Welding, Casting and Joining Processes
Welding questions test process selection, heat-input control, hydrogen management and the ability to read consumable and procedure designations.
Core rules
- Process matching: SMAW for site and repair work, GMAW for productivity, GTAW for quality and thin sections, SAW for thick plate with high deposition, FCAW for outdoor structural work, resistance spot for sheet assembly, EBW and laser for deep, narrow, low-distortion welds.
- Heat input HI = V·I/v (J/mm), where v is travel speed in mm/s. High heat input coarsens the HAZ and lowers toughness; low heat input raises cooling rate and cracking risk in hardenable steels.
- Carbon equivalent (IIW): CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15. Values near or above 0.40 require preheat, controlled heat input and often post-weld heat treatment.
- Hydrogen control: use low-hydrogen consumables, bake electrodes, clean the joint, apply preheat, and PWHT where the procedure requires it.
- Consumable example E7018: 70 ksi minimum tensile strength, all-position, low-hydrogen iron-powder coated electrode.
Worked example
A steel with C 0.15, Mn 1.20, Cr 0.10, Mo 0.05, Ni 0.10, Cu 0.10 gives CE = 0.15 + 0.20 + 0.03 + 0.013 = 0.39, so weldability is acceptable but preheat is prudent for thick sections. Welding at 25 V, 250 A and 5 mm/s gives HI = 25 × 250/5 = 1250 J/mm = 1.25 kJ/mm, mid-range for a controlled procedure.
Traps that cost marks
Exceeding CE limits and waiving preheat because the steel is called mild. Reversing GTAW polarity conventions (DCEN for most metals, DCEP for cleaning aluminium). Assuming thicker is stronger - an over-reinforced weld creates a stress concentration. Ignoring that heat input has units per unit length, not per pass.