TIG with back purging on stainless 304 car exhaust manifold

[Wildman10]

I'm getting back into hobby welding after learning 40 years ago, initially with a gasless MIG then MIG with gas. I know my limitations so am about to farm out some TIG work where I'd appreciate some advice.

I'm fabbing up a car exhaust manifold using a mix of cast and drawn stainless 304 and one carbon steel flange. The engine is a turbo-charged rotary/Wankel so the manifold will have to cope with mounting a turbo on the end and higher-than-normal temperatures, leading to my needing the best possible welds from the outset. Furthermore, I want to get the manifold ceramic coated so will need to get the welding right first time. The images show an example completed manifold and the start of my fabbing in a jig.

I'd love to buy a proven manifold off-the-shelf to avoid the hassle, effort and risk of building my own. However, none are suitable for my design and I'm having to do the work myself.

I'm therefore looking for a welder who can TIG weld with back purging. However, I'm having trouble finding a TIG welder who will back purge and the only one I've found so far near me (say, within 20 miles of Redhill, Surrey) wants to charge a minimum of £130 (+ VAT) for a bottle of Argon for the back-purging. I've looked for a specialist further away without success.

Can anyone advise me:

• Why are professional TIG welders seemingly shy to take on work with back purging?
• Is £130 + VAT reasonable for back purging my manifold? To be clear, I'm happy to pay a fair price and am concerned by wanting not to be taken for a ride rather than the ultimate cost.
• Any tips?

Thanks in advance for any help.

 

[Richard.]

Why are professional TIG welders seemingly shy to take on work with back purging?

I'm not sure id agree with that as a professional Tig welder shouldn't be welding a full pen stainless joint without underside protection. Allowing sugaring on the backside of an SS weld is producing a defective joint in my opinion.
Price wise is that £130 for welding it up . If so that's a decent price I couldn't better it. There is a fair bit of welding. Or is that price just for the purging? And welding is on top.

 

[Wildman10]

Thanks.

Maybe I'm just unlucky, but 5 of the 6 professional welders advertising TIG on stainless I tried said they wouldn't take on welding requiring back purging.

That's £130 + VAT for just the back purge gas. They said the total would be that + materials + £40+VAT for labour (best described as working from an untidy Nissan Hut on an old industrial estate where I expect a lower rate than from someone with modern premises). They wouldn't agree a fixed price.

 

[hotrodder]

Reading between the lines it sounds to me like the place who'd want £130 +vat to purge have next to zero experience of purging

...for a bottle of Argon for the back-purging.

sets off more alarm bells for me than an untidy hut

Going by the pictures we've seen no one's gonna be able/willing to quote a job price... there's 19 or 20 joints in the finished example but things like fit up and joint prep etc make a fairly big difference beyond a welders individual preferences

Is the finished example a proven design? I'm looking at that and thinking it's gonna be prone to cracking regardless of weld quality unless the turbo is supported in a way that allows for thermal expansion/contraction. Known as 'turbo dampers' in the world of Ford Cosworths as they were as much about dealing with vibration as anything, here's a stolen pic from the old F1 turbo era



showing the triangulated support and spherical joints to allow for thermal growth.
Related to both joint prep for welding and longevity what sort of wall thickness are we talking? Schedule 10 pipe fittings?

 

[Brad93]

Thanks.

Maybe I'm just unlucky, but 5 of the 6 professional welders advertising TIG on stainless I tried said they wouldn't take on welding requiring back purging.

That's £130 + VAT for just the back purge gas. They said the total would be that + materials + £40+VAT for labour (best described as working from an untidy Nissan Hut on an old industrial estate where I expect a lower rate than from someone with modern premises). They wouldn't agree a fixed price.

It depends on how many joints. Can't see that manifold requiring half a bottle of gas let alone a whole one. Probably a days labour in prep and weld, if I was making that I might imagine £230 all in and purged if you've got a jig for the manifold. Have you tried a guy, not that I know personally, but I've seen his work, Hardknocks Speed Shop. He does a lot of custom manifolds now.

 

[Paul G]

I'd be last seen disappearing over the horizon in a cloud of dust if someone brought me a "fabbed up" ( I hate that phrase ) manifold
in a wooden jig.

I'd also be none too impressed if a customer suggested I should be payed less based on the type of premises I choose to work from.


Just my opinion.
Paul G

 

[Chris Stevens]

I'd be last seen disappearing over the horizon in a cloud of dust if someone brought me a "fabbed up" ( I hate that phrase ) manifold
in a wooden jig.

I'd also be none too impressed if a customer suggested I should be payed less based on the type of premises I choose to work from.


Just my opinion.
Paul G

Wooden jigs spell no end of problems, which customer would no doubt expect welder to sort out FOC after welding work was done.

