Ferrite, modifications

These are chromium steels with more than 13% of chromium and a low carbon content (less than 0.08%). The chromium content stabilizes the ferritic modification. Ferritic steels are magnetic and are highly resistant against corrosion. 

Iron (qv) exists in three aHotropic modifications, each of which is stable over a certain range of temperatures. When pure iron free2es at 1538°C, the body-centered cubic (bcc) 5-modification forms, and is stable to 1394°C. Between 1394 and 912°C, the face-centered cubic (fee) y-modification exists. At 912°C, bcc a-iron forms and prevails at all lower temperatures. These various aHotropic forms of iron have different capacities for dissolving carbon. y-Iron can contain up to 2% carbon, whereas a-iron can contain a maximum of only about 0.02% C. This difference in solubHity of carbon in iron is responsible for the unique heat-treating capabilities of steel The soHd solutions of carbon and other elements in y-iron and a-iron are caHed austenite and ferrite, respectively. 

A number of ferrites have been subjected to shock modification and studied with x-ray diffraction as well as static magnetization and Mossbauer spectroscopy [87V01], Studies were carried out on cobalt, nickel, and copper ferrites as well as magnetite (iron ferrite). 

The barium ferrite was found to have an increase in magnetic anisotropy, as in the nickel ferrite, but its overall effect on magnetization was less because of greater magnetocrystalline anisotropy. The shock modification caused reduced crystallite size and local damage that resulted in increased microwave absorption. 

Fig. 8.3. The yield of shock-synthesized zinc ferrite is found to be strongly dependent on the early loading history. This characteristic is thought to be an indication of shock modification on subsequent chemical reaction.

The conversion process (developed by Outokumpu) is a modification of the jarosite process and involves simultaneously zinc ferrite dissolution and jarosite precipitation in the same reaction vessel. The overall reaction may be represented in simplified form as ... 

Hydrothermal Method. Iron [Fe(III)], barium, and the dopants are precipitated as their hydroxides and reacted with an excess of sodium hydroxide solution (up to 6 mol/L) at 250-350 °C in an autoclave. This is generally followed by an annealing treatment at 750-800°C to obtain products with the desired magnetic properties. Many variations of the process have been described [5.36]-[5.40], the earliest report being from 1969 [5.41], In later processes, hydrothermal synthesis is followed by coating with cubic ferrites, a process resembling the cobalt modification of iron oxides (see Section 5.1.2). The object is to increase the saturation magnetization of the material [5.42]-[5.44],... 

Fig. 1.7 Portions of XRD powder patterns of clinkers containing (A) cubic, (B) orthorhombic and (C) pseudotetragonal modifications of the aluminate phase. Peaks marked A and F are due to aluminate and ferrite phases, respectively, and arc rc-indexed, where necessary, to correspond to axes in the text and Table 1.7, and to calculated intensities. After Regourd and Guinier (Rl).

Fig. 2.8 The pseudosystem CaO-C2S-C]2A7-C2p modified by the presence of 5% of MgO, showing the phase volume of CjS and tie lines for the ferrite phase, the compositional range of which is represented by the hatched line. For details of invariant points P1-P8, see Table 2.1. After Swayze (SIO), with later modifications.

The results will be less accurate for slowly cooled clinkers, as the compositions of the ferrite and possibly also the aluminate phases may differ significantly from those assumed here. At present, there are not enough data to deal with this problem. The method is not applicable without major modification to clinkers made under reducing conditions. It is doubtful whether the procedure is applicable to white cements, both for this reason and because they may contain glass. 

The foregoing results are usually interpreted as indicating that iron is capable of existing in four allotropic modifications, designated respectively as a, / , y, and 8 ferrite, the points A2, A3, and A4 representing their transition temperatures (that is, the temperatures at which the... 

Nalbandyan, V.B., and Shukaev, I.L., New modification of lithium ferrite and the morphotropic series AFe02, Russ. J. Inorg. Chem., 32, 808, 1987. 

The particular advantage of diffraction analysis is that it discloses the presence of a substance as that substance actually exists in the sample, and not in terms of its constituent chemical elements. For example, if a sample contains the compound A By, the diffraction method will disclose the presence of A B as such, whereas ordinary chemical analysis would show only the presence of elements A and B. Furthermore, if the sample contained both A B, and Aj Bjy, both of these compounds would be disclosed by the diffraction method, but chemical analysis would again indicate only the presence of A and B. To consider another example, chemical analysis of a plain carbon steel reveals only the amounts of iron, carbon, manganese, etc., which the steel contains, but gives no information regarding the phases present. Is the steel in question wholly martensitic, does it contain both martensite and austenite, or is it composed only of ferrite and cementite Questions such as these can be answered by the diffraction method. Another rather obvious application of diffraction analysis is in distinguishing between different allotropic modifications of the same substance solid silica, for example, exists in one amorphous and six crystalline modifications, and the diffraction patterns of these seven forms are all different. 

One material that has wide application in the systems of DOE facilities is stainless steel. There are nearly 40 standard types of stainless steel and many other specialized types under various trade names. Through the modification of the kinds and quantities of alloying elements, the steel can be adapted to specific applications. Stainless steels are classified as austenitic or ferritic based on their lattice structure. Austenitic stainless steels, including 304 and 316, have a face-centered cubic structure of iron atoms with the carbon in interstitial solid solution. Ferritic stainless steels, including type 405, have a body-centered cubic iron lattice and contain no nickel. Ferritic steels are easier to weld and fabricate and are less susceptible to stress corrosion cracking than austenitic stainless steels. They have only moderate resistance to other types of chemical attack. 

Silanized and glutaraldehyde-activated sub-micron ferrite particles have been used to immobilize j3-D-galactosidase. The immobilized enzyme was used for the hydrolysis of lactose in whole milk, from which the enzyme is readily recovered magnetically. The immobilized j3-D-galactosidase is of potential industrial importance hydrolysis improves some processed dairy products and there are also nutritional products requiring modification for people with lactose intolerance. 

Iron always contains carbon. A part of the phase diagram of the iron/carbon system is shown in Figure 10.5. The carbon has different solubilities in the different iron modifications, which form mixed crystals (solid solutions). In a-iron the solubility is only 0.04% (ferrite) and in 5-iron the solubility is 0.36%. In the y-modification with its fee struemre, carbon and iron form an intercalation lattice as a solid solution called austenite with the maximum solubility of 2.06% carbon at 1147 °C. Iron with more carbon is called cast iron. Iron with less than 2.06% carbon is called steel. During slow cooling of a melt (above 1147 °C) iron solidifies either as austenite (carbon content of the melt <4.3%) or as cementite (FcjC, carbon content of the melt > 4.3%). At 1147 °C the melt solidifies in a eutectic mixture of both these phases called ledeburite. 

Steels with high chromium content but lesser nickel content (4-6%) are a combination of ferritic and austenitic modification. Their chemical stability is similar to ferritic steels but they are not so sensitive to inter-crystalline corrosion. 

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