Well, its interesting. All the 913 materials are QST materials. And yet Grade 50 is not included in the heat input exemption of Note a for 913's. My only thought would be that the strength of the higher alloyed 913 QST's is not as effected by the heat of welding.

Greg,
I suspect (I hope at least) the logic is based in emperical experience, if you'll excuse the moderate redundancy of that (as opposed to research experience).
It sorta reminds me of a comment I made once about a certain material list (it had to do with some special exemptions or such-been too long to remember specifically since I didn't really deal with those materials) in ASME III when I was working nukes. I commented that the list seemed rather random. I received a somewhat snooty response from a committee member (thankfully buffered by a friend who thought it humorous) that the list was anything but random.
But for the life of me I couldn't see the common logic in the list of materials.
The logic was apparently too deeply buried in service history. And one needed to be a Section III archivist to dig it out. But those guys are either dead or sippin Margaritas on a beach in Boca Raton.