The Stellites (6 and 12 specifically, utilize Co, W, and Cr carbides as hardening agents) used predominantly as hardfacing fillers are naturally prone to cracking. The resulting hardnesses achieved (specified mins of 42/48 HRC respectively) would indicate such. In many applications such 'cross checking' is not undesirable. However, porosity may be. Jon is correct, to even come close to eliminate weld related problems high preheats are necessary. It sounds as though your preheats are far too low. I do not know it this will influence the formation of porosity.
Deloro Stellite has a help contact on their website. I would suggest dropping them an email, and would be interested to read what their response is.