Ferrite

[sheuer]

Will passivation or pickling remove ferrite from the surface layer of 316L stainless steel? I'm not talking about free iron contamination, but the actual ferrite as part of the grain structure of the metal.

I have a customer that I'm building a vessel for and the product is sensitive to ferrite levels above 5 FN. My base material is at 2-3 FN but the welds go up to 9-13 FN, even using special "low ferrite forming" weld rod gives me a 5-6 FN. Since I'm only worried about the exposed/wetted surface ferrite level, I was wondering whether passivation or pickling will lower this surface ferrite level.

 

[stanweld]

No.

 

[sheuer]

I didn't think so but it was worth asking the question. We are going to do some samples with the low ferrite weld wire as a cover pass and potentially a welding gas with some nitrogen content (we read some articles that this will help to inhibit the formation of ferrite) to see if we can better control the final FN we are reading.

Thanks!

 

[stanweld]

Nitrogen is an austenite former and has been used successfully in reducing weld ferrite content. You may also wish to consider ER316LMn. This is a fully austenitic filler metal with FN < 0.5. The added Mn aids in resistance to hot cracking.

 

[EdStainless]

Good ideas Stan, these are the things to try.

I have seen people use 904L filler when they needed low weld ferrite levels.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[sheuer]

All good ideas. I have to be careful as this is not only a pressure vessel under ASME code & PED certification but it is a vessel for pharmaceuticals and so has other material constraints besides the low ferrite. We tried MIDALLOY ER316L LF (Low Ferrite) CRYO BARE WIRE, I'll look into the ER316LMn you mentioned. I'm trying to limit the changes from our current weld procedures, otherwise I'll need to requalify the procedures as well as our welders.

 

[metengr]

I believe the recommendation by stanweld regarding ER316LMn will work.

 

[sheuer]

Thanks for the help and recommendations! I'll give them a try and post the results in case someone else has this situation come up.

 

[EdStainless]

Can you use 5% Nitrogen in your weld gas? That will help as well.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[stanweld]

Ed,
Thanks for reminding me about 904L; forgot about that one. Used it over 30 years ago for a pharmaceutical application on 316L vessel.

 

[Guest102023]

First off, 2-3 FN in the base metal, although possible, is staggeringly high, and brings the test method/inspector into question. That is not a good starting point when trying to keep the weld deposit FN down (dilution factor). The only instrument I can recommend is the Fischer Ferritscope, and make sure the surface is properly prepared.

No chemical or mechanical surface treatment will change the ferrite content.

For welding you don't need to mess around with fancy gases or other metallurgical tricks like cover passing or even alternative filler metal classes. You just need to source consumables formulated for cryogenic applications, and request deposit ferrite on the MTR. I have specified Metrode products for a couple of clients recently. Standard issue E316L will consistently deposit 5-6FN.

Caution: low-FN covered electrode may not weld as smoothly as the regular stuff.