In the iron–carbon system (i.e.
plain-carbon steels and
cast irons) it is a common constituent because
ferrite can contain at most 0.02wt% of uncombined carbon. Therefore, in carbon steels and cast irons that are slowly cooled, a portion of the carbon is in the form of cementite. Cementite forms directly from the melt in the case of
white cast iron. In carbon
steel, cementite precipitates from
austenite as austenite transforms to ferrite on slow cooling, or from
martensite during
tempering. An intimate mixture with ferrite, the other product of austenite, forms a
lamellar structure called
pearlite. While cementite is thermodynamically unstable, eventually being converted to austenite (low carbon level) and graphite (high carbon level) at higher temperatures, it does not decompose on heating at temperatures below the
eutectoid temperature (723 °C) on the metastable iron-carbon phase diagram. Mechanical properties are as follows: room temperature microhardness 760–1350 HV; bending strength 4.6–8 GPa,
Young's modulus 160–180 GPa, indentation fracture toughness 1.5–2.7 MPa√m. The morphology of cementite plays a critical role in the kinetics of phase transformations in steel. The coiling temperature and cooling rate significantly affect cementite formation. At lower coiling temperatures, cementite forms fine pearlitic colonies, whereas at higher temperatures, it precipitates as coarse particles at grain boundaries. This morphological difference influences the rate of austenite formation and decomposition, with fine cementite promoting faster transformations due to its increased surface area and the proximity of the carbide-ferrite interface. Furthermore, the dissolution kinetics of cementite during annealing are slower for coarse carbides, impacting the microstructural evolution during heat treatments. ==Pure form==