When welding rebar to structural steel, the provisions of both AWS D1.1 and D1.4 have to be satisfied.[Ref: D1.4 1.2.1]
Make sure welders are qualified per D1.1 and D1.4 for the positions, processes, etc
Check D1.1 Table 3.1 and D1.4 Table 5.1 for filler metal selection. Make sure you read also D1.1 Table 2.3 and Sec 3.3 and D1.4 Table 2.1 and Sec 5.1.2
For preheat, D1.1 Table 3.2 and D1.4 Table 5.2, the higher value will govern. For the rebar try and switch to low alloy A706 steel as it has better weldability properties. If you stick with A615 you'll need the CE and if it is not available you'll have to use a preheat of no less than 500 degrees F, which is certainly higher than all the preheat values in D1.1 table 3.2
Two questions for you.
What type of steel are the HP piles to be fabricated from?
Do you already have a WPS submital backed up with a POR from the fabricator/contractor?
As I understand it, normal rebar has a fairly high carbon content and isn't really intended for welding. There IS low-alloy rebar that is better suited for welding, check into it. ASTM A706, maybe? Not sure about the number.
I'm not that familiar with them, but they do make mechanical splicing devices- I wonder if they make something you can weld to the piling then clamp the bars in it? Or alternatively, use the more-weldable rebar, stub it up and splice to the 615 rebar.
Yes, there are splices which are intended to be welded to structural steel, then the rebar is threaded into the splice. Dayton RIchmond makes a splice like this called the D-106 Welding Half Coupler. They list it for #7-#18 bars.
I notice that the Dayton Richmond coupler is fabricated from steel conforming to ASTM A 108 Grade 1018 or 1020 which amongst other things, is a material used for making headed studs. This material is not listed as an approved base metal in D1.1 Table 3.1 which might necessitate qualification testing per Section 4 to come up with a WPS.
