The invention relates to a method for welding rotors for power generation having a plurality of rotor discs arranged along a rotor axis. The method includes providing forged and NDT tested rotor discs and machining the discs for weld seam preparation. The weld seam preparation has an inner narrow TIG welding gap and an adjoining outer SAW welding gap. The method further includes stapling the discs on top of each other; and checking the run-out of the stapled discs relative to each other and as a whole and, if necessary, adjusting the staple. In addition the method includes melting the root of the weld without weld filler using TIG welding; increasing the weld height by narrow gap TIG welding with parent metal weld filler to allow tilting of the rotor in horizontal position; tilting the rotor in horizontal position; finalizing the welding by filling the outer SAW welding gap using SAW welding; and checking the welds of the rotor by NDT using ultrasonic testing. A weld seam preparation machining step that includes preparing an optimized transition geometry of TIG welding gap to SAW welding gap transition with a first opening having a first opening angle and a second opening having a second opening angle greater than said first opening angle.