Dear Dong Won Ho
I read your reply with interest, because in South Africa ER 70S-6 is the default wire used to weld carbon steels with the GMAW process. Almost always we (in the pressure vessel industry) use mixed gases for the shielding.
I have been involved in the welding of a number of 40mm thick test plates with this filler, and mixed gasses, and to date I have never experienced hard, brittle weld deposits. The welds are always below 200 HB in hardness.
Perhaps you can tell me a little bit more about the conditions under which you experienced the hardened weld deposits.
I also doubt that the cracking is hydrogen induced, because usually one would find that the visible cracks are substantially longer than the 1,5cm that Charlie mentioned. Perhaps Charlie can do a hardness test on the weld deposit and tell us how hard this is.
Regards
Niekie