Crack
A crack is a separation in the material that occurs under tensile stress. In welding, you distinguish hot cracks (occur during solidification) and cold cracks (occur during cooling due to hardening and hydrogen).
Physicochemical Mechanism of Action
A crack needs two things at once: tension and a spot that can't handle it. During solidification it goes like this: grains grow together, liquid residual melt remains between them, and shrinkage pulls them apart. That's the . The comes later and from three directions at once: hard microstructure, trapped hydrogen, . If one is missing, usually nothing happens.
Metallurgical Relevance & Welding Risks
The timing tells you which kind you're dealing with. Hot cracks appear right away, usually running longitudinally in the middle of the weld, and sulfur and phosphorus in the material help them along, as does a deep narrow weld. Cold cracks and take their time, sometimes a whole day, and they prefer the . That's why a test right after welding says little. If you check off immediately, you check off too soon.
Workshop Practice & Procedures
First determine the type, otherwise you're curing the wrong cause. Against hot cracks: make the weld wider, put in less heat, use filler with low sulfur and phosphorus. Against cold cracks: , dry electrodes, cool slowly. And a crack is never welded over. Drill the crack ends, grind out completely beyond the end, then rebuild. If you weld over it, it'll be back in two weeks, only deeper.