Materials and their behaviour during welding
The Materials domain tests whether you can predict what the weld thermal cycle does to a steel or alloy - hardening, cracking and loss of properties - and choose preheat, heat input and consumables accordingly.
Carbon equivalent and preheat
CEV = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15. Above about 0.40% the HAZ hardens readily and hydrogen control becomes critical; Pcm is used for low-carbon micro-alloyed steels. Preheat slows cooling, lowers HAZ hardness and lets hydrogen escape.
Cracking mechanisms
- Hydrogen-assisted cold cracking: hard microstructure + diffusible hydrogen + restraint. Controls: low-hydrogen consumables, preheat, minimum heat input, post-heat and delayed inspection.
- Solidification (hot) cracking: high S and P, deep narrow pools, austenitic stainless. Control weld-metal composition (ferrite 5–10 FN in 304/316), bead shape and heat input.
- Lamellar tearing: through-thickness strain in T- and corner joints of thick plate. Use Z-quality steel (EN 10164 Z25/Z35) and improved detail.
- Reheat cracking: Cr-Mo-V steels during PWHT; limit residual elements and control heating rate.
Heat input and t8/5
Q = (U × I × 60)/(1000 × v), with v in mm/min, gives kJ/mm. Higher Q and preheat lengthen t8/5, softening the HAZ but increasing distortion.
Worked example
24 V, 200 A, 200 mm/min: Q = (24 × 200 × 60)/(1000 × 200) = 1.44 kJ/mm. At 150 mm/min, Q = 1.92 kJ/mm - longer t8/5, lower hardness, more shrinkage. Heat input per unit length is inversely proportional to travel speed.
Traps
Using CEV alone and ignoring hydrogen and restraint; assuming more heat input is always better; applying carbon-steel preheat rules to austenitic stainless; forgetting that t8/5 also depends on thickness and preheat.