Hi
S275 is an European Standard and basically states for a low carbon steel with 275 MPa minimum yield strength, same goes for
S355. (355 Mpa minimun Yield Strength). besides that designation, there are some "suffixes", that states for the impact requeriments
and other process characteristics of the steel, i.e S275-J2G3 or S355 K2G3. Since these are mostly a strenght plus impact specification, not
related to other characteristic such as the "pipe", or "plate", or "boiler" or "PV" aplication in ASTM standards, the equivalence can be tricky and depends
on what type or product or application is the steel for.
You can see some equivalences in this page
http://www.mesteel.com/cgi-bin/w3-msql/goto.htm?url=http://www.steelmillsoftheworld.com/qualities/cs/equivalent_3.htmAnyway, both steels can be readily welded with E7018 (good quality thought, good rule would be to search for a "3Y" classification from
a shipping classification society such as Lloyd's register, ABS, DNV, Bureau Veritas, Germanisher Lloyd or such, usually stated on the electrode box)
, but for S355 you will probably need preheating, which depends of the thickness of the
parts. Very broadly and just from my memory, you can use the following preheat temperatures for S355
t< 6 mm , no preheat
6<t<12 mm, 50°C
12<t<18 mm, 100°C
18<t<30 mm, 150°C
30<t , 175°C
Hope this help, best regards