Materials and Their Behaviour During Welding
Domain 2: Materials and their behaviour during welding
This domain tests metallurgical reasoning: why a joint cracks, what happens in the heat-affected zone (HAZ), and which consumable, preheat or interpass control reduces risk.
Carbon equivalent and cold cracking
For carbon-manganese steels the IIW carbon equivalent is CE = C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/5, in wt%. Higher CE means greater hardenability and a harder HAZ. Cold cracking needs four conditions together: hydrogen, a susceptible microstructure, tensile restraint and time. Preheat, low-hydrogen consumables, interpass control and post-weld heat treatment attack different parts of that combination.
Stainless and aluminium
Austenitic stainless steels can sensitise between about 450 °C and 850 °C as chromium carbides precipitate and chromium-depleted zones form. Low-carbon and stabilised grades limit this. A ferrite content of a few percent in austenitic weld metal helps resist solidification cracking; it is measured as ferrite number. Aluminium's tenacious oxide film must be removed mechanically or by AC/electrode-positive cleaning action; hydrogen is the main cause of porosity.
Worked example
A steel has C 0.16, Mn 1.30, Ni 0.25, Cu 0.15, Cr 0.08, Mo 0.03 and V 0.02. CE = 0.16 + 1.30/6 + (0.15+0.25)/15 + (0.08+0.03+0.02)/5 = 0.16 + 0.217 + 0.027 + 0.026 = 0.43. That is above 0.40, so weldability is only moderate; a thick, restrained joint would need preheat and low-hydrogen practice, not just a change of current.
Traps
Mixing CE formulae (IIW, Pcm, AWS) and their limits. Assuming stainless steel cannot crack. Confusing porosity with lack of fusion. Forgetting that hydrogen comes from moisture, grease, paint and consumable coatings as well as the arc atmosphere.