This article provides information about A number in welding procedure specification based on requirement of ASME Code Section IX.
A number gives a similar chemical composition in a “as weld” condition. The A number is essential variable in most of welding process in ASME Code Section IX. The Table QW-442 in ASME Code Section IX provides the list of A number. Please note the A number in WPS does not refer to the filler wire or electrode chemical composition; it refers to the deposited weld metal chemical composition. The filler metal and deposited weld metal have different chemical compositions.
It can be obtained from procedure qualification coupon. The small piece of welding can be removed and chemical analysis test to be performed. Then the obtained chemical composition must be compared against the table QW-422 and A number to be selected.
Alternatively if your welding process is one of SMAW, GTAW, LBW and PAW processes you may use the filler metal specification or test report of filler metal manufacturer.
Similarly you can do same assessment for GMAW and EGW processes but the shielding gas should be as the same of welding procedure specification.
Again, similarly you can do the same for SAW but the flux should be as the same of welding procedure specification.
As mentioned in above, the A number like P number and F number is an essential variable in most welding processes. It means if you have a WPS and you need to use that in other job with different A number (it means you are using different filler metal) then you need to re-qualify your WPS because it is essential variable. The A number 1 and 2 are exceptions. You can consider it as nonessential variables.