welding take offs on pressure vessels

[aboard_epsilon]

Is there a secret to this ..

'Cause no matter how neat my weld is ..it.s porous .....when subjected to 100 psi test

I then end up chasing it around ..and around ..for the leak just to follow to the end of the weld ..

Is this why pressure vessels sometimes have brazed takes offs ..
I honestly cant figure this fault out ...

I've turned the gas up ...im using argoshield light and mild steel welding wire...my welds to start with are flowing and not lumpy...and look good.

all the best.markj

[camdigger]

Most pressure vessel welding I'm familiar with is stick or tig, not mig. Low hydrogen 7018 rod fill over a 6010 root gives best results. Mig and tig processes need back purging to avoid contamination from the back side. Once the weld has porosity, it can't be cured by additional material over top in my limited experience. Stick is often preferred because there is no requirement for purging and it is more portable.

My $.02 YMMV
Cam

[MTNGUN]

Where I come from, pressure vessels and certain steam headers have to be welded by someone who is certified at the pressure vessel game. Non-certified "farm" welders such as myself are not allowed to weld, cut, or grind on a pressure vessel. But, since this is the Home Shop forum, I presume this is some small home project where nobody cares, as long as it works.

I'm not the welding expert, but tell us more about this vessel. How thick is the vessel wall ? How big is the take-off fitting ? What kind of mig welder are you using ?

Cuz unless this "vessel" is made out of sheet metal, I'd reach for the stick welder and do the traditional 6011 root and 7018 cap. I would be leery of mig unless the machine had enough power to allow "spray" welding.

But maybe mig is all you've got ?

[aboard_epsilon]

I've got a tig 250 amp.but it's three phase ..and i don't have enough amps three phase to power it..........so its wired single UK phase and is now only 125 amp .no good for this job .

my MIG welder is a migatronic professional MIG welder 180 amp ......i was using as much amps as i possibly could without blowing holes through the vessel wall ...7 settings ..i was on number 5...so probably about 150 amps

The pressure vessel is one of these..will hold about 2 litres...ive drilled out the centre brass core and replaced with steel.3/8 bsp inlet welded on ...........plus it now has 25 mm filler ...and 3/8 outlet with dip tube.

http://www.bristol-gas.co.uk/camping_gaz_refillable_cylinder_901.jpg

I'm making into a fuel injection flow tester .......see post "air over hydraulic" a week or too ago

wall thickness is about 2.5 mm

The take-offs are made on the lathe.........basically 8 mm high disks with 3/8 bsp thread in them

it will have a working pressure of 45 psi ...so not that dangerous...........i was testing for leaks at 100 psi .......

funny thing is ........you can fill it with air ...and submerse it ...and you get a string of tiny bubbles from one or two places on the welds

fill it with water ..and pressure it up t o 100 psi.......... and no leaks.

I'm quite happy with it ..even though, because all the chasing, Ive been doing the welds look ugly...i wont be making it again

so I'm asking for next time...so i can get it right ..next time .

I've no experience with a stick welder.

what do you mean by purging ..

All the best.markj

[jeremy13]

I have heard that and seen. That the small disposable fuel containers have oil in them. To counter act the corrosive nature of the gases in them. This might be contaminating the weld. The cylinder gets hot and vaporizes the oil and is forced out in front of the weld.

[aboard_epsilon]

i washed the cylinder out with strong detergent before i welded it ...because there was cutting oil in it from the drilling.

all the best.markj

[hardtail]

Purging is generally cleaning the gas side of the vessel but here back purge refers to your shielding gas, depending on the setup sometimes you have to run an inside purge as well, one starts to see the difficulties encountered and costs and theres yet another reason stick is preferred........I would agree with the root and cap choices previously mentioned when using stick. The weldolets (vessel fittings) are usually tapered or beveled at the end where they fit to the vessel allowing full filler thickness, these are usually fitted with a dime or penny gap with tacks before running your root pass.

As you say you don't have stick so I'm wondering if your leaks are occuring everytime where your bead starts/stops?, with limited mig amps/penetration you can encounter problems where the bead first starts (fusion) and I would reverse your bead back into the pass when finishing.

Pressure testing should always be done with cold water, disastrous results have occured from using air...........

[NickH]

Purging refers to purging all air in contact with the metal using shield gas.
So in this case they mean to ensure the cylinder has pure shield gas inside with no air,
Regards,
Nick

[aboard_epsilon]

aha ..i see .........so you mean were the weld met fresh air on the inside ..it made it porous .. and the reaction manages to come strait through to my side of the weld.


if that's so will make sure ..if i do it again ..that the vessel is filled with argon co2 mix ...this is heavier than air is it not.

all the best.markj

[jeremy13]

I have seen a tee placed after the shield gas regulator and a another small valve and hose. Just place the hose in the pressure vessel and let the shield gas flow freely just a small amount. Yes they are heaver than "air"

[aboard_epsilon]

OK thanks for the informative replies guys.
what i cant understand is ..putting layer upon layer of weld on-top of the leaks does not cure the problem.....leaks appear then in a different place ..perhaps i should clean the slight soot off the weld, before i add another layer ..

guess, MIG, is unsuitable for this purpose .

would silver solder or braze do a better job in the future.

i have a stick welder ......but my experience is zero .in-fact, Ive had it now for 5 years and never connected it up... :o

all the best,,,markj

[macona]

I have seen many weld on fitting migged in place.

