Allen and Gizmo make several good points, but I don't fully agree with them on a couple.
Hydrogen is indeed present in certain fluxes. 6010 and 6011 have a high cellulose (wood powder/flour) flux because at welding arc temperatures the cellulose produces a large quantity of gas that both completely shields the weld pool against atmospheric contamination and makes the arc penetrate more deeply due to the blowing effect of the gas and the hydrogen produced by the cellulose (which increases the strength of the arc). These fluxes have about 5% moisture which is necessary for the flux to function properly.
6013 replaces a large part of the cellulose in 6010/6011 with more rutile (titanium dioxide), and does not penetrate as deeply. The flux has about 2% moisture. The reduced cellulose and moisture in 6013 produce about only half the hydrogen as the 6010/6011.
7018 uses a primarily lime flux with iron powder to furnish more weld metal. It has virtually no moisture and the lime flux produces about 1/8 the hydrogen of the combined moisture and cellulose flux of 6010. Unfortunately the lime flux readily absorbs moisture from the air, and will absorb enough moisture to increase the hydrogen available to above the limits for embrittlement of high carbon steels.
This was demonstrated to me once in the 1960's by a Lincoln instructor. He took two pieces of mild steel and two Lincoln 7018 electrodes (I think they were called "Fleetweld Blue Dot" back then). One electrode had been in proper storage (250deg F, I think) and the other had been lying on the bench in the open shop for several days. He ran a heavy bead on each piece and doused it in water instantly. The he put both pieces in clear oil (mineral oil, I think). The bead made with the 7018 that had been exposed to the air for several days begin to emit small bubbles and continued to do so for quite some time. The bead made with the "fresh" rod emitted no bubbles. He explained that the gas bubbles were hydrogen resulting from the decomposition of water into hydrogen and oxygen as it was exposed to the arc. That was proof enough for me of the need to properly store low hydrogen electrodes if they were going to be used on critical work.
It will make the professional welders cringe, but before I use old 7018 electrodes that I know are almost certain to have absorbed moisture on medium to high carbon steel (like fork lift forks, scraper blades, ), I stick the electrode and let the weld current flow through it until it begins to steam. That's not scientific, but it seems to work for my jackleg welding.
I was trying to show the original poster how he might get by with cellulosic flux rods (6011) on his splitter wedge. My experience with Northern Tool suggests that the wedge is not heat treated because of the significant additional expense and because the subsequent welding would be likely to remove the heat treatment in the heat affected zone. If the wedge is 0.50% carbon (typical for an ax-head), he may be able to use 6011 rods if he allows the weld to stay above 450 deg F long enough for much of the hydrogen to diffuse out of the weld. At 450deg F and above the hydrogen is able to diffuse through the weld metal fairly rapidly. By preheating to around 400 deg F (which will not materially affect the heat treatment, if any, if done for only several minutes) the weld itself, particularly with the multiple passes Allen and Gizmo recommend, can stay "hot"long enough for much of the hydrogen to diffuse and escape. We were tought to make the cap pass a wide weave for the very purpose of keeping the underlying weld metal hot for a longer period to allow more hydrogen to escape. Recall that I suggested to the original poster that the cover the completed weldment with sand. That was to slow the cooling.
Not long ago when welding brackets onto some 3" X 6" forklift forks I left a propane weed burner on them to maintain the weld at (about) 500 deg F for about 30 min. I used 6010 for the root pass to get the penetration I needed, and then used 7018 for the hot passes and cap (with a "temper bead"). According to the charts, the hydrogen in the root pass diffused up through each of the upper weld layers during the welding process (because of the elevated temperatures) and the post weld heating allowed the residual amounts to diffuse out of the cap. I guess time will tell.
As Allen and Gizmo correctly point out, fresh 7018 rod produces the strongest weld, and preheating is advisable for several reasons: it drives any moisture out of the metal, its reduces stress cracking from uneven cooling, it reduces the chance of forming large needle martensite in the HAZ, and it keeps the weld metal above 450 deg F for longer to allow more hydrogen to diffuse out of the weld.
Spydylk: I am not sure why the flux sometimes cracks. I seems most likely that we get it too dry. It might also come from differential expansion. I suspect it is initially put on the rod as a slurry and then dried to the required moisture content. The rod and slurry are probably at room temperature. But when drying at 400-700 Deg F (depending on the material) the metal rod may expand more than the flux and create stress concentrations (particularly axially) that cause the cracks. Slow heating might allow the stresses to build more slowly and to dissapate without cracking.
As you have probably guessed, I use the rods as long as at least a reasonable amount of the flux is left, particularly if no is going to see the weld.
Rhett: 30W motor oil for underwater welding. Wonder what that does. If I am out of cutting/chamfering rods and must cut or gouge metal with the stick welder, I use 6010 that has been soaked in water. All that extra moisture adds to the blowing effect that gets rid of the molten metal
