316/316L EBW difficulties and MTR discrepancies 2

[Dyskolos]

Hello,
I have a couple of questions regarding welding and materials I was hoping to get input on.

Background info:
We are electron beam welding a thin-walled (.030" wall thickness) tubing component to a "head" that is machined from barstock. Both parts are 316/316L material; the tube is welded, bead-reduced, and bright solution annealed, and the barstock is solution annealed bar.

Problem 1:
The weld has what looks like undercut around the circumference on the side of the machined component. In addition, there are depressions about the size of a ballpoint pen roller ball. I can see no issues with the fit-up (joint is quite tight and clamped together securely prior to weld. No tacking.) or cleaning procedures (liberal use of acetone to clean parts). Please see attached photo. I would greatly appreciate any suggestions.

Problem 2:
While investigating problem one, I noticed that our 316L barstock was attracted to a magnet, albeit fairly weakly. The tubing (also 316L), has no such attraction. I performed several hardness checks (Rockwell C, since that is what I have in-house), and the hardness ranges from well below the Rockwell C scale in the center to 21 HRC at the OD. Using the tables in ASTM A370 to "convert" this to Rockwell B makes this somewhere around 100 on the B scale. The MTR gives the hardness at 153 HB (80 HRB), which jibes with my center measurements, but is way off from my OD measurements. According to the MTR, this bar is solution annealed, but this does not appear to be the case. Am I correct in concluding that the ferromagnetism and excessive hardness are indicative of cold working, and that the MTR is incorrect?

Please let me know if I can provide any more information.
If it helps clarify, my background is manufacturing engineering; we do not have a welding engineer on staff unfortunately.

ETA: both the raw barstock and the machined components exhibit the magnetism and hardness issues. This is not as a result of machining.

 

[EdStainless]

Annealed just mean that it meets the spec requirements.
Check tensile properties if you want to know.
The surface could be slightly worked from straightening, this is fairly typical and in no way violates the spec.
What is the S content of each material? S has a huge impact on the fluidity of the weld metal and small amounts can matter.


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P.E. Metallurgy, Plymouth Tube

 

[Dyskolos]

Ed,

I'm not sure what you mean by annealed meaning that it meets spec. The spec requirement is for annealed bar, but I have what I suspect to be annealed and cold-worked bar.

Tensile properties from MTR:
39.3ksi YS, 87.2 UTS
This is in line with what I would expect for annealed bar, but it is likely this was from the center of the bar, where I know the hardness is good, so not much help.

I don't believe this is from straightening, as I have measured a hardness gradient showing the excessive hardness quite deep into the bar. At mid radius it is about 90 HRB. This would seem to indicate deliberate cold working to me.

The sulfur is something I had not thought to check. It does happen to be as it was in spec, but at the top of the allowable range. Something to consider.

 

[EdStainless]

When we autogenously weld 316 with 0.002% S it is difficult to weld unless it is very thin, at 0.012% the welds are very fluid and smooth, and at 0.025% the sulfide inclusions in the welds are a serious issue.

Some bar specs allow 'cold sizing', and as long as the full section or mid-radius tensile pass it is still considered annealed. Either way this is not having any impact on your welding.

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P.E. Metallurgy, Plymouth Tube

 

[Dyskolos]

Ed,

We are at .026% (max allowable is .03%)on the sulfur for this heat, so I think you may have nailed it. I don't see any obvious inclusions, but perhaps the "depressions" or "pits" are a result of inclusions? I will source some material with a sulfur content in the .01 range and see if our welding improves.
This link suggests between .005% and .017% sulfur content for maximum weldability, which lines up with what you have observed. It talks about orbital GTAW, but I think it will translate to EBW.

http://www.arcmachines.com/news/case-studies/effects-sulfur

Interestingly, we are also welding this heat to a 718 inconel piece with beautiful results.

I agree the cold working is not related to the welding issue; it is a separate issue I found while investigating the weld problem. I have made some inquiries at various material suppliers and it appears to be very common to cold finish these materials, to the point where true annealed stock is not available off the shelf. Unfortunately, our customer requires fully annealed stock, as the presence of ferromagnetic material can affect their instrumentation. I will most likely pursue getting our material annealed and re-test the mechanicals at that point.

