Not as much as one would think. In Table 1 of Section 5 of API-1104, note at the bottom states "Other electrodes, filler metals, and fluxes may be used but require separate procedure qualification". You see, API-1104 groups various materials by ranges of tensile strengths not matching tensile strengths. You can use any strength pipe, for instance within the ranges listed without requalifying your procedure. In some cases may even deviate (API-1104 Paragraph 5.1) from that provided it is "specifically authorizd" by the company and the PQR passes the required destructive tests and any nondestructive tests required by the company. I do not have immediate access to ASME Section II, but I believe both X60 and X65 pipe have tensile strengths of at least 75,000 psi. API groups base materials as "greater than or equal to 65,000 psi" for one group. This is the range allowed by the code. Any material above 65,000 psi is within that range, although any change in materials above 65,000 psi from that which was used for qualification, must be requalified. The bottom line is "You must qualify the procedure by means of mehanical tests". By the way, the filler metal we are using is actually ER-70S-6 (API Group 6). It has been qualified and has passed mechanical testing, NDT and hydrotesting. It is perfectly acceptable for service. If you are truly concerned about matching the tensile strengths of the pipe and filler metal, depending on the application, you may want to take a look at using A-106 Grade B (tensile strength of 60,000 psi). That is also acceptable, again provided you qualify your procedure.