Sometimes welding Stainless Steel (S.S.) to Carbon Steel(C.S.) is necessary. For instance, there might be a S.S. valve that has to be installed in a pipe line, so therefore, the flanges should be S. S., but the pipe might remain C.S. Too weld S.S. to C.S., we would use E309 for SMAW or ER309, if it is GTAW process.