P91 to SS Weld
P91 to SS Weld
(OP)
Does anyone have experience joining SA335 Grade P91 to SA312 Type 304L stainless? In the absence of other advice, I plan to butter the P91 with ERNiCr3 filler metal and apply PWHT, then weld stainless to the buttered section again using ERNiCr3. Am I headed in the right direction? Thanks.





RE: P91 to SS Weld
Have welded A217-C12A to A376-TP316
GTAW Buttering using Metrode 20.7Nb and 250 Deg. C Preheat
PWHT @ 750 deg. C
SMAW weld using Metrode Nimrod 182KS
RE: P91 to SS Weld
If so, that is ERNICr-3. The Metrode Nimrod 182KS is NiCrFe-3. I think either combinations would be fine. However, since P91 has nitrogen, I would use the one that has the lowest Nb available. Nb is supposed to cause brittle zones where it formes nitrides. It isn't always a problem, as super austenitic and duplex has often be welded sucessfully with Inconel 625 (NiCrMo-3), but why not aviod the chance if possible?
RE: P91 to SS Weld
RE: P91 to SS Weld
Normally P91 is used for high temperature creep strength, and that would imply comparable use of 304H not 304 L. Also, because of the high cyclic shear stress imposed on the weld after cyclic heating to design temp followed by cool down to ambient, cracks may eventually appear in the mid wall , detected by angle UT. One important measure to prevent failure is to limit stresses caused by bending moments at the weld to less 20% allowable stress for the weaker alloy.
RE: P91 to SS Weld
Sorry you are correct 20.70Nb
RE: P91 to SS Weld
While Inconel 617 and Inconel 625 fillers may also work, I would not suggest them. As you are welding the P91 to 304L, you apparently don't need the high temperature strength these alloys provide. The ERNiCr-3 is as strong or stronger than the 304L, and is therefore strong enough for your application.
Even at higher temperatures, there are some cracking mechanisms that sometimes make the use of Inconel 617 and Inconel 625 problematic. It's also hard to imagine that the dlution effect of the niobium in Inconel 625 will be helpful to the P91 weldment.
RE: P91 to SS Weld
RE: P91 to SS Weld
I,however, do not recommend such joints for use in todays combined cycle HRSG power generating stations or any other application where thermal cycling is anticipated. Weld failure by thermal fatigue is inevitable. Dissimilar butt welds in pipe have failed in fewer than 50 full temperature cycles. Repairs using best EPRI practices have failed in less than 10 cycles.
If the P91 material is operating below 1000 F, I would recommend welding a P22 transition to the P91 alloy and then welding the 304L to the P22 transition. This way you avoid having to postweld heat treat the P22 to 304L weld.
RE: P91 to SS Weld
"first nickel base weld metal is deposited on one side of the T91 piece followed by a NT treatment similar to T91 base material. This transition piece can tehn be welded on site to T91 and to the austenitic steel. The T91 side is a regular similar weld with matching consumable and local PWHT, whereas the austenitic side is welded to the nickel base weld deposit using a nickle base consumable without PWHT"