Grey iron

There are two basic types of cast iron white, and grey. The phases in white iron are a and FejC, and it is the large volume fraction of FejC that makes the metal brittle. The name comes from the silvery appearance of the fracture surface, due to light being reflected from cleavage planes in the FejC. In grey iron much of the carbon separates... 

Berenson and Wranglen have suggested that the presence of magnesium sulphide inclusions in ductile cast irons may act as corrosion-initiating sites and may therefore make ductile iron more liable to corrosion than grey iron but this view has been contested by Collins . 

It should be noted that it is extremely difficult to predict service lives of buried pipelines from the results of controlled trials with small specimens, whether in the laboratory or in the field. For example a study on the comparative corrosion resistances of ductile and grey iron pipes carried out jointly by European pipemakers in 1964-1973 indicated a mean pitting rate of 0 -35 mm/y for uncoated ductile iron pipe exposed in a typical heavy Essex clay of 500-900 ohm cm resistivity for 9 years. This is clearly at odds with the rate of 1 mm/y normally found on a corroded service pipe from such a soil. The discrepancy appears to be due to the use of specimens that were only a third of a pipe length each and were buried separately. It may reflect the contribution of the total surface area of the pipe as a cathode to the corrosion current at the anodic area at the pitting site. 

The addition of about 20% nickel to cast iron produces materials with a stable austenitic structure these materials are sometimes known as austenitic cast irons but are more often referred to commercially as Ni-Resist cast irons. The austenitic matrix of these irons gives rise to very different mechanical and physical properties to those obtained with the nickel-free grey cast irons. The austenitic matrix is more noble than the matrix of unalloyed grey irons and it was shown in the early work of Vanick and Merica that the corrosion resistance of cast iron increases with increasing nickel content up to about 20% (Fig. 3.42). 

La Que, F. L., Cast Iron in the Chemical and Process Industries, Grey Iron Founders Society Inc., Cleveland, Ohio (1945)... 

The data available suggest that the high-chromium irons do not offer any better resistance to alkalis than unalloyed grey iron, which would normally be preferred in view of its lower cost and its mechanical properties. 

High-silicon irons are inferior to unalloyed grey irons in their resistance to attack by alkalis. For example, boiling 20% caustic soda solution or 50% caustic soda at 80°C will attack high-silicon irons at rates of the order of l-27mm/y (grey iron would be attacked at rates not greater than 0-25 mm/y). 

For cast iron enamelling the so-called grey iron is preferred. Its composition varies somewhat depending upon type and thickness of casting, but falls within the following limits 3-25-3-60% total C, 2-80-3-20% graphitic C, 2-25-3-OOVo Si, 0-45-0-65% Mn, 0-60-0-95% P and 0-05-010Vo S. 

The requirements for ductile iron have been developed with time and experience. The key property that differentiates grey iron from ductile iron is the sulfur content. There is a large body of information that relates the sulfur content of cast iron to the physical properties. Individual manufacturers have researched the properties needed for their products, and have established specifications for sulfur content and other related properties so that product requirements are met. 

The emission control residuals sometimes exceeded EP Toxicity hazardous waste limits for lead and cadmium because the melting point of grey iron is approximately 2,700°F, where the melting point for lead is only about 620°F. As the metal is melted, the lead and cadmium will tend to volatilize and be collected by either the baghouse or wet dust collection emission control system. 

Fig. 3.60 The crystal structure of Fe4N (a = 3.795 A) in space group Pm3m. The large nitrogen atom occupies the very center (Wyckoff position U), and the dark/light grey iron atoms are found at the corners (la) and face centers (3c).

CORROSION, GRAPHITIC - Corrosion of grey iron in which the iron matrix is selectively leached away, leaving a porous mass of graphite behind. This type of corrosion occurs in relativel mild aquous solutions and on buried piping. 

Due to the intimate contact of the liquid metal with the coke, cupola melted iron shows a relatively high sulphur content. Sulphur pick-up in the cupola gives rise to a lower viscosity of the liquid metal, which is an advantage in some cases, e.g. for thin walled grey iron castings. 

Through good design of the connection points, the feeding system may even get broken-off during shake-out. This is mainly possible for grey iron. 

Medium sized grey iron jobbing or small batch 65-75... 

Mechanised repetition, general quality small to medium sized grey iron engineering and municipal castings 65-75... 

Small or very small grey iron repetition 45-55... 

Grey iron cast iron with grey-coloured fracture may be found with lamellar, nodular and compact graphite but the term is most often used for lamellar iron... 

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