Materials and their behaviour during welding
This domain tests what the welding thermal cycle does to the parent metal and how cracking and loss of properties are prevented. Expect carbon equivalent, hydrogen, preheat, heat-affected zone microstructure, and the behaviour of stainless steels, aluminium and other alloys.
Carbon equivalent and preheat
For structural steels the IIW formula is CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15, valid for carbon up to about 0.25%. A higher CE means a harder, more crack-sensitive HAZ. Preheat slows cooling and helps hydrogen diffuse out; EN 1011-2 combines CE, thickness, heat input and hydrogen level. Consumable hydrogen is graded (H5, H10, H15), with basic electrodes typically achieving H5 after correct baking.
HAZ and special materials
High heat input coarsens grains and lowers toughness; fast cooling in steel produces martensite. Austenitic stainless steels can sensitise between roughly 450 and 850 degrees C as chromium carbides precipitate, so low-carbon 304L/316L or stabilised grades are used. Aluminium carries a tenacious oxide and conducts heat strongly, so AC GTAW and appropriate cleaning are needed.
Worked example
A steel has C 0.16, Mn 1.30, Cr 0.05, Mo 0.02, V 0.01, Ni 0.08 and Cu 0.10. CE = 0.16 + 1.30/6 + (0.08)/5 + (0.18)/15 = 0.16 + 0.217 + 0.016 + 0.012 = 0.405, about 0.40. Reasoning: at this CE and a section of 25 mm, EN 1011-2 indicates preheat in the order of 100 degrees C together with a hydrogen-controlled consumable and controlled heat input; ignoring thickness or hydrogen would understate the risk.
Traps that cost marks
- Applying the IIW CE formula to austenitic stainless steels or to very low-carbon micro-alloyed steels where Pcm is used.
- Treating preheat as a function of thickness only, ignoring hydrogen and heat input.
- Routinely preheating austenitic stainless steel; interpass temperature control is the issue there.
- Forgetting 475 degrees C embrittlement and sigma phase in duplex and ferritic stainless steels.