AWS does allow, simply qualify a procedure per ASME Section IX. What you have to do is determine if the right design is to have a dissimilar metal joint in a structural member. There are differences in strength, thermal expansion, rigidity and possible galvanic corrosion aspects that should be fully analyzed before you decide to more forward with the design welding.
Since your question refers to AWS D1.1 Structural Welding Code/Steel, you can qualify the procedure to Clause 4 parts A and B. The alternative is to qualify the procedure to AWS D1.5 Structural Welding Code/Stainless Steel as a combination of dissimilar metals. The requirements of either code are the same relative to how the welding procedure is qualified.
If you are fabricating a pressure vessel or pressure piping, ASME Section IX and the applicable construction code, i.e., Section I, VIII, B31.1, etc., would be appropriate.
If the structure does not involved pressurized components, AWS would be more appropriate.
In any event, you need to know exactly what alloy of stainless steel you are welding, whether it is martensitic, ferritic, austenitic, duplex, or precipitation hardened stainless steel. You also need to know the specific chemistry of the stainless steel and the carbon alloy steel being joined. From the chemistries of each, you can approximate the chrome equivalency and nickel equivalency to estimate the ferrite number you can expect using different filler metal/base metal combinations. I suggest using the WRC diagram as modified by Kotecki and Lippold as the means of estimating the ferrite number of the weld. You will need sufficient ferrite to minimize the potential for developing hot cracks. At the same time, too much ferrite will increase the potential for forming Sigma phase, which is an embrittling phase that is not desirable.
If you are welding an austenitic grade of stainless to a carbon or low alloy steel you will most likely find that either a type 309 or 310 filler metal will provide the proper ferrite number. Still, it is best to estimate the ferrite number using the chrome and nickel equivalencies before actually welding a test piece for the procedure qualification record (PQR).
AWS D1.1 or D1.5 will provide a listing of the essential variable that has to be addressed when qualifying the welding procedure. Sample forms that can be used and an example of the data to be recorded are provided in the Annex of D1.1.
One small thing...
Structural Welding Code for Stainless Steel is D1.6. I agree with gtaw, I think he just noted the wrong AWS Code.
The pre-qualified welding details are very similar in D1.6 and D1.1. Use D1.6 to select the appropriate filler metal. For most cases probably 309. Use D1.1 to determine if preheat is required for the carbon steel. The SWPS's do not overlap for D1.1 and D1.6, but qualification should not be difficult.
