Welding Metallurgy, Materials, and Weldability
Welding Metallurgy, Materials, and Weldability
This domain tests why welds crack, harden, or lose corrosion resistance. The inspector uses metallurgy to recognize risk before a test fails. Key controls are heat input, preheat and interpass temperature, hydrogen control, and restraint.
Core rules
- Heat input in kJ/in = (60 × volts × amps) / (1000 × travel speed in in/min). Faster travel lowers heat input; higher amperage or voltage raises it.
- Carbon equivalent (IIW) = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15. Higher CE means greater hardenability and cold-cracking risk; preheat and low-hydrogen practice are common controls.
- Hydrogen cracking is delayed and needs hydrogen, a hard microstructure, restraint, and time. Low-hydrogen electrodes must be stored and used correctly; rust, moisture, and paint are hydrogen sources.
- Hot cracking occurs during solidification and is promoted by sulfur, phosphorus, and high restraint. Lamellar tearing occurs in through-thickness restrained joints with inclusion-rich plate. Reheat cracking can occur during postweld heat treatment.
- Austenitic stainless steels can sensitize when held roughly in the 800–1500°F range; low-carbon L grades or stabilized grades reduce risk. Aluminum's tenacious oxide film requires cleaning and appropriate current.
- Preheat is a metallurgical control, not just cold-weather comfort.
Worked example
A procedure uses 200 A, 25 V, and 6 in/min: heat input = (60 × 25 × 200)/(1000 × 6) = 5.0 kJ/in. If travel speed rises to 12 in/min, heat input falls to 2.5 kJ/in. The faster pass cools more quickly, increasing HAZ hardness and hydrogen-cracking risk in a restrained, higher-CE steel. The slower pass may improve hydrogen escape but can increase distortion and reduce toughness. The inspector checks the WPS range, not a single reading.
Traps
Using amperage alone as a proxy for heat input; forgetting travel speed; applying the IIW carbon equivalent formula directly to austenitic stainless steel; assuming preheat is only for ambient temperature; and treating all stainless steel as immune to sensitization.