Welding metallurgy and weldability
This domain tests solidification, phase transformation and cracking mechanisms, and the use of quantitative weldability tools. The candidate must connect composition and procedure to cracking risk with defensible numbers.
Solidification and hot cracking
Solidification cracking depends on the terminal liquid film, restraint and composition. Sulphur and phosphorus raise susceptibility in steel; a Mn/S ratio above roughly 20 is beneficial. In austenitic stainless steel, delta ferrite of about 3–10 FN disrupts continuous solidification boundaries and resists cracking.
Cold cracking and carbon equivalent
Hydrogen-assisted cold cracking requires hydrogen, a susceptible hard microstructure and tensile stress. The IIW carbon equivalent CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15.
Worked example
Composition: C 0.16, Mn 1.30, Cr 0.20, Mo 0.05, V 0.03, Ni 0.15, Cu 0.20. Then Mn/6 = 0.217; (Cr + Mo + V)/5 = 0.28/5 = 0.056; (Ni + Cu)/15 = 0.35/15 = 0.023. CE = 0.16 + 0.217 + 0.056 + 0.023 = 0.456. That value, with plate thickness and hydrogen scale (ISO 3690 H1–H5), drives the EN 1011-2 preheat recommendation: CE around 0.46 in thick sections typically calls for preheat and a controlled hydrogen level.
Other mechanisms
Reheat cracking in Cr-Mo-V steels relates to residual stress and grain coarsening; sigma phase in stainless steels forms around 600–900 °C with long exposure; nitrogen pickup in duplex steels disturbs phase balance.
Traps
Using CE for stainless or aluminium; ignoring hydrogen scale; assuming ferrite number is irrelevant; treating preheat as a cure for all cracking.