Cementite, 1.28

As for the cemented coating constitution, the carbon stability in iron is very low. This addition element is essentially present under the form of cementite. The presence of carbon atoms in iron leads to an increasing of the resistivity and diminishes the magnetic permeability. 

Cementgrout Cement, hydraulic Cementing agents Cementite... 

The carbon content of DRI depends primarily on the direct reduction process used and the way the process is operated. Carbon content can be adjusted within limits by operating changes within the DR process. Most steelmakers prefer slightly more carbon than is required to balance the remaining FeO in the DRI. DRI from gas-based processes typically contains 1 to 2.5% carbon, mostly in the form of cementite [12169-32-3] Fe C. DRI containing approximately 6 to 7% carbon in the form of cementite is called iron carbide. DRI from coal-based, rotary-kiln processes contains very low (ca 0.5%) levels of carbon. 

Nickel—Iron. A large amount of nickel is used in alloy and stainless steels and in cast irons. Nickel is added to ferritic alloy steels to increase the hardenabihty and to modify ferrite and cementite properties and morphologies, and thus to improve the strength, toughness, and ductihty of the steel. In austenitic stainless steels, the nickel content is 7—35 wt %. Its primary roles are to stabilize the ductile austenite stmcture and to provide, in conjunction with chromium, good corrosion resistance. Nickel is added to cast irons to improve strength and toughness. 

Cementite, the term for iron carbide in steel, is the form in which carbon appears in steels. It has the formula Fe C, and thus consists of 6.67 wt % carbon and the balance iron. Cementite is very hard and britde. As the hardest constituent of plain carbon steel, it scratches glass and feldspar, but not quart2. It exhibits about two-thirds the induction of pure iron in a strong magnetic field, but has a much lower Curie temperature. 

A nonaHoyed carbon steel having 0.76% carbon, the eutectoid composition, consists of austenite above its lowest stable temperature, 727°C (the eutectoid temperature). On reasonably slow cooling from above 727°C, transformation of the austenite occurs above about 550°C to a series of parallel plates of a plus cementite known as peadite. The spacing of these plates depends on the temperature of transformation, from 1000 to 2000 nm at about 700°C and below 100 nm at 550°C. The corresponding BrineU hardnesses (BHN), which correspond approximately to tensile strengths, are about BHN... 

If small specimens are prepared in which the austenite can be cooled to 250—500°C sufficiendy rapidly to avoid the above microconstituents, and transformed at temperatures in this range, the formation of a completely different phase, a bcc a-phase supersaturated with carbon and containing small cementite particles (bainite), which is both strong and tough, occurs. Bainite is rarely found in plain carbon steels, but it can be obtained in commercial practice by judicious alloying and is increasing in importance. 

When a eutectoid steel is slowly cooled from the austenite range, the ferrite and cementite form in alternate layers of microscopic thickness. Under the microscope at low magnification, the diffraction effects from this mixture of ferrite and cementite give an appearance similar to that of a pearl, hence the material is called peadite. 

Microstmcture and Grain Size. The carbon steels having relatively low hardenabihty do not contaia martensite or bainite ia the cast, roUed, or forged state. The constituents of the hypoeutectoid steels are therefore ferrite and peariite, and of the hypereutectoid steels, cementite and peariite. 

The iron—carbon system contains the orthorhombic iron carbide (3 1) [12011 -67-5] which melts congmendy and represents the cementite in... 

Iron carbide (3 1), Fe C mol wt 179.56 carbon 6.69 wt % density 7.64 g/cm mp 1650°C is obtained from high carbon iron melts as a dark gray air-sensitive powder by anodic isolation with hydrochloric acid. In the microstmcture of steels, cementite appears in the form of etch-resistant grain borders, needles, or lamellae. Fe C powder cannot be sintered with binder metals to produce cemented carbides because Fe C reacts with the binder phase. The hard components in alloy steels, such as chromium steels, are double carbides of the formulas (Cr,Fe)23Cg, (Fe,Cr)2C3, or (Fe,Cr)3C2, that derive from the binary chromium carbides, and can also contain tungsten or molybdenum. These double carbides are related to Tj-carbides, ternary compounds of the general formula M M C where M = iron metal M = refractory transition metal. 

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... 

The austenite phase which can contain up to 1.7 wt% of carbon decomposes on cooling to yield a much more dilute solution of carbon in a-iroii (b.c.c), Fenite , together with cementite, again rather diaii the stable carbon phase, at temperatures below a solid state eutectoid at 1013 K (Figure 6.3). 

The higher solubility of carbon in y-iron than in a-iroii is because the face-ceiiued lattice can accommodate carbon atoms in slightly expanded octahedral holes, but the body-centred lattice can only accommodate a much smaller carbon concentration in specially located, distorted tetrahedral holes. It follows that the formation of fenite together with cementite by eutectoid composition of austenite, leads to an increase in volume of the metal with accompanying compressive stresses at die interface between these two phases. 

In the most frequently used steels, having less dran the eutectoid content of carbon (about 0.8 wt%), die vaiious forms in which the cementite phase can be produced in dispersion in fenite depend upon die rate of cooling to... 

Figure 6.3 The iron-carbon phase diagram showing the alternative production of iron and cementite from the liquid alloy, which occurs in practice, to the equilibrium production of graphite...

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... 

FejC (also called "iron carbide" or "cementite") Complex A hard and brittle chemical compound of Fe and C containing 25 atomic % (6.7 wt%) C. 

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