I would direct the question to the Engineer for resolution.
Welder qualification a a demonstration the welder can deposit sound weld. Once qualified, the welder is qualified to weld a range of base metals. In some cases, a code will allow the welder to weld base metals from several families based on a single welder qualification test. For example, ASME allows the welder, qualified on carbon steel, to weld both carbon steels, austenitic stainless steels, and nickel alloys in production.
I am not familiar with JIS G3101 SS400, so I can not say whether it is similar to ASTM A36 based on chemistry or mechanical properties. Nor can I predict whether the Engineer (representing the owner) would permit the substitution.
Al
