Ferrite acicular

Microstructure ferrite + acicular ferrite Yield stress low... 

Microstructure ferrite/acicular ferrite + small quantity pearlite Yield stress high... 

The material properties used in the simulations pertain to a new X70/X80 steel with an acicular ferrite microstructure and a uniaxial stress-strain curve described by er, =tr0(l + / )", where ep is the plastic strain, tr0 = 595 MPa is the yield stress, e0=ff0l E the yield strain, and n = 0.059 the work hardening coefficient. The Poisson s ratio is 0.3 and Young s modulus 201.88 OPa. The system s temperature is 0 = 300 K. We assume the hydrogen lattice diffusion coefficient at this temperature to be D = 1.271x10 m2/s. The partial molar volume of hydrogen in solid solution is... 

Similar comments are applicable to weld metal hydrogen cracks, with the further observations that cracking in weld metals is frequently transverse to the weld and that in weld metals of the acicular ferrite type, cracking is most common in the primary ferrite (if it is present) at the prior austenite grain boundaries, as in Fig. 1.3(a). In relatively hard, alloyed weld metals (e.g. in the Cr-Mo steels) any transverse weld metal cracking is usually perpendicular to the weld surface, but in C-Mn weld metals the chevron type of cracking (Fig. 1.3(c)) at 45° to the surface is common. 

The change of crystal structure which occurs in some materials in which different crystal structures are stable over different temperature (and pressure) ranges. In ferritic steels, the most important transformation is from the high temperature form, austenite, to lower temperature transformation products, such as ferrite, pearlite, bainite, martensite and, in weld metals, acicular ferrite and ferrite with aligned second phase. 

Quenching of steels containing < 0.08 wt% C such that acicular ferrite or low-carbon bainite is formed. This microstmctural state provides an excellent combination of high yield strengths of 275 to 690 MPa, ductility, formability, and weldability. 

The microstructure of the welding metal is based on acicular and eutectoid ferrite. The acicular ferrite has blunter rods due to the annealing of the welding splice. 

Stmctures that form as a function of temperature and time on cooling for a steel of a given composition are usually represented graphically by continuous-cooling and isothermal-transformation diagrams. Another constituent that sometimes forms at temperatures below that for peadite is bainite, which consists of ferrite and Fe C, but in a less well-defined arrangement than peadite. There is not sufficient temperature and time for carbon atoms to diffuse long distances, and a rather poody defined acicular or feathery stmcture results. 

Dehydroxylation of goethite produces the ferrite reds - extremely colour fast and pure hematite. With low temperature calcination the acicular shape of the goethite precursor is retained, whereas high temperatures lead to a sintered product. 

Towards the end of the middle period, a renewed growth of AFt crystals takes place (D27,P3I,S4I) (Fig. 7.6d). They are markedly more acicular than those formed earlier their lengths are typically 1-2 pm, but sometimes up to 10 pm. Their formation is associated with a shoulder on the heat evolution curve (Section 7.5.1). Their formation implies an increase in the rate of reaction of the aluminate, or less probably the ferrite phase, which is probably related to the reaction of the alite (S68). 

Kakizaki, K., and Hiratuka, N., Magnetic properties and crystal structures of acicular barium ferrite particles, J. Magn. Soc. Jpn., 22(suppl. SI), 129, 1998. Lemke, J.U., Ultra high density recording with new heads and tapes, IEEE Trans. Magn., 15, 1561, 1979. 

High-iron clinkers contain well-developed acicular or prismatic crystals of the caldtrm alirminoferrite phase, as well as dicalcium ferrite. The A/F molar ratio in the alirminoferrite phase may be as low as 0.11 (Stark et ai, 1997b). 

Y HIDA et ak, Influence of Partial Pressure of Oxygen in the Atmosphere on the Formation of Acicular Calcium Ferrite, Transactions of the Iron and Steel Institute of Japan, 24( 1984), B331. 

An excellent overview of the area of structural damage to RPV steels was published in [37]. The microstructures of RPV steels are typical of those of annealed bainite and martensite and low-alloying carbon steels [38]. In the structure of the base material, traces of native acicular composition (termed sorbite ) are evident (Figure 4.10a). Ferrite can occur locally at grain boundaries (Figure 4.10b) at levels of 1% (at the surface on the side of the RPV) to 1% (in the middle of the wall of the RPV). The austenite grain diameter is distinctly average at 76 pm [39]. 

Figure 4.10 Sorbidic micro structure of the base material with a traces of native acicular composition and b local ferrite isles...

In the practice of ferrite production proper particle sizes are obtained by proper conditions of precipitation, drying, and calcination. Ferric oxide, for instance, prepared in less than 1 n size and of acicular shape is more reactive than larger sized oxide of oblong shape( ). Sizes below 1 /i are generally preferred for the other components as well. Precipitation of barium carbonate onto ferric oxide, for instance, has also been suggested to optimize the contact for the ensuing reaction in the solid state. 

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