Steel iron, ferrite

Soft magnetic materials are characterized by high permeabiUty and low coercivity. There are sis principal groups of commercially important soft magnetic materials iron and low carbon steels, iron—siUcon alloys, iron—aluminum and iron—aluminum—silicon alloys, nickel—iron alloys, iron-cobalt alloys, and ferrites. In addition, iron-boron-based amorphous soft magnetic alloys are commercially available. Some have properties similar to the best grades of the permalloys whereas others exhibit core losses substantially below those of the oriented siUcon steels. Table 1 summarizes the properties of some of these materials. 

Hydrogen transport in a metal can occur by lattice diffusion, dislocation motion, or short-circuit diffusion along grain boundaries. Hydrogen can diffuse rapidly through the lattice in many metals, particularly those with a body-centered cubic (bcc) structure such as a-iron, ferritic steels, and )3-titanium alloys. However, grain boundary diffusion and dislocation... 

Main crystalline constituents in carbon steels are ferrite, cementite, perlite, and, depending on heat treatment, bainite and martensite. Below approximately 723 °C, austenite in carbon steels is transformed into perlite and, according to the carbon content, ferrite or cementite. Thus, austenite is only present at room temperature in alloyed steels and not in carbon steels. The iron-carbon phase diagram shows the metallographic constitution for unalloyed carbon steels depending on the carbon content and the temperature. Fig. 2. 

Fe—Cr. The Fe—Cr phase diagram. Fig. 3.1-106, is the prototype of the case of an iron-based system with an a-phase stabilizing component. Chromium is the most important alloying element of corrosion resistant, ferritic stainless steels and ferritic heat-resistant steels. If a-Fe—Cr alloys are quenched from above 1105 K and subsequently annealed, they decompose according to a metastable miscibility gap shown in Fig. 3.1-107. This decomposition reaction can cause severe embrittlement which is called 475 C-embrittlement in ferritic chromium steels. Embrittlement can also occur upon formation of the a phase. 

The reader probably knows already that real motors have more coils and poles, and more complex commutators. There is more than one pulse of attraction, and sometimes the current is reversed to also cause repulsion. The coil almost always has a small soft iron core, and the permanent magnet is a large piece of hard steel or ferrite (iron oxide and barium oxide ceramic). Because the permanent magnet is heavier than the coil, it is usually the stationary part, not the way it is shown here. However, this diagram communicates the main ideas. 

Unalloyed and low-alloyed steels/cast steel 193 Unalloyed cast iron and low-alloy cast iron 224 High-alloy cast iron 226 Ferritic chromium steels with < 13% Cr 228 Ferritic chromium steels with >13% Cr 229 High-alloy multiphase steels 235 Ferritic/pearlitic-martensitic steels 235 Ferritic-austenitic steels/duplex steels 235 Austenitic CrNi steels 237 Austenitic CrNiMo(N) steels 239 Austenitic CrNiMoCu(N) steels 244 Nickel 260... 

Ferritic chromium steels with < 13 % Cr 320 Ferritic chromium steels with > 13 % Cr 320 High-alloy multiphase steels 320 Ferritic/pearlitic-martensitic steels 320 Ferritic-austenitic steels/duplex steels 320 Austenitic CrNi steels 323 Austenitic CrNiMo(N) steels 323 Austenitic CrNiMoCu(N) steels 323 Nickel-chromium alloys 339 Nickel-chromium-iron alloys (without Mo) 339 Nickel-chromium-molybdenum alloys 339 Nickel-copper alloys 339 Zinc 343 Bibliography 344... 

Austenitic Stainless Steels Ferritic Stainless Steels Iron- and Nickel-Base Alloys... 

Figure 3 Picture of the inspected part (plasma-sprayed chromium cast iron on ferritic steel). The surface presents several cracks I to 15 pm wide.

Ferritic stainless steels depend on chromium for high temperature corrosion resistance. A Cr202 scale may form on an alloy above 600°C when the chromium content is ca 13 wt % (36,37). This scale has excellent protective properties and occurs iu the form of a very thin layer containing up to 2 wt % iron. At chromium contents above 19 wt % the metal loss owiag to oxidation at 950°C is quite small. Such alloys also are quite resistant to attack by water vapor at 600°C (38). Isothermal oxidation resistance for some ferritic stainless steels has been reported after 10,000 h at 815°C (39). Grades 410 and 430, with 11.5—13.5 wt % Cr and 14—18 wt % Cr, respectively, behaved significandy better than type 409 which has a chromium content of 11 wt %. 

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. 

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. 

Ferritic Stainless Steels. These steels are iron—chromium alloys not hardenable by heat treatment. In alloys having 17% chromium or more, an insidious embrittlement occurs in extended service around 475°C. This can be mitigated to some degree but not eliminated. They commonly include Types 405, 409, 430, 430F, and 446 (see Table 4) newer grades are 434, 436, 439, and 442. 

Many stainless steels, however, are austenitic (f.c.c.) at room temperature. The most common austenitic stainless, "18/8", has a composition Fe-0.1% C, 1% Mn, 18% Cr, 8% Ni. The chromium is added, as before, to give corrosion resistance. But nickel is added as well because it stabilises austenite. The Fe-Ni phase diagram (Fig. 12.8) shows why. Adding nickel lowers the temperature of the f.c.c.-b.c.c. transformation from 914°C for pure iron to 720°C for Fe-8% Ni. In addition, the Mn, Cr and Ni slow the diffusive f.c.c.-b.c.c. transformation down by orders of magnitude. 18/8 stainless steel can therefore be cooled in air from 800°C to room temperature without transforming to b.c.c. The austenite is, of course, unstable at room temperature. Flowever, diffusion is far too slow for the metastable austenite to transform to ferrite by a diffusive mechanism. It is, of course, possible for the austenite to transform displacively to give... 

Austenitic steels have a number of advantages over their ferritic cousins. They are tougher and more ductile. They can be formed more easily by stretching or deep drawing. Because diffusion is slower in f.c.c. iron than in b.c.c. iron, they have better creep properties. And they are non-magnetic, which makes them ideal for instruments like electron microscopes and mass spectrometers. But one drawback is that austenitic steels work harden very rapidly, which makes them rather difficult to machine. 

Thermal Expansion. Alloys differ in their thermal expansion, but the differences are modest. Coefficients for the ferritic grades of steel are perhaps 30 percent below those of the austenitic steels at best, while expansion of the nickel-base austenitic types may be no more than 12 to 15 percent less than tho.se of the less expensive, iron-base, austenitic, heat-resistant alloys. Unfortu-... 

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