Hello Tom;
One must consider that D1.1 is intended for carbon steels or high strength low alloy steels joined to the same. Likewise, D1.6 is intended for stainless to stainless. Dissimilar base metal combinations, they are a tiger of a different stripe.
ASME has a different philosophy from AWS. Whereas AWS codes typically endorse the concept of prequalification, ASME has the philosophy of "show me it will work."
ASME operates under the presumption the person making the decisions is knowledgeable and familiar with the technology and processes they are using. AWS, not so much.
The A number is an additional constraint that comes into play when welding similar base metals, but involved additional complications when dissimilar metals.
Not only do you see A numbers in ASME, but you used to see the weld deposit chemistry as an essential variable in the old MIL-STD-8604 for welding aluminum alloys. The reason was due to the exact factors you mentioned.
AWS B2.1 used to have different F numbers for all the ferrous filler metals differentiated by the welding processes. It wasn’t until AWS tried to harmonize B2.1 with ASME Section IX that the multitude of ferrous F numbers were combined into the F6 grouping. That was a mistake from my point of view.
It has been my longstanding view that ASME is one of the few codes that allow the unwary to write a code compliant WPS that simply will not work. In that respect, I believe the AWS structural codes are more relevant and more conducive to developing a “workable” document the welder can actually implement.
I always write a WPS with the welder in mind, something ASME seems ignores. That shouldn’t surprise anyone familiar with welding, few of the people on ASME’s committees have ever struck an arc. An engineer’s perspective on welding is different from a welder’s perspective. The engineer is interested in the mechanical properties and whether the weld will perform as expected. They expect the welder to be trained, skilled, and knowledgeable. Those attributes that are skill related are not usually addressed by the ASME WPS in sufficient detail. Why, because many of the engineer tasked writing the WPS are clueless about those aspects of welding. Actually, I see AWS structural welding codes drifting in that direction as well.
All that being said, I find the A number, if properly used, a valuable means of ensuring the wrong filler metal isn’t substituted for the production weld. And for many of the reasons you’ve noted, the person tasked with writing or implementing the WPS must know when a chemical analysis of the deposited weld metal is more relevant than the chemistry provided by the manufacturer.
In reality, some of the element you mentioned, i.e., silicon, does affect the mechanical properties. Silicon and manganese are effective deoxidizers and are added to filler metals for that reason as well as others. With a change in shielding gas, i.e., CO2 to ArCO2 for example, those elements are not utilized as deoxidizers and they become alloying constituents. As such, the tensile strength and yield strength goes up at the expense of ductility. The change in weld deposit chemistry may result. Am I going to blindly use the A number provided by the manufacturer that qualified the filler metal using CO2 or as the engineer in charge consider the chemistry of the deposited weld metal?
One must also consider the fact that ASME Section IX also considers the shielding gas to be an essential variable. A change for one gas to another is something that must be considered and usually requires a supporting PQR to demonstrate the resulting weld will still provide the properties required.
Section IX considers the groove detail to be a nonessential variable. However, when welding dissimilar base metal combinations, the amount of dilution can change the deposit chemistry. So, should one blindly use the A number determined by the manufacturer when qualifying the filler metal on matching base metals, or base the A number on the actual deposited weld metal chemistry? Where should the sample be taken; from the center of a multipass groove weld, from the weld interface, if the weld interface, which one? Things can get pretty complicated very quickly. The point really is whether the groove detail has an affect on the deposited weld chemistry. I believe you and I agree that it does if the amount of dilution is substantially different based on thickness, and groove detail. That’s really when the A number shines. If the dilution substantially affects the resulting chemistry of the deposited weld metal, a change in A number results and a new supporting PQR is in order.
Much of this is beyond the purvey of many CWIs, so while a CWI may be able to write a simple prequalified WPS, additional education and training may be warranted for someone tasked with developing WPSs for combinations of dissimilar metals.
Just my ramblings and my opinion on the subject. I expect there will be blowback from some that work with ASME Section IX, but this is how I view the requirements of A number. Simply put, it attempts to prevent the mindless substitution of filler metals simply because they are from the same F number group. Consider that an ER70S-3 belongs to the same F group as ER309. Just because they are from the same F number grouping, we can agree that the chemistry, thus A numbers are going to be different. It is the A number that keeps us from substituting ER70S-2 in place of ER309 when welding carbon steel to stainless steel.
What about substituting ER308 in place of ER309 in the case of a single pass weld joint 1/8-inch carbon steel to austenitic stainless steel? Would the A number be affected?
Let’s assume the Cr and Ni are reduced by dilution so that only 60% of the Cr and Ni are contained by the fully mixed deposit, that’s an assumption, but reasonable for a square groove with no root opening. The ER308 has roughly 19% Cr and 9% Ni. The ER309 has roughly 23% Cr and 13.5% Ni. Both filler metals are grouped as being A-8. The chemistry of the weld deposit is going to be roughly 10% Cr and 6 Ni using ER308. The chemistry of the deposited weld is roughly 13% Cr and 7% Ni using ER309. The resulting weld deposit chemistries are sufficiently different that the A numbers would be different, Thus if the WPS was qualified using ER309, one could not substitute ER308 and expect the same A number. One would have to qualify a new WPS before making the substitution. If one blindly uses the A number derived by the manufacturer, the results would not be so favorable. This conclusion is bourn out if a solution is derived using the WRC diagram or the WRC diagram as modified by Kotecki and Liphold. The metallurgy of the deposited weld would not have the necessary Ferrite and the morphology would be unfavorable if one was to use the ER308 filler metal in place of the ER309.
Best regards - Al