Cementite decomposition

The iron-carbon solid alloy which results from the solidification of non blastfurnace metal is saturated with carbon at the metal-slag temperature of about 2000 K, which is subsequendy refined by the oxidation of carbon to produce steel containing less than 1 wt% carbon, die level depending on the application. The first solid phases to separate from liquid steel at the eutectic temperature, 1408 K, are the (f.c.c) y-phase Austenite together with cementite, Fe3C, which has an orthorhombic sttiicture, and not die dieniiodynamically stable carbon phase which is to be expected from die equilibrium diagram. Cementite is thermodynamically unstable with respect to decomposition to h on and carbon from room temperature up to 1130 K... 

Since the rate of formation of cementite is determined by nucleation, and therefore proceeds more rapidly in fine-grained steels, it follows that the T-T-T diagram will show a more rapid onset of austenite decomposition than in steels of the same composition, but a coarser grain size. The shape of the T-T-T curve is also a function of the steel composition, and is altered by the presence of alloying elements at a low concenuation. This is because the common alloying elements such as manganese, nickel and clrromium decrease... 

Iron carbide (cemenite, FejC) is thermodynamically unstable. Decomposition of cementite in carbon and C-l/2Mo steels is negligible at <850°F (455°C), but accelerates with temperature exponentially. At 900°F (480°C), 50% conversion to graphite occurs in about 10,000 hours, but at 1100°F (595°C) in about 1000 hours. 

The understanding of the MD of selected pme metals and alloys has been summarized in recent reviews such as that presented by Ramanarayanan and Chun [30]. The formation of a surface layer of cementite that is further destabilized by carbon deposition is the main mechanism for the corrosion of iron. On decomposition Fe nanoparticles are formed that catalyze... 

Ferrite that is formed directly from the decomposition of hypoeutectoid austenite during cooling, without the simultaneous formation of cementite. Also called proeutectoid ferrite. 

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