stress relieving the job

[W.olly]

Hi all, i have been practicing with the Tig, not doing to bad, i am seeing if i can the nack so i can make a SS exhaust myself, now one thing i dont know is what ` stress relieve on completion` been told to do this but wasnt told what it is. i dont think he meant me with a bottle of cider though lol

anyone tell me please.

cheers

wolly

 

[BrokenBiker]

the only stress releif ive done after welding is to heat it all up to about 250c and let it cool slowly in air. That was on stainless plates that had yo be welded back to back so both pulli g in oppo directions

 

[Welderpaul]

Why would you need to stress relieve an exhaust?

 

[W.olly]

Why would you need to stress relieve an exhaust?

thats what has got me know, i will send the chap a message to see if he will reply again.

 

[BrokenBiker]

ah I didn't read about the exhaust, I assumed it was for a bracket or something. never heard of anyone stress relieving an exhaust, and I wont be doing anything to mine when I weld it up next weekend either

 

[W.olly]

what does stress relieving do, i will be making the manifold aswell if that matters.

 

[W.olly]

got this back to the reason to stress relieve

`Fit it to engine, you may find you have to Adjust it :-) pull it up on the bolts etc. Then you have to heat the whole thing up around the welds, this softens the welds and stops it from cracking`

 

[biffa]

If it has pulled the method he suggest will not relive the stress as heating all the way around will just cause more shrinkage which probably would result in more stress, you would be better of only heating one side of the weld to negate the effects of weld distortion

 

[hotrodder]

Sounds like he's got flame straightening and stress relief half confused. If you, say, 'banana' a gasket flange then a stress relief heat treatment won't straighten it as it redistributes any 'locked in' stresses evenly through creep. Flame straightening as Biffa said involves heating specific areas to counter physical distortion caused by weld shrinkage... heat metal evenly and it expands in all directions, heat one spot and when it tries to expand it's partially restrained by the surrounding cool areas so it expands more where it can during heating and then shrinks evenly as it cools which can be used to bend (or straighten) stuff

As said, there's no need to stress relief an exhaust which, being rubber mounted, has to be very badly made to crack. Exhaust manifolds... far more prone to cracking, especially if there's a turbo hanging off them, but still no need to stress relief*. In the case of a turbo manifold they can run hot enough for creep to be an issue in use which in turn can result in cracks as they're bolted solid and have stuff hanging off them

If all the joints fit well then distortion shouldn't be an issue when welding. If the fit up is bad then it can be harder to keep the thing straight when welding, especially if you say lay the thing on a welding table, weld everything you can reach and then roll it and repeat. Gasket flanges can be kept straight by various methods.... in the case of a manifold the flange for the primaries pre-setting the flange by clamping or tackwelding it to something heavy with a shim under the centre to counter the shrinkage from welding. Another way is to pass the primaries through the flange and weld them from the inside closer to the neutral axis of the flange. Smaller 3 bolt flanges won't distort if it's done right but bolting them together or welding closer to the neutral axis can be used to ensure they stay straight. Google any of the phrases in italics and add welding for a bunch of more detailed info


* properly expensive ultra thin wall inconel manifolds are sometimes shot peened which does improve resistance to cracking

 

[W.olly]

Thanks for that Hotrodder, very helpfull.
My plan to help keep things good was to bolt the header flanges on the head and tack the bends/section on untill all spot on where i want them, then whilst all in situation weld all i can get at before i remove the header and pipes down to the collector and weld up on the bench.

will this work as i can get good access to the engine as the body can come off.

 

[langy]

Thanks for that Hotrodder, very helpfull.
My plan to help keep things good was to bolt the header flanges on the head and tack the bends/section on untill all spot on where i want them, then whilst all in situation weld all i can get at before i remove the header and pipes down to the collector and weld up on the bench.

will this work as i can get good access to the engine as the body can come off.

Flanges are usually 8mm or thicker, tube is usually 1mm - 1.6mm, if you warp the flanges welding them together you have way to much heat in the job, I weld all my flanges on the inside for a neat job

 

[hotrodder]

As long as the flange is bolted solidly to the head (or something else) when welding the primaries to it that'd work. If you tack everything and then make the primary to flange welds with it sat on the bench the flange WILL bow. There's loads of ways to skin a cat and even more methods used for fabricating manifolds, i use several of 'em depending on the design...

