 

[claymore]

What kind of engine and is the fractured piece a standard or modified component?
regards
js

The fairytale ends here

 

[Aja]

Seems to be a very common problem. Sort of thing you can expect where hot salt water is involved.

Happened with my father's old Volvo MD2B and also on my old Yanmar YSM12

A bronze casting would help.

 

[ccscott49]

The best way is to re-weld it, stainless that is heated and coolled regularly, tends to crack just behind the weld. How old is this elbow, mine cracked after about eight years, don't consider flexible stuff, it's worse. You might investigate if the exhaust is mounted too rigid, with slightly flexible mounts on the engine, this is what caused my problem, which I will have to live with, double gasket the join, use bronze bolts with two nuts and only just tighten the flange until there's no leaks, then use the two nuts to lock them together, this will help to relieve any stresses being set up. Periodically check for leaks and retighten when required. You could also use "thackray" washers (like a double spring washer) behind the nuts, but I dont know if they are available in bronze, but stainless nuts& bolts might be OK if you went that route. In short you need to stop the flange being in a stressed condition, like myself!!

 

[vyv_cox]

As others say, this is a very common failure but it has nothing to do with the seawater that is usually injected downstream of the fracture. The cause of failures immediately adjacent to welded joints is invariably fatigue, assuming nothing unusual has been done during the welding procedure. The fatigue is either generated thermally, due to different rates of expansion of the flange or pipe, or mechanically where a long length of fitting is hanging off a flange and being shaken by engine movement. In your case it sounds like the latter, possibly with some contribution from the former.

What you need to do is to prevent this reciprocating mass from trying to bend the attachment to the flange. Either provide some support between the fixed part of the exhaust and the engine and/or gearbox or cut the exhaust pipe fairly close to the engine, but after the water injection, and insert some rubber hose.

 

[Guest]

With all welding there is an area adjacent to the weld known as the Heat affected Zone (HAZ) Because the bulk of the weld can cool relatively slowly after welding whereas the tube alongside the weld cools very quickly due to low mass. The problem here is that this creates hardness that invariably leads to fracture within 10mm or so if vibration is involved.

Your welder should be aware of this and either arrange for the weld to be lagged to slow the cooling or if he has the facility to heat treat the weld, do so!. The other posts regarding the flexibility of the exhaust are all just as important.

Hope it gets repaired ok!

PZ

 

[vyv_cox]

There is no significant heat affected zone in a weld between low carbon and stainless steels. A hard martensitic structure will only be created if there is a significant level of carbon, normally reckoned to be above 0.22%. Neither of your materials has anything like as much as this unless they are highly unusual.

 

[Sinbad1]

Big thanks to everyone who has responded. I now understand the causes and hope to rectify this weekend.

 

[Peterduck]

Stainless steel has, I have been told, a low tolerance to "Heat Shock", which might also be termed thermal stress. I have also been told that when cars are fitted with stainless steel exhausts, the engine pipe [the first pipe in the line after the manifold] is usually plain steel. I suggets that you examine how you can support the after end of the pipe, as it would appear that it is flexing. This is what has caused the crack. The support must be anchored back to the engine or gearbox, as this will be moving as one unit relative to the hull. If you anchor the pipe to the hull, the problem will be exacerbated. All movement must be confined to the hose which is "downstream" of the end of the pipe. I hope that this helps.
Peter