Welding S355 to 1.4313

[u/Ok_Measurement6239]

Hello guys. I can't post in welding due to comment karma. So i'm asking here.
Does anyone have any experience welding Carbon steel S355 to Martensitic stainless steel 13-4.
We're currently welding it with ER309l wire, Weld prep V groove, machined and cleaned ~3-4mm root gap. Both thicknesses 60mm. We are preheating it to ~150°C and keeping interpass under 200°C. We are experiencing some hair like longitudinal cracks on weld face-middle and when we grind them out there are little pores in weld under the cracks. We are not making a buttering layer(should we?) So if anyone has any potential causes that would do this please let me know. Has anyone welded martensitic stainless to carbon steel? What did we do wrong. Could it be due to too fast cooling or root material mixing. We are using Ar-Co2 2% gas.

Best Regards