Why steel cracks after you go home

Inspectors are not metallurgists. You still need the story, because the WPS preheat, the rod oven, and the delay-before-inspection rule all come from it.

• Carbon equivalent (CE): a weighted sum of C, Mn, Cr, Mo, V, Ni, Cu. Higher CE → more hardenability → harder HAZ if you cool fast → more hydrogen-crack risk. Thick sections cool faster at the core than you think.
• Hydrogen cracking (cold crack, delayed crack): appears at room temperature, often in the HAZ, sometimes hours later. Needs hydrogen, a susceptible microstructure, and stress. Kill any one of the three.
• Preheat and interpass: slow the cool, let hydrogen diffuse out as atomic hydrogen while the metal is still warm.
• Low-hydrogen consumables and dry joint surfaces: starve the hydrogen.
• Hot cracks (solidification): in the weld metal as it freezes — crater cracks, centreline cracks, high sulphur/phosphorus, deep narrow beads. Different animal from hydrogen cracks.
• Lamellar tearing: terraced cracking in the base metal under a T-joint, from inclusions in the rolling plane plus through-thickness strain. Material and joint design problem.
• Toughness / CVN: ability to absorb energy at temperature. Required on many bridges and seismic members. A pretty weld with no Charpy is still the wrong weld if the WPS demanded it.