Simple, 7018's will drive you nuts when your instructor insists on you using them on AC on the 50V setting 'as it's good to learn how to re-start them' I never did strangle him amazingly
I believe the first 2 numbers are the tensile strength, the next number is what position it's designed for, 1 being all position, flat, overhead etc and the 4th number is the coating on the rod.
7018 is a low hydrogen rod which needs to be used from a sealed packet or dried in a oven as they absorb moisture from the air. I think 7018's also have better penetration than 6013.
6013's are better for thin stuff and are classed as a general maintenance rod or farmers rod.
its different steel,the 7018 has a higher tensile strength 70,000lb vs 60,000lb and the flux coating is different. the 7018 have to be kept warm and dry and are harder to start. If you don't keep the rods hot then you mayas well just use the 6013s
the low hydrogen is designed to stop embritlement in the weld
6013s are far from useless rods.
"Farmers rods"??
Not at all. Youd be suprised how much pipework and structural in this country is filled and capped with Vodex or Fincord M 6013s, wether it be Fleet 6010 root or tig root, and how many storage tanks, silos, and things like that are welded with rutile rods.
Once you get over 20mm thick then low hy yes more often than not.
A lot of places are now going over to Mag (which is a low hydrogen process by default) and Gas shielded Fluxcore due to higher deposition rates anyway.
Dont take everything you see on american youtube channels as gospel.
A weld deposited properly with a Vodex (60,000 ksi yeild strength)for example will give xray quality welds, stronger than the mild steel. If s275 grade, approx 36,000 ksi, s355 (high yeild structural steel) = 50,000 ksi.
Dont get so hung up on Low Hydrogen welding, as unless you land yourself a job in a boilershop, welding high pressure vessels and pipe, seriously heavy structural work, or 2inch thick steel, you wont come accross them as often as you might think unless most of the work is over engineered. Low hydrogen rods require baking, preheating in quivers, different techniques, less forgiving overall and therefore prone to problems once someone gets a bit out of practice.
7018s, much nicer to weld with, you can see the weld pool so much easier. If you are just practicing with them there's no need to worry about drying them or keeping them in sealed packets. All that faff is only required for welding something important to a required spec.
I could be putting my foot in it here! But 90 percent of the time at work I never see a vac pac, or quiver just pull them out of a packet and away we go. That goes for the contractors we bring on to do jobs for us to.