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API 5L Grade X65 steel pipes, internally clad alloy 625, are commonly utilized in pipelines and risers for subsea oil and gas extraction. Gird welds in such pipes are conventionally made using alloy 625 filler metal. However, alloy 625 weld metal cannot meet the base metal yield strength overmatching requirement for subsea reel lay installation. This study explored materials selection and process development for low-alloy steel girth welds in API 5L Grade X65 steel pipes, internally clad with alloy 625. Welding with a higher melting point filler metal over a lower melting substrate, i.e., low-alloy steel over Ni-based alloy, is impractical due to increased susceptibility to solidification cracking and solidification shrinkage porosity. Pseudo-binary phase diagrams developed for various combinations of low alloy steel filler metals and Ni-based alloy substrates identified good compatibility between ER80S-G filler metal and alloy 686. The solidification temperature range and the tendency for partitioning of alloying elements were significantly lower throughout the entire ER80S-G/alloy 686 dilution range than in the low alloy steel filler metals/alloy 625 combinations. Extensive process optimization effort to reduce the dilution of alloy 686 root pass in the low-alloy steel weld metal and avoid incomplete fusion defects allowed for the production of defect-free girth welds. These welds met the yield strength and ductility requirements for subsea reel lay installation of pipelines. Process optimization for bead tempering significantly narrowed the high hardness region in the ER80S-G/alloy 686 partially mixed zone.