Carbon Martensitic Hardening
The increase in hardness through the interstitial solid solution of carbon in the body-centered tetragonal martensite lattice. The lattice distortion is the primary reason for the enormous strength increase.
Martensite formation is basically a hardening game: you heat the part up, then quench it quickly, and the carbon gets "frozen" in the lattice. That makes the material hard, but also brittle. In welding, this is the classic: when the heat-affected zone cools too fast, it becomes glass-hard and cracks. If you cool too slowly, the structure becomes soft and mushy. The problem is you don't see it until it breaks. In practice, you have to control the cooling rate. For thick parts or cold weather, you need to keep the heat in the part, for example with preheating or a thermal blanket. If you're in a hurry and cool with water, you'll ruin the part – it will come back to bite you at the next impact. Better to let it cool controlled than to repair later.