If you are getting soot and stuff on the weld you are getting contamination from something. Is the tank galvanized or somehow coated? Are you grinding down to bright metal before welding?

[aboard_epsilon]

well not soot just the usual discolouration ..

i was wondering if i had to wire brush the weld before i deposited more on top ..when after this seemingly impossible gas tight weld.

all the best.markj

[10KPete]

150 amps sounds way too hot for such thin material. I don't know your MIG machine but that just sounds wrong.

If the parts are clean, inside and out, the shielding gas is right (not contaminated and sufficient flow), the parts fit is decent and the operator good then such a weld should not be a problem.

MIG is used for most of the production welding of pressure vessels. I've done hundreds, and not by a 'robot' or automatic machine either. Strictly 'manual'. All sizes and shapes.

From your description you're burning thru and contaminants are 'sucking back' into the weld.

And the guys are right; once the junk is in there you're lost unless you grind out all the junk.

My two cents anyway.

Pete

[boslab]

bin the argo light, use cougar [air products] change your wire and contact tip, purge with argon, dont pressurise, give a good clean with emery weave at about 1/2 weld width slow travel to keep heat in the pool so the crap floats out, its worked for me on steel radiators that always seemed to leak
regards
mark

[seattle smitty]

Mark, I'm guessing you are a machinist first, and only occasionally a welder. Imagine that you saw an amateur put a crankshaft between centers on a new lathe he had just bought, and proceed to turn a deep, sharp-edged, stress-concentrating notch in the crankshaft right next to the flywheel flange. Would you say something, or would you let him take a chance on him or a passenger losing their legs when the flywheel broke free at 4500rpm?

I'm not one of those safety-first people that are always pestering everybody in the shop, but Mark, you REALLY need to get into a basic welding class, day or evening, at your local tech school. If you make bad welds on pressure vessels, it isn't likely to hurt anyone but you and yours (a friend of mine saw two walls blown out of the side of a steel building from an auto shop air compressor tank failure). But since you seem not to know your own limitations, sooner or later your are going to apply your welding skills to something that could endanger others and/or get you sued. A welding class is a huge bargain, and you will be amazed to find out how much there is to know about something that appears so simple.

I wonder how many blue-collar guys said, "I've been doing stuff like this for years and never had a problem," right before losing a finger, a hand, an eye, . . . .

[aboard_epsilon]

Mark, I'm guessing you are a machinist first, and only occasionally a welder. Imagine that you saw an amateur put a crankshaft between centers on a new lathe he had just bought, and proceed to turn a deep, sharp-edged, stress-concentrating notch in the crankshaft right next to the flywheel flange. Would you say something, or would you let him take a chance on him or a passenger losing their legs when the flywheel broke free at 4500rpm?

I'm not one of those safety-first people that are always pestering everybody in the shop, but Mark, you REALLY need to get into a basic welding class, day or evening, at your local tech school. If you make bad welds on pressure vessels, it isn't likely to hurt anyone but you and yours (a friend of mine saw two walls blown out of the side of a steel building from an auto shop air compressor tank failure). But since you seem not to know your own limitations, sooner or later your are going to apply your welding skills to something that could endanger others and/or get you sued. A welding class is a huge bargain, and you will be amazed to find out how much there is to know about something that appears so simple.

I wonder how many blue-collar guys said, "I've been doing stuff like this for years and never had a problem," right before losing a finger, a hand, an eye, . . . .

dont know mate ..
here are some done on stainless about 4 of years ago ..and i was in a hurry when i did these ...could have done with a few more amps ..but stainless tends to flow less.
you dont get worse at this welding as the years roll by ..do you..if so im an Alzheimer's welder :o

http://img.photobucket.com/albums/v190/aboard_epsilon/jones%20shipman/weld2.jpg

http://img.photobucket.com/albums/v190/aboard_epsilon/jones%20shipman/weldone.jpg

it's just a mystery to me ..the way its going i could depoisit an inch of weld on that vessel job and it will still be porous.

ps ..i dont know what sort of steel these take-offs were ..could very possibly be leaded
all the best.markj

[10KPete]

ps ..i dont know what sort of steel these take-offs were ..could very possibly be leaded
all the best.markj

Whoa, hold the truck.

Seattle Smitty gives some really good advice.

Then I read this about unknown steel. That's the breaker.

