Welding Metallurgy and Materials Behaviour
What you must know
Understand the heat-affected zone, its peak temperature gradient, and how it produces hardening, softening, or sensitisation. Know carbon equivalent and its role in hydrogen-assisted cold cracking, plus the interaction of hydrogen, restraint, and hard microstructure. Understand preheat and interpass control, post-weld heat treatment purposes, and the difference between stress relief and normalising. Know basic behaviour of carbon steel, low-alloy steel, stainless steel (ferrite-austenite balance, sensitisation), aluminium (oxide film, thermal conductivity), and the effect of sulphur and phosphorus on hot cracking.
Traps in questions
- Assuming preheat eliminates the need for low-hydrogen consumables.
- Confusing interpass temperature limits with preheat minimums.
- Believing PWHT always improves toughness.
- Forgetting that austenitic stainless steels are sensitive to sensitisation in a temperature band.
- Treating carbon equivalent as a pass/fail number without considering restraint and diffusible hydrogen.
Worked example
A 25 mm low-alloy steel with a carbon equivalent near 0.45 is welded with cellulosic electrodes in high restraint. Cold cracking risk is high because hard microstructure, hydrogen, and restraint coincide. Controls: low-hydrogen consumables, preheat, controlled interpass, and prompt PWHT where permitted. Rule of thumb: remove at least one leg of the cracking triangle.
Inspection link
Verify preheat and interpass with calibrated instruments and record readings at defined points.