According to my box of E8018, the manufacturer’s directions say to “dry rod at 600 degrees F for one hour, then store at 300 degrees F until ready for use”.
For chit-chat –
Steel contains dissolved hydrogen, similar to how water contains dissolved oxygen. And like water, when steel is heated, its capacity to retain dissolved hydrogen decreases.
More specifically - the reason you preheat high-yield steels prior to welding is to allow the dissolved hydrogen to freely migrate to the steel’s edge, ultimately driving it out of the steel and into the air. At 70 degrees F dissolved hydrogen will migrate through the steel’s lattice structure at a rate of approximately 1 inch per 24 hours. At 250 degrees F the rate increases to 1 inch per hour. Heat is the driving force.
So what happens if you don’t preheat?
Imagine knee-deep water represents the normal dissolved hydrogen content present in steel. And during welding, the intense heat rapidly pushes the hydrogen away from the weld area – forming a giant tidal-wave of hydrogen. The problem is that the weld doesn’t stay red-hot for long, and when the steel cools, that tidal wave is locked into place (drifting slowly again at a rate of 1 inch per 24 hours). Problem - a tidal-wave concentration of hydrogen (sitting right under the weld within the heat affected zone) will crack the lattice structure of the steel. Aka hydrogen embrittlement cracking.
By preheating (250 degree F min – for 1 hour per inch of thickness) the knee-deep water turns into ankle-deep water. Instead of getting a giant tidal wave of hydrogen, you get a much more manageable knee-deep ripple (below the threshold for initiating hydrogen-cracking).
Which brings us back to the rod. When you weld with damp rod, the arc will literally split the water molecule, and you’ll be dumping high concentrations of free-hydrogen directly into your weld. It completely negates any benefit of the preheat. The higher the yield-strength of the steel, the greater the risk of hydrogen cracking (generally speaking).
I have had the privilege of observing a hydrogen embrittlement weld-demonstration. Two samples of 1” thick HY-100 plate-steel were joined together (double-bevel – 100% penetration). Everything was done by the book – proper preheat, dry 11018 rod, proper current setting, etc. One sample was welded up, then another just like it, but with intentionally damp 11018 rod.
After both samples cooled, we put the properly welded piece into the vise, and beat the living crap out of it with a 10 pound hammer. The metal folded over – but no cracks. The sample welded with damp rod completely fractured in half upon the first strike with the hammer – and it wasn’t even a hard hit.