Thank you sir, you have been most helpful!

 

[WKTaylor]

Any reason You couldn't specify CRES 321 or 347 Annealed... these alloys weld consistently better/cleaner than 316 or even 316L [or any other 3xx alloy CRES].

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can b

 

[Dyskolos]

I would not mind doing that at all, but it would require a concession from our customer. In my experience they are reluctant to do so for established product lines.

 

[EdStainless]

I actually prefer to keep S between 0.006% and 0.012%. Yes it applies to TIG but the weld fluidity will still be the same for your application.
Don't use a stabilized 321 or 347, the addition of Ti and Nb crates a lot of weld trash. And they don't do anything that >0.015% in 304L won't do except have lower corrosion resistance.

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P.E. Metallurgy, Plymouth Tube

 

[WKTaylor]

EdStainless...

Please explain Your objection to 321/347 fusion welds... with ER347 weld filler... or filler-free using excess edge material for melt-down. I am really confused.

Perhaps the 347 [less commonly specified for welded CRES parts now-days] is Your real objection.

Or am I otherwise brain dead, IE: missed an important function of this particular weld Assy that is incompatible with 321/347 alloys??

We use a lot of 321 [preferred] or 347 welded to 321 [preferred] or 347 [NO OTHER COMBINATION ALLOWED for welded parts] that produces beautiful/consistent/durable welds for high temp air duct applications including aircraft engine exhaust parts [turbine and piston engines, hot carbonized gasses, high-cyclic pressure/sonic pulses].

https://www.sandmeyersteel.com/321-stainless-steel-aerospace.html

The only other alloys that perform better in this regime are nickel alloys [Inconel] 625 [using ER625 weld filler].


Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can b

 

[EdStainless]

The reason for 321/347 is to prevent intergranular sensitization (localized Cr depletion due to the formation of chrome carbides in grain boundaries). This was important when getting below 0.05% C was nearly impossible.
The sensitization data for very low C 304 (<0.015% C) is just as good as the data for 321/347.
And with the stabilized alloys you have a lot of Ti or Nb carbo-nitride in the structure. Not mention the significantly higher tendency for weld slag to form from the highly reactive alloy additions.
I am not a fan, would rather work with clean 304 any day.


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P.E. Metallurgy, Plymouth Tube

 

[Dyskolos]

All,

Thank you for your assistance so far; can I trouble you with some additional questions?

We are unable to source material with the sulfur content in the required range. We have tried all of our usual suppliers, as well as anyone else we could think of. Carpenter was the only company to provide a quote. Unfortunately, they quoted a leadtime of 10 weeks, and we would have to buy a mill run. Needless to say, this is something of a setback.

1) Can anyone recommend a potential source for 316L with a restricted sulfur content? Looking for 1.75 diameter barstock.

2) Can you give any suggestions for reducing the weldability issues with our existing material, at .026% Sulfur.

 

[EdStainless]

You should be able to find very low S (<0.005%) easily. It would work.
The bar is made because fittings for sanitary piping systems are machined from it.

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P.E. Metallurgy, Plymouth Tube

 

[metengr]

WKTaylor
From the power generation perspective, the 321/347H materials are just fine to use as tube materials and have no significant weldability issues. There is plenty of this material in reheaters and superheaters in remaining fossil units across the US, still performing after 50 years. Unless they close more plants, this stuff will outlast me.

 

[Dyskolos]

Ed,
The only place I can even find restricted sulfur 316 online is with Carpenter. Can you advise any specific suppliers?
Perhaps we are looking in the wrong places. Our industry is Oil and Gas and this seems to be an unusual requirement in this industry.

 

[EdStainless]

The bar that you commonly fins should be very low S, below 0.005%. It will be hard to machine but it will weld fine.
The high S stuff is free machining, and the weldability and corrosion resistance both are poor. You don't want it.
Check with major SS bar distributors such as TW Metals or Ryerson.

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P.E. Metallurgy, Plymouth Tube

 

[Dyskolos]

TW already no-quoted us, but that was asking for .005 to .017%. We will amend our RFQ to see if they have <.005

Thanks