Metallurgy and Materials Behavior
Weldability and phase control
Welding metallurgy explains why a sound procedure can still produce cracking or low toughness. Candidates must understand the weld pool, the heat-affected zone (HAZ), cooling rate, and phase transformations. In carbon and low-alloy steels, fast cooling can form hard martensite; hydrogen from moisture, grease, or consumables can then cause cold cracking. Preheat slows cooling and helps hydrogen escape. Carbon equivalent (CE) estimates hardenability. The IIW formula is CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15. Higher CE generally means greater preheat and interpass control.
Other concepts include solidification and segregation, hot cracking, lamellar tearing, temper embrittlement, and ductile-to-brittle transition. In austenitic stainless steels, sensitization from chromium carbide precipitation can cause intergranular corrosion; too much heat input can also promote sigma phase or distortion. Interpass temperature control is critical for duplex and austenitic grades. Post-weld heat treatment can relieve stress and temper hard microstructures, but it is not automatically beneficial for every alloy.
Traps
- Assuming thicker steel always needs more preheat without checking chemistry and CE.
- Using a carbon-steel PWHT cycle on austenitic stainless steel.
- Ignoring hydrogen sources such as paint, oil, or damp electrodes.
- Treating hardness limits as the only cracking criterion.
Worked rule
Calculate CE before selecting preheat. For a steel with C 0.15, Mn 1.20, Cr 0.10, Mo 0.05, Ni 0.10, and Cu 0.10, CE = 0.15 + 0.20 + 0.03 + 0.013 = about 0.39. If the procedure or code requires preheat above a threshold, follow the more restrictive requirement. Chemistry, thickness, restraint, and hydrogen all matter.