If i'm using a transition piece to match the diameter or shape of the ports to the primaries then these get solidly welded to the flange right at the beginning. As Langy I prefer to pass the primaries part way through the flange and weld from the inside as this places the welds closer to the neutral axis amongst other things and negates distortion. If i'm using a fillet weld to the face of the flange i clamp it to something heavy with a thin (less than 1mm but depends on flange length)* shim under the middle which presets the flange i.e. bends it slightly in the opposite direction to any distortion that'll occur from welding. Shim thickness depends on several things including the length of the flange and welding style (how hot/fast/arc length etc), i know what works for me but it may not for others, it's a trial end error deal and when you get it right a small tickle on a belt linisher will show even contact over the entire flange

Pipework itself, pretty much as you say but (especially if fit up isn't spot on) i always complete butts one at a time rather everything i can reach before repositioning/moving stuff about. Depending on design you often have to remove a primary to physically reach some of the welds. Obviously this alters things with regards to welding order some, especially at the collector end, but typically means the final welds at the flange end are either directly to the short port transition pieces or even the flange itself

* more than 5 pots and i would always split the gasket flange i.e. two piece for a 6 cylinder engine due to expansion/contraction as it's heat cycled in use. Individual flanges have some benefit in this regard but are a relative ballache during fabrication

 

[W.olly]

Hi steve, is it not the matter of when you weld the rest of the down pipes and it distorts the flanges out of line/level?

 

[langy]

Hi steve, is it not the matter of when you weld the rest of the down pipes and it distorts the flanges out of line/level?

Not if its clamped down.

 

[BrokenBiker]

clamp it all down so it cant move and it will be exactly as you set it up when it finished cooling. if I need something to stay exact I clamp it down, weld it, let it cool then unclamp and measure again

 

[W.olly]

Thats why i will be bolting on the flanges then tacking sections inplace, then welling as much as i can before i remove to finish the welds. just gets me thinking when folk say certain things it gets all confusing at times, folk say it needs stress relieveing but then folk say it doesnt..

anyway thanks for the help chaps, much appreciated, i will bolt on the flanges then tack and weld what i can , then if i can brace or clamp before i do the rest off the engine i will.

 

[langy]

Thats why i will be bolting on the flanges then tacking sections inplace, then welling as much as i can before i remove to finish the welds. just gets me thinking when folk say certain things it gets all confusing at times, folk say it needs stress relieveing but then folk say it doesnt..

anyway thanks for the help chaps, much appreciated, i will bolt on the flanges then tack and weld what i can , then if i can brace or clamp before i do the rest off the engine i will.

Planning it all out will help, sometimes you can't get to certain joints, you need to think about how to get all the primaries the same length also, the pics below look different lengths but are actually within 1/4" of each other, I like to weld short stubs to the flanges first.

 

[W.olly]

So complex the exhaust system isnt it, i knew they were a little but when i got into it my god, so what i found out is the header pipes ( is that what you mean by `primaries`) need to be the same length down to the collector which needs to long tapered nt short as the will cause bad back pressure? then under the prop/diff intput i will add a X pipe then to the silencers, as oyu know i want to fit the silencers under the foot wells, or just back a little, i was thinkign to make my own as i need then 3 inch high so i was going to get some 4-5 inch OD pipe and shape/squash down to 3 inch high to fit under the car, these will be minimum of 18 inch long. idea is is that il will still have near the same area as the 5 inch just different shape.
Will it help if i join the flanges as you have while making the header, or are yours like that in the pic anyway?

 

[langy]

So complex the exhaust system isnt it, i knew they were a little but when i got into it my god, so what i found out is the header pipes ( is that what you mean by `primaries`) need to be the same length down to the collector which needs to long tapered nt short as the will cause bad back pressure? then under the prop/diff intput i will add a X pipe then to the silencers, as oyu know i want to fit the silencers under the foot wells, or just back a little, i was thinkign to make my own as i need then 3 inch high so i was going to get some 4-5 inch OD pipe and shape/squash down to 3 inch high to fit under the car, these will be minimum of 18 inch long. idea is is that il will still have near the same area as the 5 inch just different shape.
Will it help if i join the flanges as you have while making the header, or are yours like that in the pic anyway?

Header science is based on what your gonna use the car for really, Headers or Manifolds as we call them here are the complete thing, primaries are the pipes that come off the head flanges. X or H balance pipes usually go just after the collector joint where it meets the main system, there is an easy way to define the balance position, build the system and spray a bit of paint on the collector and run the motor, where the paint burns off is where the balance tube goes

Not sure what you mean about joining the flanges ? Do you mean the one piece header flanges ? If so I prefer to keep them one piece as it stops the tubes spring slightly out of line (it happens no matter how carefully you weld them on separate flanges)

Once you have mastered the basics you can move on to this type not my cup of tea but notice how symmetrical they are