Anyone give me some pointers or guidance as to what to be aware of when welding thick 1045 shaft material to A36 plate?

Assuming this is not a code project and it is a one of a kind weldment assume the following: AISI 1045 is a medium carbon steel. The carbon content of the AISI 1045 is between 0.4 and 0.5% carbon. Assume the worst case and go with 0.5% carbon.

The hydrogen introduced into the molten weld pool is dependent on the welding process you select. You can minimize hydrogen pickup by using 7018 with SMAW, fresh 10 pound tins, and you open the cans just as you need them to reduce the potential for moisture pickup. Better yet, GTAW or GMAW (spray transfer), using bare wire/electrode, minimizes the potential for introducing hydrogen as well.

I recommend assuming high restraint since it is the most conservative condition relative to the need for preheat.

Based on the limited information you provided, I would use 600 degrees F for the minimum preheat and interpass temperature for this job. Wrap the parts once you are done welding to ensure slow cooling and then post weld heat treat the completed weldment.

I once had a similar job. As my post weld heat treatment, I lit up a 50 pound bag of charcoal before I started welding. It was burning pretty good by the time I finished the welding. As soon as the welding was completed, I buried the part in the charcoal and left it overnight. Worked very nice! The outcome was no cracks and a long service life (20 plus years).

I would also recommend having someone check it with MT after it has cooled.

Good luck - Al