Interstitial Solid Solution Hardening by Carbon

Carbon is the most important alloying element for the hardenability of steel. The more carbon it contains, the harder and stronger the material becomes, but also more brittle. During welding, this is the critical point: rapid cooling can form hard, crack-prone martensite in the heat-affected zone. The higher the carbon content, the more careful you must be and let the component cool more slowly, for example by preheating.