PWHT/stress relief question

[KernOily]

Hi guys. Dumb question here. To remove the HAZ in thin-wall 316L pipe, what procedure do I need to do? The item in question is a pipe spool made from 3" schedule 10 316L pipe. It is a short spool about 3' long with an ANSI flange on one end and a weld cap on the other end. We have experienced cracking in the weld area after the item was placed in service for a year. The cracking is not SSC. I am thinking the loss of ductility in the HAZ could be alleviated by PWHT or annealing. Any suggestions? Before I just gin out an annealing procedure from the handbooks, I thought I'd ask you fellers. The fabricator is clueless.

Thanks! Pete

Thanks!
Pete

 

[Carburize]

Exactly what service is it in?
Can you be more precise about where in the weld zone the cracking is taking place?

 

[stanweld]

Was the pipe purchased in the solution annealed condition.

 

[sayee1]

Was the cracking in the weld or the HAZ? the original post mentions cracking in weld, but you state that you are asking about removal of HAZ in a thin pipe. What was the nature of the crack? Were the crack(s) logitudinal/ transverse and were they in weld root/ HAZ? Typically welds in stainless steels especially low thicknesses would not be expected to be PWHT unless part of a bigger equipment which needed to be fully PWHT. In any case, unless a micro examination carried out on the cracked section to determine if the cracking is due to PWHT, it cannot be really determined otherwise. What were the service conditions?

Thanks and regards
Sayee Prasad R CEng MWeldI MIOMMM


If it moves, train it...if it doesn't move, calibrate it...if it isn't written down, it never happened!

 

[KernOily]

OK that's what I get for getting in a hurry... sorry for the incomplete initial post.

The crack is in the weld metal, not the base metal so it is not in the HAZ.

The pipe is not solution-annealed as far as I know.

This spool piece is one of 28 gas distribution laterals of different length that are installed in the bottom of a vertical sparge vessel (a reactor). The lateral has (17) 3/4" diameter holes, about 2" o.c., on the bottom CL all along its length.

A gas flows through the lateral and out the holes where it bubbles up through a liquid iron chelate solution. The laterals are submerged in the solution. There is about 3' of solution level above the laterals. The gas is 60% CO2, 25-30% CH4, water-saturated, with 15,000 ppm H2S. THe process removes H2S from the inlet gas stream by converting it to solid elemental sulfur. The process conditions are T = 110° F and 6 psig. This is the normal service condition.

Twice a week, each lateral is cleaned in-situ in service of solid sulfur by passing utility gas (methane) at 200 psig through it for about 10 seconds. This is required because the solid sulfur that precicptates out during the reaction is sticky and collects on the laterals, causing backpressure and reducing the effectiveness of the process. This 'gas blast' knocks off the solid sulfur that collects on the lateral holes. This blast does cause a cyclic load on the lateral but I do not think the magnitude is high enough to cause the cracking in and of itself, although I have not calc'd its magnitude and I do need to do that. Assuming the stress magnitude is low enough, would not an annealed condition in the weld increase the ductility enough to prevent cracking? Thanks - Pete

Thanks!
Pete

 

[stanweld]

The base pipe should be in the solution annealed condition. If not, as you have indicated, I would suspect some manufacturing problem as the cause of failure. Do you know the pipe's materials specification?

 

[stanweld]

Just one more question. How have you determined that it is not SCC? SCC at low stress and at ambient temperature is quite possible in sulfurous acid and this acid might be present under the conditions that you have described.

 

[Carburize]

More questions - do you have any record of what type of electrode/filler wire was used to make the welds?

 

[KernOily]

Guys thanks for your time and help...

OK. The pipe is sch 10S, either EFW or seamless ASTM A312 TP 316L or A358 Gr 316L. They used both types on this job.

Filler metal is ER316L root and E316L hot pass/cap, flux-core. MIG process, done in the shop. Since this is sch 10S I think there are only two passes in there.

I have not ruled out SCC or SSC. Sulfurous acid MIGHT be present but it is not usually. I am checking on this. There also might be hydroxyl present. To tell if this was SCC/SSC for sure I would have to do the testing. As it is, unfortunately, they ground out the crack and welded it back together with an ER316L stick process in the field since the process unit was down at the time. (The things that get done when the engineer isn't around...) So it is too late for this one, and that is why I am trying to see if I can engineer this out for the next one. Thanks!!! Pete



Thanks!
Pete

 

[sayee1]

The HTed condition of the pipe is ruled out as the cause for the failure because the cracks were in the weld. If MIG was used for the root, what was the shielding and the purging gas compositions?
Stanweld/Carburize,
Would oxidation due to improper shielding or the extent of ferrite in weld metal be possible contributors to the failure considering the prevailing environments?

Thanks and regards
Sayee Prasad R CEng MWeldI MIOMMM


If it moves, train it...if it doesn't move, calibrate it...if it isn't written down, it never happened!

 

[unclesyd]

You should look at the MIG welding process for lack of fusion (LOF) in the original weld. If there was LOF in the original welding the subsequent flow induced vibrations will quickly cause the weld to fail.

 

[aybee]

74Elsinore what you have is polythionic stress corrosion cracking caused by the H2S and water at low temperature. I think your answer is to change from an austenitic ss to an Inconel grade maybe 800 or 825 . need to do a bit of book lookin to check this..Aybee

 

[EdStainless]

acid and H2S, sounds like SCC to me.
The original pipe should be solution annealed. You can verify this by checking the residual ferrite in the longitudinal welds. The spool welds will need to have some residual ferrite in order to prevent hot cracking, but too much will make them very suceptible to pitting corrosion.
My thought is that either small cracks were present originally or small pitts formed in service. This, combined with your shock treatment, cause the cracks to grow.
You probably should use an over-alloyed filler such as 317L. This would give the weld better corrosion resistance.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[KernOily]

Ed thanks for the reply. As I said I was not able to do a failure analysis so I don't know if if was SCC or SSC for sure. The conditions are right, yes.

I have to check the spec to see if that pipe grade is solution-annealed.

I suspect you are right about cracks or voids being present initially. I will check into 317L as a filler wire.

Thanks!
Pete