 

[zeppelin builder]

would be worth pricing it in inconel that would stand up better, even in stainless you need a knowledgeable welder to control the distortion not some nugget that welds it up and pulls it all over the shop ,it would ring alarm bells to me if they dont want to know about purging i would think about 1/4 bottle would do it
for a pro welder id say around 200 to 250 quid dont dissmiss the guy in the tin shed lots of good work comes out of em
btw love them rotarys fancy puttin a mazda in a mcycle pity norton stopped makin em imagine what they would be like now
your jig could only be used as reference from flange to flange as welding progressed it wouldnt hold it might have to temp tack bars box section to stay it and remove after welding then heat shrink by reheating cap of welds to pull in desired direction to match mounting holes and true up flanges to jig, them wee jobs need time and patience

 

[hotrodder]

Thousands of light aircraft fuselages have been assembled/tacked using simple wooden jigs & fixtures over the years. It's still pretty much the go to method for example http://www.sonerai.com/construction.html Even with a production quality/beefy jig it's possible to end up with something that won't fit back into the jig let alone fit the car (or whatever the part's destined for) if the fit up was poor and/or the welder was a numpty

Personally i don't see anything wrong with the jig in the OP... even if it was a butch chunk of steel that didn't block access to any of the joints pretty much everyone i know woud tack it up before seperating the primaries to attack most of the butts before reassembling it for the last couple of welds

 

[squazy84]

what I know of stainless and ex manifolds in turbo applications is that the commonly available stainless steels are actually unsuitable for the purpose because the seem to get brittle and just crack,and most definitely will unless braced in some way,ive seen regular gun barrel (as we call it) heating pipe basically out perform stainless, as far as i know the stainless used on jet engine hot sections is 321 but it was always too expensive for me to try out and i never got to rob it from work but ive even see that crack on aircraft,i suppose my only point is brace it properly and you will need deep pockets..as if you didn't already know that lol

 

[hotrodder]

Bracing a manifold can make things worse... the spherical joints in the pic of the Renault F1 engine allow things to move due to thermal expansion while supporting some of the weight and killing some of the vibration. Brace a manifold rigidly and it's forced to expand more in the directions it still can which can be enough to crack it

The low carbon grades like 304L etc have lost much of their strength at the operating temps of turbo manifolds. With enough attention to detail they can work even with relatively thin wall tube but there ain't much margin for error. Higher carbon flavours have better creep strength but don't like being constantly heated and cooled as the carbon reacts with the chromium leading to sensitisation aka carbide precipitation. Stuff like 321 fixes that by adding a little Ti which has a greater affinity for carbon and sacrifices itself. Like 304 it's not immune to sigma phase though, explained here along with sensitisation http://www.gowelding.com/met/austenitic.html
One of the reasons 321 mandrel bends are spendy is 'cause they usually being life as sheet instead of buying tube to bend, dunno what the price or availability is like off the top of my head but 321 exists as pipe fittings including weld elbows but that kinda defeats the purpose in that usually you go to a thinner wall if your pockets are deep enough for 321, or to inconel and even thinner wall if your pockets are bottomless

 

[Brad93]

http://www.goodfabs.com/bends-and-pressings/pressing.aspx

 

[squazy84]

I didn't mean solid bracing,you see that on the cheap Chinese 6cylinder turbo manifolds and they crack everywhere including right beside the welded bracing

 

[zeppelin builder]

i made headers for my 27 litre meteor engine doubled up 2inch stubs per port got the flanges cut on a cnc plasma the stubs feed into 4 inch pipe they are made in two halfs bolted back to back after tacking on the engine once welded they were cut into 4 sections 2 per bank partly to make them easy to fit over the studs you try fitting the flanges over 24 studs at once 12 lots easier i made a sprung split clamp arangement to cover the split just as well as it moves about 6 mm when its red hot the material is 316 l its on youtube meteor engine v12 dont know how it would stand up in the long run welded up lots of light aero exhausts cracked to bits that were made from 304

 

[hotrodder]

I didn't mean solid bracing,you see that on the cheap Chinese 6cylinder turbo manifolds and they crack everywhere including right beside the welded bracing

my bad and yep, cheapnese manifolds are often a good lesson in how not to do things http://www.mig-welding.co.uk/forum/threads/its-just-a-tiny-hole-can-you-weld-it-up.9428/

 

[arther dailey]

Probably of no use, but light years ago I had a escort mexico, new engine fitted, a 4 2 1 manifold from a well known maker in salisbury, that manifold was for ever cracking, in the end I asked the boss if I could use work shop kit and braze it, never cracked again......Arther .ps for the enthusiast lowered bilstien world cup suspension, which meant you couldnt go over anything higher than 2" tall,tyres I found out later were race track use only, geared to do 105 flat out so accelerated like the clappers.

 

[squazy84]

my bad and yep, cheapnese manifolds are often a good lesson in how not to do things http://www.mig-welding.co.uk/forum/threads/its-just-a-tiny-hole-can-you-weld-it-up.9428/

yep they are the ones ive come across, just add two more cylinders,not even worth fitting..proceed directly to the recycling bin,it pains me to see such a waste of materials and time,often the cast iron items they replace are superb quality compared them!