Ferrite-carbide aggregate

The ferrite-carbide structure forms when cooling from austenitizing. Ferrite is soft and tough, carbides make the steel hard and wear-resistant. In welding, cooling rate matters: too fast cooling hardens the HAZ and can cause cracks, too slow coarsens carbides and makes the material brittle. So you must control cooling to hit a middle ground – usually with preheating and controlled post-cooling. If you stress-relieve after welding, you get a good, tough structure.