Welding undercutting

[LD1]

Another vote for 7014 for me.

Back in the day, my dad always swore by 6013. Super easy to weld.
Myself, after trying different types of rods, I liked the 7014. It welds as easy as 6013 and has 10,000psi more tensile strength. I now have him hooked on it as well. I have never noticed it being brittle.

I never had any luck with 7018 though. Maybe it is the welder. It is just an old craftsman and I'm not sure wether it's ac or dc.

To everyone that swears by the 7018 as being the ultimate rod, for the home DIY'er, a good weld with 7014 will hold better than a poor weld with 7018.

 

[BigEddy]

Couple comments
7018 is a Lo-Hydrogen rod that is a medium to deep penetrating rod (Not low penetrating as stated earlier). Probably one of the highest volume rods of all. As a Lo-Hydrogen rod - for code work it needs to be kept in a rod-oven once removed from the sealed package. It absorbs water otherwise and eliminates the Low-Hydrogen properties of the rod. Thousands are burned every day that are not dry - but that's not saying it's right. Rod holders extend the "open" time, but are not a replacement for a rod oven. 7018 has a (well deserved) reputation of being hard to start, and really hard to restrike - for an AC buzz-box - ONLY buy the 7018AC version which is specifically formulated for AC machines.

7014 is a shallow penetrating rod. Not a Lo-Hydrogen rod, it does not need to be kept in an oven. Easy running, it is a favourite of many welders but can be brittle and if not used by a skilled welder can result in a pretty but shallow weld. It is well suited to material under 3/8". After that, there are better alternatives.

As for undercutting - you have too much heat for the speed you are travelling. Undercutting is incomplete filling of cavity formed by the weld current. Watch the toes of the puddle and wait for the puddle to fill before moving away. Turning down the heat will reduce the size of the cavity, moving slower will allow more rod to fill the cavity. It's a blend of both.

You said you're making forks - I assume you have 3/4" or thicker material. In this case, you would need to bevel the material and make multiple passes, tieing the pieces together with the first, then filling in the bevel with either stringers or a weave, with proper cleaning between passes. I'd start with 6010 or 6011 then move to 7018 for the fill and cap myself, but you could do the whole thing with any of them. Just make sure you clean well between passes, and watch to ensure you have good tie in to the parent material on each pass.