Hello Julius, if the materials are made up of some specific alloys that are heat sensitive or have other issues related to their welding then the PQR sequence definitely has a purpose. There is also a process of welding referred to as "Temper Bead Welding", if this has been employed during the set-up of the PQR, changing things would defeat the purpose of the process and cause detrimental effects to the weld and parent material.
     If the materials aren't sensitive to the above mentioned type of parameters then slightly altering weld sequence or number of passes due to operator differences(one welder runs smaller beads than another one, or runs filler passes that utilize the maximum allowable width and thickness values allowed by the code), may not cause detrimental effects and be considered acceptable. You will likely have other responses and hopefully have your question answered fully. My $.02 worth. Regards, aevald

You didn't specify what welding code or standard you are working with. Your question was specifically on carbon steel. You didn't define the wall thickness, carbon equivalency, application, etc., so the following reply is very generic.

My experience is that most welding standards or codes use the PQR as a demonstration that the base metal / filler metal combination will produce the minimum mechanical properties required by the welding standard. Most people actively involved in welding recognize that varying the welding parameters, i.e. voltage, amperage, travel speed, etc. has minimal affect on meeting the minimum mechanical properties, i.e., tensile strength, provided a sound weld is achieved. That is not to say they have no affect on the mechanical properties.

Other properties, such as notch toughness, are affected by the welding parameters that  influence the heat input and grain size. That being the case, the welding parameters are controlled when toughness is a design consideration.

Groove details do not affect the strength of the weld provided the welds are sound. The purpose of the groove is to provide access to the root and to achieve the necessary joint penetration.

Weld bead placement is not going to have a major impact on meeting the minimum tensile strength required by the welding standard. Bead placement can affect the final distortion of a welded component, but that is not relevant to the discussion of mechanical properties.

If the discussion pertained to high strength quenched and tempered steels, bead placement relative to the "temper bead technique" would be relevant. However, you simply stated you are welding carbon steel, a rather broad range of materials, so a discussion on Q&T steels isn't necessarily relevant.

The production WPS is based on production requirements and the results of the PQR and any lessons learned while going through that process. The individual (a welding engineer perhaps) tasked with writing the WPS must understand and appreciate all the influencing factors affecting the final welding operation. A WPS that meets all the "code" requirements may be useless as a welding work instruction if the individual writing it doesn't fully appreciate or understand what is being welded and how it will be used. The groove details of the production WPS may not be the same as that used for the PQR. As an example; the PQR may have utilized a single V-groove, while the production weld is a single pass fillet weld. Clearly, bead placement isn't a factor. On the other hand, if the component being welded is Q&T steel, bead placement and heat input may affect the tensile strength and notch toughness of the completed weld.

Best regards - Al