Cementite, structure

The borocarbides FesB jCi also have the cementite structure. As x increases to 0-54 the average magnetic field increases to 240 kG (partly due to an increase in Tq) and the lines broaden and indicate some structure [86]. The effect of boron or carbon neighbours on the iron atoms are predominantly short-range, and the effective 3[Pg.318]

In the solidified melt in cast irons, a ledeburite (mixture of austenite and cementite) structure generally forms. Hardening is caused by graphite dissolution for form cementite and austenite transformation to martensite. 

The carbides of Cr, Mn, Fe, Co and Ni are profuse in number, complicated in structure, and of great importance industrially. Cementite, FcsC, is an important constituent of steel (p. 1075). Typical stoichiometries are listed in Table 8.3 though it should be noted that several of the phases can exist over a range of composition. 

On tempering or annealing martensite, bainite or even pearlite at even higher temperatures (about 970K) a structure consisting of coarse cementite spheroids (readily visible in a light microscope) in a ferrite matrix is obtained. This is the most stable of all ferrite/cementite aggregates, and it is also one of the softest. 

At temperatures in the range of 850 to 950 °F (454—510 °C), permanent structural changes, such as spheroidization, take place in the boiler steel. In this process, the pearlite phase component disappears as the laminar cementite gradually changes into spherical grains. 

The nature of the solidification process in these cements has received little attention. Rather, attention has focussed on the crystalline components that form in cements which have been allowed to equilibrate for some considerable time the nature of such phases is now quite well understood. Gelation is reasonably rapid for these cements and occurs within a significantly shorter time than does development of crystalline phases. The conclusion may be drawn that initial cementition is not the same as crystallization, but must occur with the development of an essentially amorphous phase. Reactions can continue in the amorphous gelled phase, but are presumably limited in speed by the low diffusion rates possible through such a structure. However, reactions are able to proceed substantially to completion, since in many cases X-ray diffraction has demonstrated almost quantitative conversion of the parent compounds to complex crystalline mixed salts, though several days or weeks of equilibration are required to bring this about. 

The fact that the initial setting process for magnesium oxychloride cements takes place without observable formation of either the 5 1 8 or the 3 1 8 phase is important. It indicates that formation of an amorphous gel structure occurs as the first step, and that crystallization is a secondary event which takes place from what is effectively a supersaturated solution (Urwongse Sorrell, 1980a). This implies that crystallization is likely to be extremely dependent upon the precise conditions of cementition, including temperature, MgO reactivity, heat build-up during reaction and purity of the components in the original cement mixture. 

Temperature-programmed reduction combined with x-ray absorption fine-structure (XAFS) spectroscopy provided clear evidence that the doping of Fischer-Tropsch synthesis catalysts with Cu and alkali (e.g., K) promotes the carburization rate relative to the undoped catalyst. Since XAFS provides information about the local atomic environment, it can be a powerful tool to aid in catalyst characterization. While XAFS should probably not be used exclusively to characterize the types of iron carbide present in catalysts, it may be, as this example shows, a useful complement to verify results from Mossbauer spectroscopy and other temperature-programmed methods. The EXAFS results suggest that either the Hagg or s-carbides were formed during the reduction process over the cementite form. There appears to be a correlation between the a-value of the product distribution and the carburization rate. 

The iron-carbon solid alloy which results from the solidification of iron blastfurnace metal is saturated with carbon at the metal-slag temperature of about 2000 K, which is subsequently refined by the oxidation of carbon to produce steel containing less than 1 wt% carbon, the 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) /-phase Austenite together with cementite, Fe3C, which has an orthorhombic structure, and not the thermodynamically stable carbon phase which is to be expected from the equilibrium diagram. Cementite is thermodynamically unstable with respect to decomposition to iron and carbon from room temperature up to 1130 K... 

Gallagher, KJ. Feifknecht,W. Marmweiler, U. (1968) Mechanism of oxidation of magnetite to Y-Fe20j. Nature 217 1118-1121 Gallias, J.L. (1998) Microstructure of the interfacial transition zone around corroded reinforcements. In Katz, A. Benier, M. Alexander, M. Arliguie, G. (eds.) The interfadal transition zone in cementitions composites. E.F.N. Spon, London, 171-178 Galvez, N. Barron,V. Torrent, J. (1999) Preparation and properties of hematite with structural phosphorus. Clays Clay Miner. 47 375-385... 

Ferrite. Iron which, in pig iron or steel, has not combined with carbon to form cementite (FeaC). It exists in a, /9, y and 8 forms, which vary in magnetism and ahility to dissolve cementite. Name also applied to compd NaFe02 (called Na ferrite), to ferromagnetic oxides having a definite cryst structure (spinels) and the formula M++Fe ++04 of which the divalent metal may be Fe, Ni, Zn, or Mn. The magnetic props vary accdg to the divalent atom present, and ferrites are now tailored for their desired effect, as Ni-Al ferrite ... 

There is, however, a well-defined solid iron carbide phase known as cementite, FesC (6.69 weight % C). Further, as the temperature is increased toward the melting point of 1539 °C, the crystal structure of pure iron changes as follows ... 

If austenite is cooled slowly toward ambient temperature, the dissolved carbon in excess of 0.022 weight % comes out of solid solution as cementite, either in continuous layers of FeaC (pearlite) or as layers of separated FeaC grains (bainite). In either case, the iron is soft and grainy, as with cast iron. If, on the other hand, the hot austenite is cooled quickly (i.e., quenched), the 7-Fe structure goes over to the a-Fe form without crystallization of the interstitial carbon as cementite, and we obtain a hard but brittle steel known as martensite in which the C atoms are still randomly distributed through the interstices of a strained a-Fe lattice. Martensite is kinetically stable below 150 °C above this temperature, crystallization of FesC occurs in time. 

Fig. 2.58 Structure of FcjC (cementite)-type, (3, 3)hcp Fej C , projected on (001). Large circles are metal atoms and small ones are carbon atoms. Heights are in units of c/100. On the left are drawn the M C prisms and on the right the empty Mg octahedra. The arrows indicate the twin planes. The unit cell is outlined.

Some other aspects of these structures may also be discussed. In cementite the carbon atoms, with ligancy 6, coordinate six iron-atoms about themselves at the corners of a trigonal pyramid. The arrangement of iron atoms is such that octahedral coordination about carbon would be an alternative possibility, and we may ask why the carbon atom with ligancy 6 prefers the trigonal prism as its coordination polyhedron, and also why the tin atom in AuSn assumes this coordination... 

Cementite, FeaC, has an interesting structure, involving both octahedral and trigonal-prismatic arrangements of six iron atoms about a carbon atom (Slrukiurbericht, II, 33). The iron boride FeB (Struktur-bericht, III, p. 12) contains trigonal prisms of iron atoms about the boron atoms, the Fe—B distance being about 2.15 A, which is approxi-... 

It has also been suggested that flow might occur at lower stresses than those predicted above by movement of material within the individual layers (Chu and Barnett, 1995). This has been observed in pearlitic structures made up of alternating layers of ferrite and cementite, and observations in other multilayer systems suggest that that deformation might occur in this way (Gil-Sevillano, 1979). Two cases have been identified the first where only the movement of a pre-existing dislocation loop is required, the second where the activation of a dislocation source within the layer is needed. Gil-Sevillano (1979) showed that the extra stress, Atm, required to move a dislocation half-loop in a layer of width 1 is... 

---