Welding of magnetized pipe 3

[MohamadKandil]

we were doing welding job for inconel (3mm SS Clad internal) spools welding 34". the welders were unable to debosit the weld metal correctly in the root. what might be the reason? Magnetization? and how to know?

 

[stanweld]

To determine whether the material is magnetized, use a gauss meter to determine residual magnetism direction and field strength. View the arc to determine arc blow (favoring a direction to one side of the joint or wandering arc).

Magnetic fields are created due to the welding current. Be sure that the pipe is properly grounded. Be sure that the welding cables are not wrapped around the pipe.

I assume that the pipe base material is stainless steel internally clad with Inconel alloy. Please confirm. Also what is the stainless steel alloy and the Inconel alloy designations?

 

[racookpe1978]

Magnetism is very unlikely to be the cause: reverse lead hookup (electrode negative/electrode position wrong?

Bad cleanliness/not grinding to absolutely clean base metal/not wiping clean the surface after grinding with alcohol or cleaner?

Wrong electrode? Wrong current setting ? What kind of welding were they attempting?

 

[SectionIX]

Get a gauss meter. The reading should be 35 or less. If close to, or above, your problem is most likely residual magnetism. One tell is welding electrodes that freely stick to the pipe surface.


Your best bet is to use a demag unit for the root pass. Once the root is in place, the residual magnetism should subside to a workable level.

Failing the availibility of a demag unit, you can tightly wrap the welding leads close to the bevel. Be sure to use an odd number of wraps, preferrably five or more. This should reduce the gauss enough so you can perform the task at hand.

One thing to note is that that persons with imbedded cardiac appliances should not be performing these operations as it could affect the operation of the appliance. It can also negatively affect credit cards and other magnetic media.

 

[SectionIX]

hmmmm...should have read the post better. Probly not magentism for inconel, but the procedure outlined will work for other carbon steel pipe welding operations...

 

[SJones]

There was a theory somewhere, and I can't remember where, that the bimetallic couple behaves like a thermocouple and creates a thermoelectric current at the joint that impacts the welding arc. I'm not sufficiently electrically orientated to work out where the thermoelectric current would flow to. Sorry about that. I do know that 16-inch, 625 clad pipe with around 1 Gauss was extinguishing a mechanised GMAW arc. Not a magnetism level that one expects arc blow. Cable wrapping didn't cure it but turning the 'juice' up did thus negating the contractor's claim that 'magnetic pipe' had been free issued.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[JJPellin]

The last time a welder tried to blame magnetism on a weld that wouldn't take, it was just a poor ground. He was grounding his welding machine to the handle on a small bleed valve out in the pipe rack. The weld would not take. Once we found and corrected the problem, it worked perfectly.

Johnny Pellin

 

[SubPipeEng]

Related to ongoing discussion on magenetism of pipes, I want to clarify my doubt.
Recently, I came across requirement of Residual Magenetism (RM) values depending upon the pipeline welding process in one of the project specification for line pipe material. It was stated that if GMAW or GTAW processes are to be used for Offshore Pipeline Installation (Field Welding), RM should be less than 15 Gauss. DNV-OS-F101 allows upto 20 Gauss (average) and 25 Gauss (individual). My understanding is that weld arc blowing can be avoided if RM is limited to 20 or 25 Gauss for Carbon Steel line pipe.

1. Is there any limitation in achieving lower value of RM, say 15 or idealistically, near to zero?

2. Also, during the process of coating pipes, pipes are usually pre-heated to around 250ºC using induction coils. This results in the induction of residual magnetism within the pipes. How much RM can be expected at that stage?

3. Can we expect higher values of RM (greater than safe limit of 20-30 Gauss) after pipe manufacturing (including MPI) and pipe coating stages?

Thanks

 

[hacksaw]

250C does not reduce the residual magnetism any great degree in steel,

inductive de-gaussing works if you really need to meet a given spec, but time consuming, there are other methods but these are not generally suited to field fab

 

[Duwe6]

Inconel is notorious for being difficult to weld, especially the root. It requires a very high degree of skill, and quite a bit of practice. If the welder concentrates on making a 'pretty' bead, he/she is almost guaranteed lack-of-fusion to one or the other sidewalls.

I'm not a Tig {GTAW} welder, but a world-class one told me that the secret to Tig-welding inconel is to never dip the filler-rod into the puddle; just keep it close and allow it to drip into the puddle while concentrating on breaking down the sidewalls.

Try to 'borrow' a welder that is comfortable making x-ray & ultrasound acceptable inconel welds. Have him/her show your best welders the 'tricks' needed to make acceptable welds.

 

[CRG]

Duwe6, how do you let the filler-rod drip up into the puddle for out-of-position-welds.

 

[Duwe6]

I presume that the rod is laid on the basemetal, and surface tension draws the filler into the puddle. The main idea is to never stick the rod directly into the puddle.

 

[davefitz]

more to Duweg's point, a manual welder will turn down the amps to avoid spatter, and that asically means insufficient penetration . For an assured weld penetratration , it is useful to to perform the weld using a robotic/ orbital welder with specified amps, and accept the spatter.

 

[Mr168]

SubPipeEng: Just to elaborate on your statements a bit, I agree with the first value more so than the latter based on prior experience.

I had an issue with a heavily magnetized separator vessel in a supercritical boiler installation that was approximately 80 gauss at the welding end preps. After initial degaussing (had to call in Gaussbusters, as we were unsuccessful using induction equipment, and I was out of ideas after a week on the scaffold), we were around approximately 25 gauss, and a GTAW arc was still flip-flopping across the root gap.

A second run had us down around 20 gauss, and it was steadier, but still not adequate to install a GTAW root pass.

Third run had us down around 12 gauss, and they were able to put in a root pass without issue (2.25 Cr. material).