ironcarbon

The iron–carbon system is a binary alloy framework consisting of iron and carbon, forming the basis for most steels and cast irons. It describes how carbon content influences phase structure, microstructure, and mechanical properties in these materials.

Carbon content in the system distinguishes steels from cast irons. Steel typically contains about 0.02 to 2.0 percent carbon by weight, while cast iron contains more than roughly 2.0 percent carbon. Two key invariant reactions organize the phase relationships: the eutectoid reaction at about 0.76 percent carbon (approximately 727°C), where austenite transforms to pearlite; and the eutectic reaction at about 4.3 percent carbon (around 1147°C), where ledeburite forms. At room temperature, ferrite (alpha-iron) and cementite (Fe3C) are common constituents, with graphite forming in certain cast irons.

Microstructures vary with carbon content and heat treatment. Low‑ to medium‑carbon steels typically exhibit ferrite plus cementite as pearlite, while higher carbon content can produce bainite or martensite after appropriate cooling. In gray cast iron, graphite flakes within a ferritic matrix give distinct properties, whereas white cast iron contains cementite and is hard and brittle. Ductile iron features spheroidal graphite, offering different mechanical behavior. Carbon’s interstitial nature strengthens iron, increases hardenability, and governs the formation of various carbide and graphite phases during cooling and processing.

As a foundational alloy system, iron–carbon informs steelmaking, casting, and heat treatment. The addition of other elements (chromium, vanadium, nickel, etc.) shifts phase boundaries and enables a wide range of steels and cast irons with tailored properties.