Ron, sulfur increases the machinability of steels, but reduces transverse ductility, impact toughness, and weldability. Sulfur in any appreciable amount promotes hot shortness in welding, and the tendency increases with increased sulfur. During welding sulfur segregates into layers which forms iron sulfide. This condition can cause cracks in the fusion line. If your weldment is a new fabrication, I would recommend changing base metal to one with lower sulfur, and with less tendency to cracking.

I must agree that you are saddling a difficult horse to ride here. Not only is the S a problem, but it is being aggrevated by the fact that you have a rather high C content as well. This combination is bound to give you cracking problems.

If however you have a good reason to want to weld this, then I would recommend you try the following: (I have not welded this steel myself, so the following would be an experiment on your part.)

1) Try to tie up the S by using a filler with a high Mn content. Mn tends to form MnS inclusions that are relatively harmless. Alternatively, you could use a filler with some Cr present. The Cr has a similar effect.
2) Due to the high C content, you will have to also use a relatively high pre-heat. (Possibly around 350°C) This pre-heat will however tend to result in greater dilution which will again aggrevate the S problem.

Remember that even if you managed to prevent the weld metal from cracking, you will still have a hot-short problem in your HAZ.

Let us know what your progress is.

Regards
Niekie Jooste