Weld porosity problem

[vks1986]

Dear welding experts,

we are manufacturing longitudinal saw weld pipes of api 5l
weld groove: double v(internal saw and external saw)
recently we have weld 50 nos(12.2Meter length)of pipe of size dia.32"Xthi.39mm
we have done 100% Rt of each pipe & results no any porosity found. but after few days we have weld pipe size dia24"Xthi.38mm & rt results found porosity in every pipes.
both pipe size grade(Api 5l x-65) is same, joint geometry is same, saw welding consumables is same,same preheating temp and also we are taking extra care like cleaning of pipe , heating of flux in pipe size 24"X38mm.
then why porosity come in pipe size 24"x38mm in internal saw welding

 

[desertfox]

I'm no expert but there seems to be more factors than you mention, here is a list of things that can cause porosity on this link:-

http://www.thefabricator.com/article/arcwelding/22-possible-causes-of-weld-metal-porosity

 

[DekDee]

vks1986,
Certainly seems like a problem with the material.
Is it from the same manufacturer ?
If so, is it same heat number ?
Do you have any off-cuts ?
If so put a 32" and 24" pup piece together with the longitudinal seams lining up.
Weld the SAW along both pipes at the same time and if one is OK and the other is full of porosity it is time to reject the 24".
Hope that helps,
Regards,
DD

 

[metengr]

Some variable had changed in your SAW process to introduce porosity.

Go to the source;

http://www.twi.co.uk/technical-know...causes-of-porosity-in-sa-submerged-arc-welds/

 

[gtaw]

I would start with checking the base metal for surface contamination, i.e, rust, oil, grease, soil, etc. Then look at the flux used. Was it contaminated by anything such as moisture from overnight condensation, i.e., dew. Next, check the welding parameters used by the welder. Did the welding operator change the depth of the flux blanket? Are you recycling the unfused flux. If so, is it becoming too dusty thereby preventing the gases generated from escaping easily. If the most common culprits are not the cause, dig into the problem a little deeper.

Best regards - Al

 

[ornerynorsk]

Same preppers and welders?

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[metengr]

I would guess because of the change in diameter, the flux burden as mentioned in 3) is the problem.

3. Insufficient flux burden can expose the arc and molten weld pool to atmospheric contamination.

 

[vks1986]

Dear all,

we are using saw three tandam arc automatic welding and following steps I have all ready taken...

1. baking of flux as permanufacture recommendation( same flux batch for both size)
2. I throuly clean the pipe from internal side and external side by grinding and buffing.
3. machin taken for operation and electrical maintenance in which we have check all parameter, instrument and littel damage part replace by new.
4. I have raise preheating temp. 125C instead of 85C.

but stil two to three points of poro is coming in each pipe.

I have doute in following points.
1. due to lower dia of pipe generation of megnetic field is effect arc and resultant in arc blow may be entrept gases.
2. joint jeometry double v (means narrow gap between upper face and lowerface of two edges and no rootface gap )

is it root cause for porosity..????

 

[vks1986]

Dear all thanks to reply...

we are using saw three tandam arc automatic welding and following steps I have all ready taken...

1. baking of flux as permanufacture recommendation( same flux batch for both size)
2. I throuly clean the pipe from internal side and external side by grinding and buffing.
3. machin taken for operation and electrical maintenance in which we have check all parameter, instrument and littel damage part replace by new.
4. I have raise preheating temp. 125C instead of 85C.

but stil two to three points of poro is coming in each pipe.

I have doute in following points.
1. due to lower dia of pipe generation of megnetic field is effect arc and resultant in arc blow may be entrept gases.
2. joint jeometry double v (means narrow gap between upper face and lowerface of two edges and no rootface gap )

is it root cause for porosity..????

 

[ornerynorsk]

Magnetic field causing the arc blows is an interesting thought, particularly on pipe (which I have no experience with, by the way). Lincoln put out a short article on this very subject.

Good luck getting to the root of the problem! These things can be exasperating sometimes.

http://www.lincolnelectric.com/en-us/support/welding-how-to/Pages/preventing-arc-blow-detail.aspx

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[vks1986]

I have perform all action which was root cause for porosity but still I got two to three pts of poro in pipe..
I realy need more guidance...