It does make one wonder.
MIL-W-8604 had a similar requirement, i.e., A-number, but they didn't call it that, for aluminum. It makes sense to be concerned with weld metal chemistry when welding aluminum because dilution can play a major role in whether the weld metal will be susceptible to solidification cracking. An example would be welding 6061 or 6063 aluminum alloy. Insufficient dilution of the base metal with the filler metal will push the crack tendency. Changes in dilution, thus weld deposit chemistry, also affects the final strength of the welded joint.
A-number does play a role in welding stainless steel as far as determining ferrite if using the WRC diagram. However, the A-number isn't what is used to calculate the chrome and nickel equivalencies directly. Instead, the chemistry of the diluted weld is the basis of determining Ceq and Neq. As you pointed out, the chemistry can easily change when a thick joint requiring many weld beads is involved. That leads to the fact that there can be several regions in the weld, each having different microstructures and potentially different ferrite numbers, and different susceptibilities to solidification cracking and potentially Sigma Phase.
I look at the A-number as a red flag warning the engineer that he needs to consider what is happening to the chemistry of the weld as we change base metals and filler metal combinations. What is actually happening with dilution in the weld as we move from the weld interface to the centerline of the weld? Without the A-number, the influence of dilution and the resultant chemistry could be easily overlooked. Where to perform the chemical analysis is something that should be considered because the maximum dilution will be the first welds deposited against the base metal. Just one more factor the engineer needs to consider.
As you noted, dissimilar base metals present a situation that is often more complex than the situation encountered with welding base metals of the same specification or alloy family.
Best regards - Al
Hi Al
We must remember that the qualification of a WPS to ASME IX does not actually tell us anything about the serviceability of the weld, other than that it has suitable mechanical properties. The corrosion resistance and service degradation is not measured in the standard ASME IX PQR tests. In this light, the intention of the A number has nothing to do with predicting sigma phase formation, or anything else regarding the service of the weld. While you can determine the A number from an analysis of the weld, you can also determine it from the chemical composition of the filler used, so it does not take into account dilution etc. This indicates to me that this is merely a manner of grouping ferrous filler materials into groups that will give similar weld deposits, to reduce the amount of PQR testing. It does not mean that from a serviceability point of view any fillers in a particular A grouping can be interchanged, it merely means that a similar type of deposit will be obtained, and from a strength point of view, a similar result will be achieved. It still remains the responsibility of the engineer/s to make sure that the weld will deliver the desired results from a service degradation point of view. It is for this reason that many client specifications call for additional testing, such as: ferrite determination; pitting corrosion testing; stress corrosion cracking testing; microstructural evaluation etc.
Despite what may be allowed by the code, the engineer needs to perform his/her calculations and determine if the weld deposit will be suitable, and then follow up with the required testing, in relation to the risks associated with the particular weld/s.
Regards
Niekie
I thought I made it clear that the A-number in itself wasn't used to determine Creq or Nieq or to determine the potential for Sigma phase. I must have missed the mark.
My apologies. Thanks for the clarification.
Al
Our company has a WPS written to Sec IX with 2 pqr's supporting it for GTAW. One is for 304L to 304L using ER308L filler and the other is for 316L to 316L using ER316L. I would like to know if I am interpreting the code correctly when I think I can use this procedure to weld 304L to 316L either using ER308L or ER316L because they are the same P-numbers and F-numbers. Would I also be correct in saying that I could weld 316L to 316L with ER308L wire if that would actually ever be needed? I'm pulling this out of QW424 (Base materials used for procedure qualification) and base materials qualified. QW404.12 for the filler metal kind of trips me up a bit and I think that would probably kill the aspect of using ER308L to weld 316L to 316L because we don't have a pqr for it. Although impacts aren’t required so would I actually fall back into the good to go category. I would greatly appreciate a little hand holding if anyone has a minute, but not in the funny sense.
