Hello:
As someone who was one of the first to use pure Ni in knifemaking (Daryl Meier, a few others and myself were experimenting with it a while back) it can be a PITA to weld, especially in a coal fire..(from the sounds of it..you are using gas or oil as a fuel source?)
Ni will pick up all the crud in a fire like a sponge and oxidize like crazy if you are not careful. Use of borax can and does work, but the anhydrous along with some boric acid and maybe some powdered red iron oxide mixed in will also help when using pure Ni ( I have been using this same recipe for 35+ years, but I have tweaked it a bit here and there) .
Now pure Ni will do nothing more than give contrast, and since it does not harden, you will get no other benefit from it. I employ it now as a "colour layer" usually in cores on a multi core composite blade like a Migrationary Period sword, or as a "contrast" in welded cable.
For contrast differences Ni can't be beat but as far as cutting goes...it sorely lacks in that department. If you are running gas you need a carburizing atmosphere..Ie, run it a little on the rich side as excess O2 in the furnace atmosphere can bugger you up and cause the Ni to oxidize heavily and then you will have problems sticking the weld. Using the right flux and the right "working habits" will help facilitate a good weld.
Make sure you are 100% soaked through on your temp as this is usually the number one cause for a weld to not stick, especialy using the thin stuff like I do...(I usually weld 0.032" sheet in 45 to 50 layers for the first weld faggot..saves time that way )..
Personally I think the pure Ni sticks like glue but it has been said I could pretty much forge weld anything except the crack of dawn... I happen to like the contrast that Ni gives but the lack of hardening really limits its use to either ornamental items or to the "non cutting" areas of a blade.
I really do like Ni and some folks seem to have a lot of problems with it for some reason...but it gives great contrast but doesn't do anything for cutting so...
All in all..keep it clean and get it soaked through..remember Ni melts about 300 degrees higher than most "welding temps" for most "Knife Steel" (Ni melts at 2650ish) and so it should really stick if you are clean and hot enough for the other materials in the faggot to weld...
Hope this helps...
NVHammerHead
