Metallurgy and Weldability of Metals
Metallurgy and Weldability of Metals
This domain tests how metal composition, thickness, heat input, and cooling affect weldability. You need to predict cracking, hardness, distortion, and property loss, then choose preheat, interpass, filler, and process controls.
Core rules
- Steel weldability is often estimated with carbon equivalent (CE). A common IIW formula is CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15. Higher CE means greater hardenability and hydrogen-cracking risk.
- Preheat slows cooling, helps hydrogen escape, and reduces hard martensite. Preheat and interpass limits are taken from the code or WPS, not guessed. Thicker sections need more preheat.
- Hydrogen cracking needs three conditions: susceptible microstructure, hydrogen, and stress. Controls include dry low-hydrogen electrodes, clean joint faces, preheat, and controlled cooling.
- Austenitic stainless steels can sensitize when held roughly 450–850°C: chromium carbides form at grain boundaries and corrosion resistance drops. Use low-carbon grades such as 308L/316L, control heat input, and avoid slow cooling through that range.
- Aluminum has a tenacious oxide layer with a much higher melting point than the base metal. Remove oxide mechanically and use appropriate cleaning/shielding; GTAW uses AC for aluminum.
- Cast iron is brittle and crack-sensitive. Repairs usually use preheat, controlled cooling, peening, and nickel-based fillers.
Worked example
A steel has C = 0.45%, Mn = 0.75%, and no significant Cr, Mo, V, Ni, or Cu. CE = 0.45 + 0.75/6 = 0.45 + 0.125 = 0.575. That is relatively high, so the steel is crack-sensitive and needs a qualified preheat/interpass procedure, low-hydrogen practice, and likely a controlled cooling plan. The reasoning is not “0.575 is bad” alone; the number predicts hardenability, and the actual preheat must come from the applicable code and thickness.
Traps
- Applying the CE formula to stainless or aluminum.
- Assuming preheat is always harmful or always required.
- Using cellulose electrodes for high-CE steel without a qualified procedure.
- Ignoring interpass temperature on stainless.
- Failing to remove aluminum oxide before welding.