I'm gonna say that you need to stop playing with dynamite and either buy your pressure vessels or have them made/modified by qualified people. If you don't then someone is gonna end up either dead or badly hurt.

The kinetic energy stored in even the smallest air pressure vessel, even at the "low" 45 psi you're talking about is huge. It's a bomb.

Please, don't pursue this unless and until you achieve the level of experience required to qualify for this type of welding. Please!

Pete

[aboard_epsilon]

bomb ...it'll go phut..ands that's it ..

it's good enough for me ..it ain't going to explode ..ive just got some problems with porosity thats all...(a trail of 1mm bubbles coming out of the weld at a rate of 3 or 4 a second)..........you'll be saying my rubber airline going to blow up in my face next ..

make sure you check your tyres for damage ..may blow your head off one day !!!

all the best.markj

[Carm]

First timer here, as far as posting. I usually read the Machinist board.
But the OP's question may have another cause.
MIG can certainly be used for pressure vessel and coded pipe work. Usually there's a qualified procedure and parameters thereof are followed.
Both short arc and spray are used.
Given the vessel in the picture, of deep drawn composition, I think you have insufficient de-oxidisers in your filler metal, ruling out incompetence. The American designation to solve your problem is E-70S3, a higher silicon wire that will tolerate rimmed steels. The same wire can be used for TIG.
If you decide to stick weld it, there are no plain carbon (mild) steels that cannot be welded with a cellulose root and low hydrogen cap. American designations are XX10 or 11, XX16 or 18, former and latter.
Retired pipefitter weldor with U & N stamp and API 650 & 1104 experience.

[wmgeorge]

First timer here, as far as posting. I usually read the Machinist board.
But the OP's question may have another cause.
MIG can certainly be used for pressure vessel and coded pipe work. Usually there's a qualified procedure and parameters thereof are followed.
Both short arc and spray are used.
Given the vessel in the picture, of deep drawn composition, I think you have insufficient de-oxidisers in your filler metal, ruling out incompetence. The American designation to solve your problem is E-70S3, a higher silicon wire that will tolerate rimmed steels. The same wire can be used for TIG.
If you decide to stick weld it, there are no plain carbon (mild) steels that cannot be welded with a cellulose root and low hydrogen cap. American designations are XX10 or 11, XX16 or 18, former and latter.
Retired pipefitter weldor with U & N stamp and API 650 & 1104 experience.

I'm going to second this, sounds like someone with a lot of experience speaking. I wonder what was inside this tank? If it was oil then maybe using a torch (outside in fresh air) to get the metal hot enough to burn off once and for all whatever is left inside. Grind out the old weld, flush again with your detergent water mix, heat again to dry and follow Carm's advice above.



"A retired UA member, but not a welder"

[boslab]

I'm going to second this, sounds like someone with a lot of experience speaking. I wonder what was inside this tank? If it was oil then maybe using a torch (outside in fresh air) to get the metal hot enough to burn off once and for all whatever is left inside. Grind out the old weld, flush again with your detergent water mix, heat again to dry and follow Carm's advice above.



"A retired UA member, but not a welder"
Carm
who the hell still makes rimming steel? i havent seen a bottle top mould since the early 80s [2 bags of ali, lid on bar through and run, popping tops were like beirut on a bad day]
mark

[10KPete]

bomb ...it'll go phut..ands that's it ..

it's good enough for me ..it ain't going to explode ..ive just got some problems with porosity thats all...(a trail of 1mm bubbles coming out of the weld at a rate of 3 or 4 a second)..........you'll be saying my rubber airline going to blow up in my face next ..

make sure you check your tyres for damage ..may blow your head off one day !!!

all the best.markj

I wasn't trying to do anything more than make sure that you (or anyone reading this) weren't simply whistling along without appreciating the potential dangers involved.

I'm not paranoid but I have been around a bit and have seen what can happen.

Just don't want anything to happen to you!

Sincerely,
Pete

[Carm]

Carm
who the hell still makes rimming steel? i havent seen a bottle top mould since the early 80s [2 bags of ali, lid on bar through and run, popping tops were like beirut on a bad day]
mark

Now that I couldn't tell you. But there's plenty of old pipe around that will get modified and re-tied in for years to come.

[boslab]

for those out there wo havent heared the term 'rimming steel' it is liquid steel in its unkilled state, ie no aluminium to scavange the oxy, when you look at it in a mould through the old cobalt blue glasses you see a rolling action around the edge or rim, similar to when milk is about to boil, most steels cast today are AK [ali killed], its impossible to continuously cast a rimming steel, ingot only [ i did try and it went big bang ]
mark

[Carm]

Good explanation, boslab. Somewhere I read/was told that deep drawn steels were rimmed and any welding process need supply the de-ox.
TIG is a sure fire way to find a rimmed steel, impossible to get an autogenous clean puddle.