Ferritic Family

The name is derived from the analogous ferrite phase, or relatively pure iron component, of carbon steels, cooled slowly from the austenite region. The 

AISI Type UNS Designation AISI Type UNS Designation 

Chromium readily forms an oxide that is transparent and happens to be extremely resistant to further degradation. It is less noble than iron and, when alloyed with steel, tends to form its oxide first. Gradually increasing the chromium content in steel above the 2% level steadily improves mild atmospheric corrosion resistance up to approximately 12%, where corrosion is essentially stopped. For exposure to mild wet environments, the addition of approximately 11% chromium is sufficient to prevent rusting of steel, hence the term stainless. 

Ferritic stainless steels are magnetic, have body-centered cubic atomic structures and possess mechanical properties similar to those of carbon steel, though they are less ductile. 

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The replacement of the MnZn ferrite family by the NiZn ferrites as the application frequency rises is partly due to the higher resistivities of the NiZn... 

In areas where steels are in contact with the product or raw materials, stainless steels are often used as the workhorse. The production of stainless steel began in the early 1900s. The original efforts in this area were presumably based on the observation that chromium-plated steels parts were highly corrosion-resistant. The end result was the introduction of the ferritic family of stainless steels. The first documentation of the development of this class of steel began to appear in the 1920s. The first American Society for Testing and Materials (ASTM) Specifications for stainless steels were published in 1935. 

Continued additions of chromium will improve corrosion resistance in more severe environments, particularly in terms of resistance in oxidizing environments, at both moderate and elevated temperatures. Chromium contents in ferritic stainless steels are limited to approximately 28%. These alloys are known as 400-series stainless because they were identified with numbers begirming with 400 when AISI had the authority to designate alloy compositions. Specific members of the ferritic families will be covered in Chapter 7. 

The two main families of magnetic additives used for polymer magnets are ferrites and rare earths, which have very different characteristics. 

The family of structurally related hexagonal ferrites contain intergrowths of these elemental building blocks S, R and T. 

When excited by an applied alternating magnetic field the magnetization vector will precess around the anisotropy field as discussed more fully later (Section 9.3.4). Resonance occurs when the frequency of the applied field coincides with the natural precessional frequency, i.e. the Larmor frequency coL = yfi0HA, with the result that the permeability falls and losses increase, as shown for a family of NiZn ferrites in Fig. 9.29. The onset of such ferrimagnetic resonances restricts the use of MnZn ferrites to frequencies of less than about 2 MHz. At higher frequencies, up to about 200 MHz, compositions from the NiZn family are used. 

Stainless alloys that contain roughly equal amounts of austenite and ferrite are termed duplex stainless. This family of alloys grew out of one basic material originally identified as type 329. They are balanced to contain relatively high chromium contents, with only enough nickel and austenitizers to develop about 50% austenite. 

Although more formable than the ferritic alloys, they are not as ductile as the austenitic family of alloys. Welding requires more care than with the austenitic alloys because of a greater tendency toward compositional segregation and sensitivity to weld heat input. Improper fabrication techniques can result in equipment that falls short of expectations for corrosion resistance and mechanical properties. 

Members of families of closely related structures, the formation of which h dependent on the growth mechanism of the crystals, are termed polytypes. The) are not normal polymorphs, and are formed only by compounds with certain type of structure. The best-known examples are SiC, Cdl2, ZnS, and certain comple oxides, notably ferrites, to which reference should be made for further details. 

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