Just ran a job here that was 1045. The shaft was 2 1/2" Dia.x 12" long and the plate was 1" x about 10' Dia. Plate was pre machined and shaft was also with a v groove on plates and a step on shaft for correct positioning.

I pre heated the 1045 between 400 - 500 degrees or so. The customer was happy with the weld quality but I was not. I had undercut on my top side of each weld. On the side that really mattered after there machine clean up, I welded a second pass on top of first pass which I laid down lower for second pass to fill in nice over top of. The weld size needed was a 3/8" I welded these on a turn table in the flat position.

I used ER70S-6 wire.