Effect on mechanical properties of welding stainless steel without gas purge

[metaleng123]

Hello everyone,

I need to qualify a PQR acc. to AWS D1.6 on TP321 base metal. In existing PQRs of my current employer i see that similar tests were made on test coupons that were not back gouged or purged whitch means that "sugaring" occurs and every destructive test( tensile, bend, macro, impact @-55C) passed with acceptable results. Is this common practice? If so could anyone point to a relevant article / study?
More info on what i think would be the correct way to qualify:
GMAW-Pulse
Base metal : A240-Tp321
Thk.: 8 mm
Filler: ø1 ER347Si AWS A5.9
"Sugaring" removed by grinding and re-weld the root pass.
NDT and mechanical tests : VT, PT, RT, 2 Tensile tests, 4 bend tests, Macroscopic examination, Hardness Tests (HV10) and Impact Tests @ +20C
Design temp. of the welded structure is 660C.

Thank you in advance for your input.

 

[kingnero]

Sugaring does not occur with GMAW, if welded fast/cold enough. Doesn't mean it's a good thing to do, though.
It's different when using GTAW, where you simply cannot avoid sugaring.

A PQR doesn't take corrosion resistance in consideration, hence why it's done regularly without gas backing. As you've seen, a bit of discoloration does not impact the mechanical properties in a negative way. I think a microscopic examination at the height of the root pass (below the back run) would be interesting, but obviously not required.

 

[EdStainless]

You will also end up with welds that are not 321.
All of the Ti will be TiO2 and they will not be stabilized.
This changes their properties and greatly effects how they will respond to high temperatures.
This is very poor practice.
Soak samples for a week at 600C and then try testing them if you really want to know.
The weld method has little impact on the oxygen uptake.
Appearance tells you almost nothing about the weld.

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P.E. Metallurgy, consulting work welcomed

 

[metaleng123]

@kingnero Thank you for your input.I'll conduct a pre-qualification test to observe the level of oxidation on the back side, just for my information.

@EdStainless Thank you for your valuable insight. If TiO2 is present then re-welding the root pass won't be sufficent as HAZ will also be affected. Unfornuately I can't do more tests as they won't get accepted. Purging is a common practice when production welds are made, but it wasn't the case when qualifing PQRs which seemed odd to say the least.

 

[gtaw]

AWS D1.6 is intended for structural welding of stainless steel. Generally, corrosion resistance isn't usually high on the list of priorities. As you observed, one can weld the austenitic stainless without gas backing on the second side while welding the first side and the mechanical properties usually will meet the code requirements for strength. However, AWS prequalified joints are either welded with backing or the second side is back gouged and back welded.
For applications where the weld joint is welded from one side, without backing or without a back-gouging operation, the second side needs to be properly shielded while welding the first side if the corrosion properties are to be maintained. The root oxidation(sugaring) represents a "dissimilar" base metal leading to galvanic corrosion in a wetted environment. In addition, if there will be halides in the mix (salt water), one must limit the time in the critical temperature range to mitigate sensitization.
The stabilized grades of austenitic stainless steels are alloyed with titanium or niobium because they have a higher affinity for the carbon, so either Ti23C6 or Ni23C6 forms leaving the Cr intact to provide corrosion resistance in the HAZ of the welds.



Best regards - Al