Materials and Their Behaviour During Welding
This domain examines how thermal cycles change microstructure, and how composition, hydrogen and restraint combine to cause cracking.
Hardenability and cold cracking
Cold (hydrogen-assisted) cracking needs four factors together: susceptible microstructure, hydrogen, tensile restraint and low temperature. Carbon equivalent predicts hardenability. The IIW formula is CE = C + Mn/6 + (Cr+Mo+V)/5 + (Cu+Ni)/15. Above about 0.40, preheat and controlled hydrogen practice become important; below that, thin low-restraint joints may need none. Preheat slows cooling, lowers the martensite fraction and gives hydrogen time to escape.
Solidification and hot cracking
Hot cracking occurs at the end of solidification when low-melting films (sulphur, phosphorus) remain between dendrites. A high Mn/S ratio improves resistance in carbon steels. In austenitic stainless steels, a small amount of delta ferrite (typically 5–10 FN, higher in duplex welds) resists solidification cracking. Segregation also drives sensitisation: chromium carbides precipitate at 450–850 °C in the HAZ of unstabilised austenitic steel.
Worked example
A steel has C 0.15, Mn 1.20, Cr 0.10, Mo 0.02, V 0.01, Ni 0.10, Cu 0.20 (wt%). CE = 0.15 + 0.20 + (0.10+0.02+0.01)/5 + (0.20+0.10)/15 = 0.15 + 0.20 + 0.026 + 0.020 = 0.396. Reasoning: the value sits just under 0.40, so preheat is decided together with thickness, hydrogen level and restraint, not by CE alone. A 30 mm restrained joint with H10 consumables would still justify preheat around 100–150 °C.
Traps
Applying the IIW CE formula to low-carbon microalloyed steels where Pcm is used. Ignoring thickness and restraint. Using CE for stainless steels. Quoting ferrite as a percentage when the diagram is in FN.