Iron carbide containing

Pig-iron or cast iron contains impurities, chiefly carbon (up to 5 ). free or combined as iron carbides. These impurities, some of which form interstitial compounds (p. I I3i with the iron, make it hard and brittle, and it melts fairly sharply at temperatures between 1400 and 1500 K pure iron becomes soft before it melts (at 1812 K). Hence cast iron cannot be forged or welded. 

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. 

The iron carbide process is alow temperature, gas-based, fluidized-bed process. Sized iron oxide fines (0.1—1.0 mm) are preheated in cyclones or a rotary kiln to 500°C and reduced to iron carbide in a single-stage, fluidized-bed reactor system at about 590°C in a process gas consisting primarily of methane, hydrogen, and some carbon monoxide. Reduction time is up to 18 hours owing to the low reduction temperature and slow rate of carburization. The product has the consistency of sand, is very britde, and contains approximately 6% carbon, mostly in the form of Ee C. 

The physical and mechanical properties of steel depend on its microstmcture, that is, the nature, distribution, and amounts of its metaHographic constituents as distinct from its chemical composition. The amount and distribution of iron and iron carbide determine most of the properties, although most plain carbon steels also contain manganese, siUcon, phosphoms, sulfur, oxygen, and traces of nitrogen, hydrogen, and other chemical elements such as aluminum and copper. These elements may modify, to a certain extent, the main effects of iron and iron carbide, but the influence of iron carbide always predominates. This is tme even of medium alloy steels, which may contain considerable amounts of nickel, chromium, and molybdenum. 

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. 

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. 

For erosive wear. Rockwell or Brinell hardness is likely to show an inverse relation with carbon and low alloy steels. If they contain over about 0.55 percent carbon, they can be hardened to a high level. However, at the same or even at lower hardness, certain martensitic cast irons (HC 250 and Ni-Hard) can out perform carbon and low alloy steel considerably. For simplification, each of these alloys can be considered a mixture of hard carbide and hardened steel. The usual hardness tests tend to reflect chiefly the steel portion, indicating perhaps from 500 to 650 BHN. Even the Rockwell diamond cone indenter is too large to measure the hardness of the carbides a sharp diamond point with a light load must be used. The Vickers diamond pyramid indenter provides this, giving values around 1,100 for the iron carbide in Ni-Hard and 1,700 for the chromium carbide in HC 250. (These numbers have the same mathematical basis as the more common Brinell hardness numbers.) The microscopically revealed differences in carbide hardness accounts for the superior erosion resistance of these cast irons versus the hardened steels. 

There is another interesting difference between the two irons. Ni-Hard (nominally 1 A Cr, 4 A Ni, 3C) has a matrix of the iron carbide that suiTounds the areas of the steel constituent. This brittle matrix provides a continuous path if a crack should start thus the alloy is vulnerable to impact and is weak in tension. In contrast, HC 250 (nominally 25 Cr, 2 AC) has the steel portion as the matrix that contains island crystals of chromium carbide. As the matrix is tougher. HC 250 has more resistance to impact and the tensile strength is about twice as high as that of Ni-Hard. jMoreover, by a suitable annealing treatment the... 

Most commercial cast irons contain 2.5-4% carbon, and it is the occurrence of some of this carbon as free graphite in the matrix that is the characteristic feature of thin material. About 0.8-0.9% carbon is in a bound form as cementite (iron carbide). 

Malleable iron It contains fewer impurities and has a lower C content (usually between 0.04% and 1.5%) and a higher melting point than cast iron. It can be forged and welded. It is prepared from white cast iron by annealing for several days (the bon carbide is decomposed into bon and nodules of graphite). 

The modern foundry process for producing nodular iron can be oversimplified by describing it as the treatment of a base iron (3% to 4% carbon, 1% to 2% silicon) having low (0,005% to 0.05%) sulfur levels and containing little (<0,05%) phosphorus. The treatment is carried out by means of the introduction of the appropriate nodulizer into this base iron. Inadequate addition of nodulizer results in incomplete spheroidization. Excessive concentrations of nodulizers promote the formation of unwanted iron carbides. The nodulizing elements include the rare earths, magnesium, yttrium and calcium. The latter two elements find little or no use today because of economical and technical problems. 

It should be recalled that the final step in the nodular iron treatment process is termed "post inoculation." The purpose of this procedure is to aid in the elimination of iron carbides and promote enhanced nucleation and proper growth of graphite spheroids. This is accomplished by the introduction of the element silicon (usually a ferrosilicon alloy) along with calcium and maybe some magnesium or rare earth. It has been demonstrated that the benefits of rare earth additions are not affected as a function of the time in the process that they are added (23). For example, the elimination of iron carbides by use of the rare earths is possible if the rare earths are introduced along with the primary nodulizer or with the post inocu-lant. In passing, it should be remarked that both the primary nodulizers and ferrosilicon inoculants contain about 1% calcium. 

The compound is a shiny brown-black crystalline solid. It is stable in air for short periods in the solid state but rapidly oxidizes in solution. The [PPN]+ salt is soluble in CH2C12, CH3CN, THF, and acetone to give intense brown solutions. The IR spectrum in CH2C12 displays strong bands at 1968 (s) and 1942(vs)cm-1 and weaker bands at 2003(w) and 1912(sh)cm- The IR spectrum of a Nujol mull contains iron-carbide stretching bands at 921 and... 

The iron—carbon system contains the orthorhombic iron carbide (3 1) [12011-67-5], Fe3C, which melts congruently and represents the cementite in steel metallurgy. The existence of other carbides, eg, iron carbide (2 1) [12011 -664], Fe2C, iron carbide (5 2) [1212745-6], Fe5C2, and iron carbide (7 3)... 

In gray iron, most of the contained carbon is in the form of graphite flakes, dispersed throughout the iron. In ductile iron, the major form of contained carbon is graphite spheres, which are visible as dots on a ground surface. In white iron, practically all contained carbon is combined with iron as iron carbide (cementite). a very hard material. In malleable iron, the carbon is present as graphite nodules. High-alloy irons usually contain an alloy content in excess of 3%. 

The best-known eutectoid reaction is that which occurs in steel where the austenite phase, stable at high temperatures, transforms into (he eutectoid structure known as pcarlitc In this transformation, the austenite phase, containing 0.8% carbon in solid solution, transforms to a mixture of ferrite (nearly pure body-centered cubic irom anti iron-carbide (Fe-.Ct. Al atmospheric pressure, the equilibrium temperature for this reaction is 723 C. This temperature is the eutectoid temperature... 

Cemeniite. This is iron carbide, FdC. containing fi.fi7( carbon. The substance is hard, brittle, and crystalline. Cenieniile rs precipitated when ausicnile cools. 

Hydrocarbon production and selectivities at comparable CO conversion are given in Table 19.2. The ultrafine iron oxide catalyst had a very poor C2-C4 olefin selectivity while the olefin selectivity of the precipitated catalyst was slightly higher than the iron carbide catalyst. This is surprising because Rice et al. report higher olefin selectivity for a similar iron carbide catalyst than a conventional Fe/Co catalyst.6 Soled et al. have subsequently reported that the conventional catalyst contains acidic sites which... 

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