Beware of 317LMN Castings!

by Paul
(New York state)

Dear Elia- We are a manufacturer of chemical processing equipment, and as such, we frequently weld base materials that must be strong and highly resistant to corrosion. On a daily basis we weld Duplex, Super Duplex,High-nickel alloys and titanium. We recently experienced a problem while welding a high molybdenum version of 317L stainless steel. The material is UNS S31726 containing 4-5% Mo. We were welding flanges or couplings to matching grade 1/2" wall pipe. Our coupling joint sees rugged service and must be able to withstand torque and bending loads. We prepare the joint by machining a double "J" groove into the coupling and sliding the coupling onto the pipe. We call the side of the coupling that is flush with the face of the pipe the "Face" side and the inboard groove is the "radius" side. We normally weld the face side complete and then start the radius side. In this case we were welding with an ENiCrMo-3 electrode, SMAW process, maintaining 300F interpass. We found that our radius side BASE metal was indicating micro-cracks alongside the weld edge. I guess it would be safe to say the cracks were in the HAZ. The cracks were oriented parallel to the axis of the weld, and only in the base metal. We were surprised to encounter the problem. Welding information for 317LMN is very basic. The only rule is to use a high moly filler metal. We started to investigate and noticed something odd on our Material Test certificates-the mechanical properties appeared questionable. Our paperwork showed an Elongation value of 26%. We expected to see a minimum of 40%. We also noticed the document made no reference to forging. We visited the vendor's website and found that the flanges were produced by a spinning casting process. We are in the process of double-checking the chemical analysis of the material at an independent laboratory. I suspect we are going to find elements such as sulfur, phosphorus and possibly lead are out of specification. I would suggest to your readers that they specify forged 317LMN if they plan on welding product forms other than plate or pipe.

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Jan 19, 2008
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Follow up
by: Paul (New York State)

Hi Elia
Thank you for the information.
We did have the chemical work done and the only element out of spec was nitrogen. That surprised me, as I expected to see sulfur or one of the other "bad actors" out of line.
It didn't matter though, as the nitrogen out of spec gave me enough evidence to convince the vendor that his certification paperwork was a tad suspect.
We recovered our $20,000 and rewelded the equipment with forgings that welded beautifully. All is well that ends well (For us anyways!)
I enjoyed reading the comments.

Jan 18, 2008
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Segregation
by: Timothy E. Volin, Ph.D.

Interesting!


I suspect that the problem is a result of the use of a casting rather than a forging or other wrought product. Stainless 317LMN is a complex alloy with a lot of alloying additions. As a consequence of this there can be a significant range of solidification temperature, that is, a range of temperature over which both solid phase and liquid phase co-exist. Segregation of the alloying additions occurs between the liquid phase and solid phase, leading to differences in composition between the initial solid phase formed, and the final solidification of the remaining liquid phase. The alloying additions that tend to depress the solidification temperature of the remaining liquid phase, particularly Silicon, Phosphorous, Sulfur, are particularly strongly segregated to the remaining liquid phase. The result is that there will be regions in the solidified structure, the last areas to solidify, that have a melting point well below what would be expected from examination of a phase diagram. These areas may be more brittle, also, causing the reduced ductility observed by the author of the contribution.


During welding, these lower melting point regions will liquify in the heat affected zone (HAZ). As a liquid, they will be unable to sustain any stresses imposed by the welding process, due to contraints and/or thermal expansion and contraction. This will cause the micro-cracks observed.


I doubt that they will find the alloy out of specification compositionally, but they are correct to check it. The problem may occur in alloys within the specification range. If the products were purchased to a spec that referenced an ASTM or other spec that required the elongation they expected, the product could be considered out of spec for this reason and rejected. Alternatively, if they referenced a spec that required the product to be manufactured from a wrought alloy, they have another reason for rejection.


The segregation that occurs in casting is reduced substantially or eliminated by heat treatments and hot and cold working in wrought products.


Micro-cracking during welding in austenitic alloys is also mitigated to some degree by adjusting the alloy composition to result in the presence of a minor amount of ferrite phase in the alloy, which tends to "capture" the temperature reducing elements. The compositions can be deduced by referring to the Schaeffler Diagram, which can be found in most discussions of the welding of stainless steel.


Hope this helps!

Jan 17, 2008
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Possible Culprit
by: Tom Barker

In my experience with these 'forgings', copper is the culprit. 17-4 has the same issues, and
some of the new SS FDM's [fused deposition modeling].

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