Magnetism induced in pressure vessel due to welding 3

[SJmetallurgist]

Dear Friends

Few years back I faced a peculiar problem in few pressure vessels, where we found that Pressure vessels have become magnetic after fabrication - all these pressure vessels were Weld overlaid & a couple of them were Heat Exchangers.

The metallurgy of the equipments was LAS-2-1/4 Cr-0.5 Mo + SS347 Weld Overlay.
Welding processes involved:
for LAS welding: SAW, SMAW, GTAW
for weld overlay: SMAW, ESSC

I could not find this phenomenon much talked about. Even in most mature user specifications.
Talked to few fabricators and they mention (casually) about induction of magnetism due to welding processes.

Q1. Can someone please take some time and give his/her based on experience.

Q2. if their is a literature/earlier work/research on the subject matter, can someone please lead me to the same ?

Warm Regards
Shashank

everything has a bit of everything, but nothing is fully explored in this world

 

[kingnero]

Due to the physical welding itself - I have no idea.
Some steels more than other, easily pick up ferromagnetism due to vicinity of magnets (think of lifting devices, magnetic earth connections of welding machines, magnetic tools like levels and those magnets used to hold pieces in place while tack welding, I don't know their name in english, ...).
Possibly even due to the work or return lead being coiled around the part, this is sometimes used to counter the residual magnetism in the steel. Possibly this also works the other way around (making parts ferromagnetic) - again, I have no practical experience.

 

[0707]

See the link below maybe they help.

luis

https://www.youtube.com/watch?v=3Sf_Jq8R7JE

https://www.youtube.com/watch?v=7xY6ZujMpyA

 

[SJmetallurgist]

@0707 : the videos are for demagnetisation.. the particular concern is the Magnetisation & how does that get induced during fabrication & weld overlays by ESSC

 

[SJmetallurgist]

@kingnero : Need practical advice.. thanks nonetheless. Pls see if any of the fabricator you know can throw some light on this

 

[3DDave]

Ferromagnetic molten metals cooling in the presence of an electrical or magnetic field freeze the magnetic component of the field.

Presently it's being explored/used in hard disk drives where a laser raises a tiny area above the Curie point while a magnetic head generates a field, a field that is locked in when the material cures. It allows the magnetized area to be far smaller than the applied magnetic field and so allows far higher density of data storage.

Since arc welding produces an electrical field I expect there is also a magnetic field along with it and if it's a DC arc then there won't be any cancellation as the field direction doesn't change.

 

[EdStainless]

This even happens when plate or bar is hot rolled on a mill that runs North-South.
When you cool steel through the Curie temp (~770C) in a magnetic field it will magnetized.
These alloys retain more magnetic field than plain steels. In fact at one point in time Cr steels were even used as permanent magnets.
Most people only notice this when they go to do more welding and are having arc defection issues.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

having arc defection issues.

Arc defection being precisely the problem when arcs deflect. (sorry)

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[SnTMan]

Ed, interesting. So do they try to avoid N-S mills?

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[EdStainless]

They de-mag the cold product.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

Post-weld heat treatment makes it go away in carbon steel. Of course you have to get that far first.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[gtaw]

Any time you pass electrical current through a conductor, a magnetic field is produced around the conductor. The strength of magnetic field is proportional the current.

The magnetic field can cause arc blow that can be severe enough to prevent welding. At that point, the part must be demagnetized before welding resumes.

There are steps that can be taken to mitigate the magnetic field produced. One of the easiest ways to reduce the magnetic field is to utilize multiple work leads connected to different locations to provide alternate current paths and to reduce the current flowing through each lead. Tack welds should be as large as practical to maximize the cross section to reduce the flux density, thus reducing the influence of the magnetic field. The back step technique can also be utilized. Switching from DC to AC will help, but not eliminate arc blow when welding at high currents.

The worst thing one can do is wrap the work lead around the part being welded. That will change the magnetic field from a circular field to a longitudinal magnetic field.

Once the part is magnetized, it must be demagnetized. I’ve made good money over the years demagnetizing weldments and then teaching the contractor how to continue welding without remagnetizing the part.


Best regards - Al

 

[gtaw]

There are a couple of articles in "Inspection Trends" published by the American Welding Society authored by Albert Moore. The articles should be available on-line in the archives of past issues of "Inspection Trends". If you are not a member of AWS, you can call AWS Headquarters in Miami, FL and ask for Carlos Guzman, editor for the publication and ask for a copy of the two articles on the subject of magnetization and demagnetization.

You might be able to find them on-line if you do a Google Search. I've never tried looking for the articles, so I can't say they are available. There has been several discussion on the "AWS Forum" that might be available on-line as well.

Best regards - Al

 

[SJmetallurgist]

Friends
Thanks a lot for your contribution and inputs. I extended the discussion with known fabricators in India. Everyone does face this problem on daily basis. Few reasons and solutions they recommend are as under:
Reasons:
a. Higher currents.
b. Current flow in single direction.
c. Direction of welding/weld overlays (N-S/E-W)

Solutions:
a. Using multiple workleads to divert the direction of current flow (also mentioned by @gtaw above) - though even this doesn't work always.
b. Taking care of direction in which welding is done, and avoid North-South (mentioned by Ed-stainless above)
c. Using PQR qualified with AC (works very well for fabricators)
d. Demagnetization techniques for unavoidable geometries, such as bottom and top spools of reactors, etc.
e. For weld overlaid nozzle/spools to shell/D-end connections, manage to first deposit the austenitic weld and then the LAS or CS parts.
f. Heat treatment: goes without saying that any material subjected to a heat treatment above the Curie point is automatically demagnetized. So the jobs going to PWHT should not have the issue of residual magnetism.

But yes it remains to be among the least discussed subjects because every fabricator has their way of resolving the issue. The only code that acknowledges this matter that i could come across is API-5L. Piping vendors complain the most about this.. Vessel fabricators face the isssue but complain little.