The closest thing I've had experience with is a 4140, 6000# part welded on to A53 18" sch40 pipe. 400°F preheat, open root with GMAW ER70S, followed up with E10018 C1 fill and cap then immediately PWHT weld and HAZ to 1100°F, hold for 30 minutes then wrapped in asbestos blankets. (Yes we used real Asbestos blankets and welding gloves back then in the 70s with Bronze reinforcing wire running thru it!)
My question is how tight are the parts? I would consider an interference fit to be superior to loose considering the differential alloys (and resultant thermal expansion) in use.
Plan "A"...
"Magic Wire" it with ER309...
OR...
Plan "B"
If it were my gig with the materials available...
1) Pressed fit. You can make a dry ice box with your CO2 and freeze the smaller part while warming up the female part. (Uuhmm Mr. Machinist, you owe me one for supplying the wrong steel!).
2) 400°F preheat and inner pass temp.
3) Peen after each pass. (I'm big on peening)
4) IMMEDIATELY PWHT after final weld profile is achieved. Pick any reasonable temp above 1000 and below 1200.
5) Bury in preheated cat litter, bentonite clay floor dry or preferably powdered Unicorn hooves. An Elfin Farrier from Madagascar can get the trimmings for you without having to slay any mythical beasts yourself.
PETA threw a bottle of ketchup on me last time I bought a bag of powdered Unicorn hoofs.
Lately I've been going the other way and doing an immediate quench in baby seal tears.
The more I read around I am finding more supporting document to PWHT.
It is a looser fit than I would like. And wouldn't you know it, The day I need Unicorn hooves the elf that works for my local welding supply has been on vacation in the Bahamas; therefore, they ran out of the powdered hooves just last week. Luckily though I did manage some bentonite clay. I will be trying again this morning trying to control my PWHT better.