Your "procedure" for EVERY different kind of welding, EVERY different filler material, first base metal, second base metal, preheat conditions, weld prep, gas type (and all of the other "essential" variables) and all of the other "non-essential" variables MUST be approved and tested FOR each different combination that you are permitted to weld.
So, we cannot answer your question. You need to read your company's weld procedures and find out what combinations are permitted to be used in this particular joint.
Now, to do an uncertified joint, just melt two pieces of metal. Let the glob of metal cool.
Lets see, what does the code of construction say...ASME B&PV Code, Section IV, Part HLW-44
HLW-440 WELDING PROCESSES
The welding processes that may be used under this Part
are restricted to the following:
(a) Arc or gas welding processes are restricted to
shielded metal arc, submerged arc, gas metal arc, gas tungsten
arc, plasma arc, atomic hydrogen metal arc, laser
beam, electron beam, oxyhydrogen, and oxyacetylene.
(b) Pressure welding processes are restricted to flash,
induction, resistance, pressure thermit, pressure gas, and
inertia and continuous drive friction welding.
(c) Definitions are given in Section IX, which includes
variations of these processes.
Now, you know the answer above, the next part requires an ASME Certificate to design and fabricate the hot water heater.
As metengr indicated, yes you can. But you need to ask the question, is the process best suited for the work (thickness of material, high production environment)?
