Not sure what an A500B is as I don’t have the spec.  However if it were a normal weldable carbon manganese steel you would use a higher alloyed stainless filler wire such as a 309.  

The problem is dilution, as you mix carbon steel with stainless steel you create a chrome steel which is very hardenable, even a 309 consumable may not be suitable if dilution is high.  Do a procedure test and run a hardness survey across the joint to make sure you have no hard areas.

You might get some useful help and guidance from consumable suppliers/manufacturers

Regards

john@gowelding.com

atlarson,
I am impressed that the aerospace specs
did not cover the type of weld.  I would
contact them.  They may be able to offer
some info, but maybe you are lucky not
to have a spec by them.  Maybe it is some
liability issue.  Do the welds have to be
certified?

i agree with welding1, that 309 is the filler of choice for most "stainless" to carbon steel welds. he also mentions that dilution can cause a problem.
my experience has been that if i am to machine this connection after welding, i will use er347.
i am not a metallurgist, but i suspect the niobium and tantalum prevent the formation of "hard spots". all other properties are similar to 309--it's just a little pricey.
finally, as diamondjim mentions, if this is a liabilty issue, let the DE's come up with a welding procedure. maybe they'll bolt it instead!

Electrode selection is determined from the base metal chemistries and the percent weld admixture. The electrode should produce a weld deposit with a small amount of ferrite (3-5 FN) needed to prevent cracking. Use 309L (including ER309LSi) when joining mild steel or low alloy steel to stainless steels.

To all---I've been using 309 wire and stick when joining SS with SAE1020 or 41whatever TS with very good results over the years.
To chph---never tried E347---any other reason for this beside what you mention?

To diamondjim---I have found it rather common to lack weld specs on equiptment from outside contractors for aerospace tooling.  It never became a problem for me but, I can see the possibility/probability of abuse.


Rod

evelrod, ever had outgassing and porosity problems when o.f. or gta welding unknown steels? try some 347. the niobium is a great scavenger and prevents carbides from forming. works better than alka-seltzer[haha].