Mike,
Some of the welded pipe specifications will designate the plate to be used in which case the plate chemistry is acceptable. I do not see that in 312. I just don't see any way to get to 240 from 312 even though everyone would understand that 240 is probably the most likely to be used. So, IMO, 312 is saying you can use any plate you want but it has to comply with these requirements of which chemistry is one.
Mike,
Neither SS nor ASME are my strong suits by any means but I would like to give you another perspective on this:
If I, as a TPI, were to come into your facility to take a look at progress and compliance then I am going to reject the use of this material as it is not what my customer specified for his job. I don't care what they made it out of. The end product has to be what my/your customer wants. It is your job to make sure that when I, as the TPI, look at it I find all aspects to be in order so that our customer is happy.
You ordered it right. They need to supply it right. And since it is not what you ordered, send it back. They need to make it right. No money until they do. They can argue till the cows come home, and even later if they want, but it is not what you ordered and it is not what the customer ordered or needs.
It actually is an open and shut case. And the ball is in your court. As others have said, they are just trying to buffalo their way through. Don't let them do it. They have an responsibility to give you what you ordered. If they can use that material to do it, fine. But with the information available thus far, it does not appear they have.
Have a Great Day, Brent