Explanation: Carbon steel is an alloy of iron and carbon. As the carbon content increases: 1. The ultimate strength of steel increases. 2. The elongation before fracture decreases. 3. The ductility of the metal decreases. 4. Increasing carbon also reduces the weldability, especially above 0.25% carbon. 5. Plasticity and ductility are similar. Generally, carbon is the most important commercial steel alloy. Increasing carbon content increases hardness and strength and improves hardenability.