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Up Topic Welding Industry / Metallurgy / 316 SS basemetal cracked during root pass
- - By poddnahh Date 11-25-2008 20:12
I have a situation of a old to new weld both 316 SS threaded olet welded to inservice phosphurus acid service. When perfoming the root pass to the olet the pipe basemetal cracked circumferentially from the olet weld. The crack goes from 12 o-clock to 3 o-clock and 12 o-clock to 9 o-clock away from the olet. Can anyone give me a reason for this to happen. All input would be greatly appreciated.
Parent - - By 803056 (*****) Date 11-30-2008 02:03
By old weld to a new weld I envision you welding a new fitting to an existing run pipe. If that is the case and the existing system contained phosphoric acid, did you grind the surfaces of the existing pipe, including the root surfaces, inside diameter of the run pipe, etc. to remove any material than may have been contaminated with the phosphorus?

Phosphorus is held to very low concentrations in base metals to minimize cracking problems. In your case, the phosphorus is possibly introduced via the residue of the phosphoric acid.

Just a thought.

Best regards - Al
Parent - By js55 (*****) Date 12-01-2008 14:38
Not just a thought Al. I think its dead on. SS base metal cracking is going to be liquation driven. Liquation will driven by the presence of low melt point constituents such a P.
Parent - By gwg (*) Date 12-13-2008 01:17
I absolutely agree with cleanliness prior to welding. One item that I am unsure of is this, was there a previous fitting in this location or was this a new fitting installed in an existing 316Lss pipe? The reason for my asking is this, if there was a fitting originally at this location and it was being replaced with a new one, I would expect there may have been a problem with the base material. Why? Because with phosphoric acid handling systems halides can be present that will result in SCC at weld locations. A simple liquid PT prior to welding the new fitting would have confirmed this.

If you drilled a hole into an existing phosphoric acid line and you developed a crack in the adjacent base metal that indeed would be hot cracking. However, I believe this would be caused by inadequate delta ferrite in the partially melted fusion zone of the root pass. It is highly unlikely that phosphorus ions from residual acid would dissolve into the weld puddle and result in hot cracking. The phosphorus would have to have been dissolved as an alloy element in the base metal for this to happen.

So, if the base metal adjacent to the weld is developing liquation cracks or solidification cracks along the edge of the root pass it is because the delta ferrite concentration in the partially melted base metal fusion zone is too low in percentage in comparison to the actual weld deposit ferrite content. As the weld deposit solidifies it results in significant tensile stresses that can result in cracks in the weaker partially melted fusion zone of the weld at it cools.
Up Topic Welding Industry / Metallurgy / 316 SS basemetal cracked during root pass

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