Consumables and Metallurgy: Classification, Hydrogen and Cracking
What to master
Learn classification systems as a decoding exercise. In AWS A5.1, E7018 means an electrode with 70 ksi minimum tensile strength, all-position capability (1) and a low-hydrogen basic coating (8). Suffixes matter: E7018-1 has improved impact toughness at −46 °C, and H4/H8 designators indicate diffusible hydrogen limits in mL/100 g. For FCAW and GMAW, know the A5.20/A5.18 classifications and the meaning of shielding gas designators. Metallurgically, understand carbon equivalent, preheat and interpass temperature, heat-affected-zone hardening, hydrogen-assisted cold cracking, solidification cracking and lamellar tearing. Consumable selection must match strength, toughness, composition and service environment, not just tensile strength.
Traps
A common error is treating preheat as a substitute for dryness and clean surfaces. Preheat slows cooling but does not remove hydrogen; low-hydrogen electrodes must still be stored and baked correctly. Do not confuse 'low hydrogen' with 'hydrogen-free'. Carbon equivalent formulas differ - IIW CE adds nickel, chromium, molybdenum, copper and vanadium in different denominators from Pcm - so a value from one formula cannot be compared with a threshold from another. Also, matching tensile strength can overmatch the base metal and reduce toughness.
Worked example
Using IIW CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15: a steel with 0.15% C, 1.20% Mn and no other additions gives 0.15 + 0.20 = 0.35. Many procedures treat CE above 0.40 as needing controlled preheat, but thickness, hydrogen level and restraint must also be considered. The calculation is a screening tool, not a qualification.