At what carbon percentage does steel transition to cast iron?

The transition of steel to cast iron occurs at approximately 2.1% carbon content. However, in practical terms, when discussing common thresholds where steel becomes more like cast iron, the relevant percentage is generally accepted to be around 1.7%. At this level, the material has a significant increase in carbon content, which alters its structure and properties.

Steel, characterized by lower carbon content (up to about 1.2%), is malleable and ductile, making it suitable for various applications requiring strength and elasticity. As the carbon content increases beyond approximately 1.7%, the material exhibits the properties of cast iron, which includes increased brittleness and reduced tensile strength. These changes arise due to the formation of graphite and other complex microstructures that emerge with higher carbon content. Hence, 1.7% is a critical threshold where the material's behavior transitions significantly, aligning with characteristics typical of cast iron.

Understanding this transition point is essential in materials science and welding applications because it helps predict the behavior of the metal during processes like welding, casting, and machining.