Ferrites structure

The grades with the 410 or 420 numerals are the basic 13% chromium type with varied carbon content. The additions of sulphur or selenium (possibly with phosphorus) to some grades (416 group) is to improve machinability. 431S29 has increased chromium content to improve corrosion resistance, but reference to Fig. 3.11 shows that such addition alone would lead to a mixed martensite-5-ferrite structure with certain disadvantages to mechanical properties. The nickel addition is to limit ferrite content. 

From Fig. 3.11, it can be seen that by increasing the chromium content while maintaining a limited amount of nickel-equivalent elements, first mixed martensite-ferrite structures are produced and then fully ferritic. This is 6-ferrite, that is a body-centred cubic structure stable at all temperatures. Relative to martensite it is soft, but it is also usually brittle. For this latter reason, usage has in the main been in small section form. This and some other disadvantages are offset for some purposes by attractive corrosion resistance or physical properties. 

These studies have produced a number of results of note. In some ways the most important of these is that ordered ferrite structures with long unit cells do exist. 

There are four main classes of stainless steel (austenitic, ferritic, ferritic-austenitic (duplex) and martensitic), and within these, a variety of different grades. The names ferritic and austenitic follow from their structures ferrite (P-Fe) and austenite (y-Fe) lattices hosting the alloying elements. The presence of Cr promotes the formation of the ferrite structure, while the austenite lattice forms when Ni is introduced. While ferritic and martensitic stainless steels are magnetic, austenitic stainless steel is non-magnetic. Further additives to some stainless steels are molybdenum (which improves corrosion resistance) and nitrogen (which adds strength and improves corrosion resistance). 

The capabilities of the spectral techniques used in the work do not allow to characterize precisely the state of all additives and their distribution in the granular ferrite structure, since signals of some elements occurred in small concentration can not be distinguished against the background. This is especially related to the dopants, which are capable to form Fe2O3-based spinel structures. 

Figure 19.3 Schematic of a partial unit cell and ferrimagnetic ordering of spinel ferrite structure.

At this point, the spinel ferrite structure MFe204, where M refers to a metal, will be briefly discussed. As a typical representative, reference is made to the ZnFe204 structure, with Fe ions occupying the octahedral sites and half of the tetrahedral sites. The remaining tetrahedral sites in this spinel are occupied by Zn [43]. Depending on the distribution of the and Fe cations between the tetrahedral... 

Ferrites are oxide materials whose main phase component is iron (Fe). In general, iron has valence -1-3, although ferrite structures are also composed by divalent materials e.g. magnesium (Mn), zinc (Zn), copper (Cu), cobalt (Co) or... 

Figure 15.1 shows the Nio.5Zno.5Fe204 ferrite structure with octahedral sites in red and tetrahedral sites in dark yellow. This ferrite is formed by a solid solution of Ni203 (nickel oxide), ZnO (zinc oxide) and iron oxide (Fe203) in haematite form (a-Fc203), where this last oxide contains about 70% by weight of ferrite composition. 

The Master Curve methodology uses a mathematical model to describe the probability of cleavage fracture initiation in a material containing a distribution of postulated fracture initiators (flaws). The model includes the temperature dependence of Kj, which was estimated empirically from a data set including various ferritic structural steels. The scatter definition based on the Weibull distribution, the size adjustment and the definition of the temperature dependence are the basic elements of the Master Curve methodology as described in ASTM E 1921. 

Cracks have been discovered in cast primary pump bodies, valves and elbows in main circulation loops. Dye penetrant testing revealed a number of cracks. The cracks were found both on the inside and on the outside of the casings. Moreover, the duplex austenitic-ferritic structure of cast steel pipes used in some NPPs is a potential cause of an embrittlement due to the thermal ageing effect on the ferritic part. It is difficult to perform reliable volumetric non-destructive testing by ultrasonic techniques of the cast material. 

Ferrite-type catalysts may constitute an alternative for perovskites for total oxidation of hydrocarbons as it was demonstrated by ferrites prepared following two routes, that is, the hydrothermal one and the calcination of oxide mixtures with the following compositions Ni >,Fe3 04 (. = 0.5) and Mn Fe3 c04 ( = 0.65) [28]. These studies demonstrated that the structure of the catalysts is very important in providing catalytic active structures. Pure XRD ferrite structures exhibit a poor activity, while mixed-phase ferrite oxide showed a good activity. 

The transition of ODS steel with ferritic structure to a two-phase ferritic-martensitic structure is of great interst [Ref 7.14], Samples of newly-developed ODS alloys with a ferritic content of 40-60% have been made and irradiated in the BR-10 reactor at temperature 650°C to study their high-temperature strength. PIE is under way now. 

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