Hello hamedian, I certainly wouldn't say that you can't weld the thicker sections with the GMAW process, however the use of Co2 as a shielding gas would definitely not be my first choice. I have only used GMAW with Co2 for a structural application one time and the weld deposits definitely weren't pretty, in order to make the proper beads the voltages had to be high enough that they resulted in operating the process in the globular transfer mode. If you are dead set on using GMAW you may want to consider using an 85Ar/15Co2, 90Ar/10Co2, or some form of mixed gas with at least an 80% or higher level of Argon. This will allow you to operate in a spray-transfer mode and likely have much better looking weld deposits and less weld spatter and bead clean-up issues.
     Is there reason that you don't want to use a gas-shielded FCAW process for the heavier sections? If you are wanting to stick with the Co2 gas selection there are a number of very clean-running FCAW gas-shielded wires that would certainly meet your needs and make spatter and clean-up issues minimal. A few items for you to consider, likely others will chime in here with their additional suggestions. Best regards, aevald

Edit: I should add however, that if you change some of these variables it may require the re-qualification of your welders or procedures if you are working to any specific codes.

hamedian,

as aevald has already assumed by the end of his excellent explanations, please let me step in here.

As I saw that you have used the old designation "St 37-2" which comes from the former German standard "DIN 17100" (Allgemeine Baustähle ~ Plain Structural Steels) I had the vague idea if you were eventually working according the European Standards (EURONORM = EN).

Resuming please allow to let me first of all "correct" the steel designation, where "St 37-2" was changed into "S 235 JR" and standardized within the DIN EN 10025.

As presuming your work is according to the EN Standards, and the parts you are manufacturing are complying with a WPS before they produced, let me assume that you are choosing your filler materials in accordance to DIN EN 14532 "Welding consumables - Test methods and quality requirements - Part 2: Supplementary methods and conformity assessment of consumables for steel, nickel and nickel alloys".
But, and this is tricky, I guess you will have to accomplish your Welding Procedure Specification in accordance to the DIN EN ISO 15614 "Specification and qualification of welding procedures for metallic materials - Welding procedure test".

Hmmm... and now it gets completely complicated in general.

The first named standard namely (DIN EN 14532) restricts the Gas Shielded Metal Arc Welding (GMAW) under using solid wire electrodes in case of welding butt joints by stating a maximum part wall thickness of 50 mm (~ 2").

The latter standard however (DIN EN ISO 15614) contains none of a restriction in both welding process (GMAW) and part's maximum wall thickness(!).

Where, you may ask, does this confusion come from.

To go not too much in detail. The restrictions within the EN 14532 are based upon the TÜV pressure vessel codes and here in particular upon the TÜV Data Sheet VdTÜV 1153. As this code has been created several large classification societies have cooperated with its generation (amongst the TÜV itself e.g. CONTROLAS and FORCE). Thus it has been decided - since 1988 - to limit the maximum wall thickness by 50 mm when using the GMAW (solid wire electrode) and welding a butt joint with pressure vessel components. The restriction in regard to the wall thickness was based thus upon the higher requirements in the welding of pressure parts. However, as well in this field of welding it was possible for the manufacturers to prove their capability to weld pressure part butt joints above 50 mm wall thickness - even by accomplishing and passing a Welding Procedure Test. And the major concern of all of this was the fear to getting lack of fusion inhomogeneities when GMAW of wall thicknesses > 50 mm.

However, due to the meanwhile certain knowledge of how it's feasible to avoid lack of fusion also with larger wall thickness parts both fillet- and butt weld, there is definitely no restriction or limitation - at least according to the European Standards - within the field of structural steelwork for the part's wall thickness in combination with GMA solid wire electrode welding. So I would venture to "refute" the statement coming from your supervisor - at least as it belongs to the valid standards and as long it would treat the "magic" 50 mm limit. What I actually not know however, is the fact what kind of requirements your employer wants to have being met by you. If your employer demands even a restriction of the maximum wall thickness with the parts to be GMA welded, by whatsoever the reasons were for this decision this should matter as a fact to be complied. Even though it were actually unnecessary according to the European Standards. However, I have honestly never heard before of a wall thickness limitation by 8(!) mm when using the GMAW both with solid or flux cored wire.

And if I am allowed to say so. This would make absolutely no sense, since it would destroy all the specific economical process benefits in general.

As a little recommendation let me say finally what follows.

In terms of product liability you should be recommended to ask your wire electrode supplier if their wire electrode would fit with the WPS requirements when using it for wall thicknesses > 50 mm. But this however, should only be an advice as long as the wire's data sheet shows no clear statement concerning this question.

Best regards,
Stephan