Reading the numbers
In both designations the first two digits give minimum tensile strength in thousands of psi — 60 000 for 6013 and 70 000 for 7018. The third digit describes welding positions, and the fourth describes the coating and current characteristics.
So 7018 is the stronger deposit. The more important difference, though, is in that final digit: the 18 coating is a low-hydrogen iron-powder type, and hydrogen control is what makes the electrode suitable for work where cracking matters.
6013: general fabrication and thin material
A 6013 is easy to strike, easy to restrike, runs smoothly at modest current, produces a light easily removed slag and tolerates less-than-perfect fit-up. It suits sheet and light plate, general repair, maintenance work and any situation where a competent but not specialist welder is doing the job.
It is also the electrode most likely to be in a workshop’s cupboard, which is both its convenience and its risk — it gets used on work that should have had a 7018.
7018: structural and code work
A 7018 gives higher strength, better ductility and notably better low-temperature toughness. Low hydrogen content is what makes it the choice on thicker sections, restrained joints, higher-strength steels and anything an inspector will sign.
The cost of those properties is discipline. The coating absorbs atmospheric moisture, and moisture means hydrogen in the weld and the risk of hydrogen-induced cracking. 7018 has to be stored in a rod oven, issued in the quantity a welder will use, and re-dried or discarded if left out. A 7018 that has been in a damp store is not a 7018 any more.
Choosing between them
- Thin material, light fabrication, general repair — 6013.
- Thick or restrained joints, structural steel, higher-strength steels — 7018.
- Poor fit-up with no strength requirement — 6013 is more forgiving.
- Anything welded to a qualified procedure — whatever the procedure names, without substitution.