American Welding Society Forum
Here is my question...
We are in the works of building a large stainless steel tank, built to ASME Section VIII Div I. We already have procedures in place to cover us, but all involve purging the root.
All i ever hear from the guys on the floor is "let me use RMD, let me use STT," etc because they do not want to have to purge it.
I am not worried about passing the mechanical requirements of Section IX, but what does the corrosion data look like involving 300 series, RMD, with no purge?
The question is, "do you have procedures qualified without root purging?"
You can add purging to a WPS that was qualified without purging, but you can't omit purging if purging was used while qualifying the procedure.
The answer to that question is no...
However, will it be worth my time getting one qualified?
I've seen root passes done with RMD/STT with no purge on 300 series stainless and they look beautiful...but am I losing anything because of it? Will i still get the corrosion resistance I am wanting from stainless? Or am i losing it and being fooled with smoke and mirrors?
That is what i am curious about.
Weld a plate without root purge and test it with an accelerated corrosion test. Many laboratories that do environmental testing have the equipment needed to perform such tests. The corrosion rate will be dependent on several factors such as the corrodents, concentrations, temperature, etc.
One test assembly can cut into several pieces that can be tested under different conditions. That would give you a sound basis for making some intelligent choices.
You can review the ASTM’s offerings on the subject of corrosion testing. NACE would also be an organization that may provide direction on corrosion testing.
Now, I’m just thinking out loud, but I suspect that if there is no discoloration on the root surface of the weld or adjacent base metal, the corrosion resistance is probably nearly as good as a root that has been purged. Consider for a moment that corrosion inside a pipe will occur if an electrolyte encounters dissimilar metals. The weld deposit is going to be different than the base metal in chemical composition or grain structure or both, either of which can lead to corrosion. It’s a question of whether the corrosion rate is low enough to provide the service life that makes it all worthwhile. Of course, we can’t overlook the material’s that are selected for the particular service. If the wrong material is used, the service life may not be as expected.
we have done a lot of work with RMD on Stainless without purge and it really depends on what the service is and what corrosion testing is done and how they are done, on how well it performs. We originally did testing for the LNG industry. We used a StainMix3 gas and used the correct Miller RMD recommended nozzle. The nozzle makes a difference by allowing more gas to tumble behind the root and give some shielding. We did our corrosion testing per ASTM A262 method A & E. We also used ASTM E562. Now this is where I might get some pushback, but on the corrosion testing we cut samples and prepped per the ASTM recommendation and samples that we kept in the as welded condition. Our thinking was to test up against real service conditions. My opinion only. The testing showed very good results and compared with our testing that we did on coupons that were purged, the results were almost identical. The root side color did not show excessive oxidation (some call sugar) but it was not perfect either. To quote our old friend of this forum Mr. Meadows (God rest his soul) "color matters". So, for the LNG industry it was satisfactory. When we sent our testing results to a major Oil & Gas company for another project that had a lot of 304L stainless they rejected it for the service that was going to be in the piping systems due mainly because the root side color. Bottom line? It is a great process for stainless as long you understand the service it will see and get client or end-user buy in.
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