ASME Section IX Question - Base Metal

[Guest102023]

Base metals are P-45 to P-45 with a permanent P-43 backing strip. I advanced the opinion that this constitutes a dissimilar weld joint, but could not find a proof text in Section IX, and I do not have access to interpretations.
Based on common sense and 'engineering judgment' I believe a new PQR is required when a different base metal is added to the joint.
Am I right?

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[weldtek]

Brimstoner,
This question will likely evoke many opinions. You didn't specify the application however virtually all of the work I'm involved in is fabricated to ASME Sec VIII so my response is based on that.
Even though Sec IX shows backing as a non essential variable, for pressuring containing applications permanent backing is considered as pressure containing material.
Considering that I agree with you. If I were faced with a similar situation I'd qualify a procedure to suit.

 

[Guest102023]

Thanks weldtek,
I've always considered Section IX's treatment of backing to be a confusing mess.

I look at it this way: suppose the proposed application had P-1 base metal on either side, with a P-8 permanent backing. Would any welding engineer seriously claim that was not a dissimilar weld, or would actually be feasible using a P-1 only WPS?

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[Duwe6]

I don't believe this problem is addressed in IX. With the basemetal in the same 'extended family' P-40's and having similar thermal expansion and corrosion characteristics, calling it inconsequential is not a ridiculous position to take. However, if it were me the fabricator would be asked to 1) -- provide a WPS that covered a dissimilar metal joint or to 2) -- rapidly run a PQR for a new WPS, or 3)-- get a written opinion from his AI that the small mismatch in backing material is indeed inconsequential.

This would make an excellent Code Interpretation inquiry.

 

[weldstan]

In a similar situation when the backing was left in service, I required a PQR for P45 to P45 and for P45 to P43. I would not want to have a weld in Alloy 20 with a 625 backing made with E/ER320LR. The weld filler must be compatible with both materials.

 

[Duwe6]

Easier solution came to mind; with a very good fitter(s) the weld gap can be made fairly uniform. With a fairly uniform fit-up gap, a good [Very Good preferred] welder can make that joint from the OD, and achieve the required efficiency. ASME VIII Div-1 Table UW-12:

"Butt joints as attained by
double-welding or by [highlight #FCE94F]other
means which will obtain the
same quality of deposited weld
metal on the inside and outside
weld surfaces[/highlight] to agree with the
requirements of UW-35.
Welds using metal backing
strips which remain in place
are excluded."

So with Full RT, this will give a 1.0 joint Eff, with 'spot' RT 0.85, and no RT 0.70
with a left-in-place backing strip: 0.90 at Full, 0.80 at 'spot' and 0.65 no RT.

A goodly number of young designers get focused on
"Single-welded butt joint Circumferential butt joints only,
without use of backing strip" being Eff = 0.60.

This does not apply with a good fitup, a good welder, and proper welding procedure. If this technique didn't work, how would we be able to weld pipe diameters smaller than the size of our welders? IMHO, backing bars are a crutch for bad fitters and poor welders. The good ones fit a reasonable gap and the welders make a weld with a root that will 'shoot' [RT].

 

[Guest102023]

Duwe6,

Your first post: I would be very surprised if nobody has asked this before. If someone with access to the full set could investigate I would greatly appreciate it.

Your second post: The joint has been designed for me. This is a repair 'situation'. The container is not technically a pressure vessel, but because of the contents we are treating the welding as if it were. In any event the WPS being developed will be used for pressure equipment in future.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[Duwe6]

Backing 'bars' [strips] have their uses, it's just that I've seen some used on Main Steam in high-pressure and super-critical coal-fired power plants. Not an appropriate use, IMHO.

Since you are using one, dissimilar metal bars need to be considered as dissimilar metals. There will be mixing and dilution into the filler metal; pretty much the definition of 'welding'. AWS calls this out plainly, you are required to use same 'M-no.' [think P-no.] for fusing backing.

 

[Guest102023]

Duwe6, dilution was my logic exactly.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"