Microstructure cast irons

Another form of microstructural galvanic corrosion, graphitic corrosion, is unique to gray and nodular cast irons. It may be encountered in cast iron pumps and other cast iron components. It is a homogeneous form of galvanic corrosion, not requiring connection to a different metal. 

The specimen in Fig. 17.15 is part of a cast iron pump impeller. The spongelike surface contours are apparent, as is the black coating that covers surfaces exposed to the cooling water. Microstructural examinations revealed preferential deterioration of the iron matrix surrounding the graphite flakes. 

O. Yanagisawa, A. Almansour, K. Matsugi, T. Hatayama. Computer simulation for sohdified microstructure prediction of spheroidal graphite cast iron of the Fe-C-Si system. J Jpn Inst Met 50 1101, 1996. 

Microstructures in cast irons are also dramatically influenced by cooling rates. If cooling is rapid, no graphite precipitates. Rather, the alloy solidifies in the metastable Fe-Fe3C state. In that state, the carbon is combined with iron as iron carbides. The fractured surface of carbidic cast iron is white. Such irons are hard and are not readily machined. Carbidic iron castings are used for some special applications, when abrasion resistance is important. 

Figure 2.13 Microstructures obtained by varying thermal treatments in cast irons (Gf = graphite flakes = graphite rosettes G = graphite nodules P = pearlite). From K. M. Ralls, T. H. Courtney, and J. Wulff, Introduction to Materials Science and Engineering. Copyright 1976 by John Wiley Sons, Inc. This material is used by permission of John Wiley Sons, Inc.

Graphitic corrosion has been observed on buried pipelines after many years of service. Gray cast iron has a continuous graphite network in its microstructure that is cathodic to iron and remains behind as a weak, porous network as the iron is selectively removed... 

Gray cast iron such as ASTM A48 material is plain cast iron, composed of ferrite containing graphite stringers. Figure 21.5 shows the microstructure. It is relatively brittle and is used for applications in which toughness is not a concern, including utility services, as pump cases, and valve bodies. 

Figure 16. The displacement of the cutting edge determines the precision of the turning operation (hard cast iron). Average grain sizes of the tool microstructures [in composites sizes of single phase subregions] are D(SHl) = 1.5pm,D(AT60A) = 0.70pm,D(AC41) = 0.56 pm (see Table 4).

Both the demands for advanced grinding tools and for the precise machining of hard cast iron and hardened steel with cutting ceramics are expected to promote the further development of powder technologies for the manufacture of highly perfect submicrometer microstructures for applications in fields where diamond or cBN tools are too expensive. 

Spheroidization Heating and cooling in a cycle designed to produce spheroidal or globular forms of carbide within the microstructure. It is used primarily in cast iron. 

Since C and Si are the alloying elements which dominate the solidification behavior and the resulting microstructures of cast irons, their phase equilibria need to be taken into account. Figure 3.1-120 shows a section through the metastable ternary Fe—C—Si diagram at 2wt%Si which approximates the Si content of many cast irons. Compared to the binary Fe—C system, the addition of Si decreases the stability of FesC and increases the stability of ferrite, as indicated by the expansion of the a-phase field. With increasing Si concentration, the C concentrations of the eutectic and the eutectoid equilibria decrease while their temperatures increase. 

Table 3.1-79 Classification of cast irons according to commercial designation, microstructure and color of fracture surface [1.89]...

Ductile Iron. This cast iron is characterized by the spheroidal graphite phase (SG) in its microstructure. Spheroidal graphite is formed during solidification if the melt has been treated by the addition of a component which promotes the particular nucleation and... 

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