Dear Experts
Auto-welding is totally new experience for me . .
RMD(Regulated Metal Deposition )seems so far only used in GMAW-short circuit transfer.
Is RMD possible to be used in FCAW-Auto wedling
Welding Engineer of Nanjing Auto, told purging/ sheilding not required in welding CS / SS pipe butt joints!
I would like know how to go about prepare pWPS to start qualifying PQRs for CS, SS, CuNi pipe materials,
using multiple weld processess- RMD(GMAW) x SAW , RMD(FCAW) x FCAW... etc.
Appreciate if you could share your experience
Thanks
Hi, Nalla, I doubt you can successfully weld using RMD for FCAW welding. FCAW is normally operated in globular or spray type transfer.
If you go to Miller Electric's website they state that one of the benefits of RMD is that the backs side of the weld won't sugar up with the RMD process (SS filler metal). However, they state that for suituations where high purity is required such as pharmacuticals or food, chemical industry, the heat tint on the weld might not give adequate corrosion resistance. For many service environments, though, the heat tint won't matter. With RMD I would expect you will get some heat tint, but little or no sugaring.
Getting started on a pWPS is a big topic. What fabrication codes are you working to ? e.g. ASME B31.1, B31.3, BS EN, etc. ?
Regards, Sol
Dear Solluz
Appl. Code- ASME Sect IX & Const Code ASME B31.3
My position is that the service conditions and base metal combinations will dictate whether root shielding is necessary.
You have a combination of two dissimilar base metals, i.e., carbon steel to stainless steel. The former has little corrosion resistance while the latter has good corrosion resistance. However, if the system is used to convey a liquid that is an electrolyte, the combination of dissimilar metals is going to result in creating a galvanic cell, thus galvanic corrosion will occur at the juncture of the two dissimilar metals whether the root is or is not shielded during welding. Nearly any fluid, unless it is dielectric, is going to promote corrosion. Even raw petroleum products often contain sufficient moisture and acids to promote corrosion.
If the system is subject to corrosion, the joining of dissimilar metals is not the ideal situation, whether they are welded or bolted, or screwed together. Corrosion is going devour the less noble metal, in this case the carbon steel.
I would side with the welding engineer in this situation. If the service environment is not corrosive; no problem. If the service environment is corrosive, the carbon steel is going to be sacrificed whether the root is protected with shielding gas or flux or no suplemental shielding is provided.
The mechanical properties of the welded joint will meet those of the "weaker" base metal or filler metal used to weld the joint. The root oxidation, whether it "looks" good or not will not be diminished because of the absence of root shielding when welding the combination of carbon steel and stainless steel. By stainless steel I am assuming you are welding austenitic stainless steel and not martensitic or ferritic stainless. Not that it would make a difference other than the degree of difficulty in welding the joint.
One consideration that must be taken into account is the expected service life of the joint. If it is relatively short, the rate of corrosion may be such that even with a limited life expectancy, it is sufficient for the duration of the need. What does it matter if the joint will corrode and leak in two years if the system is only expected to be in operation for 6-months?
Best regards - Al
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