Cast irons graphite formation

A second, more recently developed use for strontium metal is as an inoculant in ductile iron castings. Inoculants provide nuclei upon which graphite forms during the sohdification of cast iron, thus preventing the formation of white cast iron. Elkem Metals Company has commercialized a range of fine-sized foundry inoculants for iron castings. These inoculants, called superseed, are ferrosiUcon alloys containing 50 or 75% Si, 0.8% Sr. Most of the balance is iron. 

The characteristic mode of corrosion of some alloys may be the formation as a corrosion product of a redeposited layer of one of the alloy constituents, as in the case of the brasses that dezincify, or of a residue of one of the components, as in the case of the graphitic corrosion of cast iron. Particularly in the case of the dezincified brass, the adherent copper is not likely to be removed with the other corrosion products, and therefore the mass-loss determination will not disclose the total amount of brass that has been corroded. This is especially important because the copper layer has very little strength and ductility and the extent of weakening of the alloy will not be indicated by the mass loss. In these cases, also, the mass-loss determinations must be supplemented by, or replaced by, mechanical tests or metallographic examination, or both, to reveal the true extent of damage by corrosion. Difficulties in obtaining accurate mass losses of heavily graphitised specimens have been reported... 

The raw material mix possibly containing small amounts of fluxes is melted in an arc furnace. The melt is then cast into sand, graphite or cast-iron moulds. On the casting side, a cavity arises (called pipe or shrinkage pipe) as a result of contraction due to recrystallization (as with the casting of metals). To suppress formation of large internal stresses, the castings are cooled slowly in loose sand, diatomite and other insulations. The crystallization starts at the surface and proceeds inwards. Its behaviour is difficult to control. 

No special strict requirements are put forward with respect to cast iron composition because in this case adhesion of enamel is of a mechanical nature. The carbon content is usually 3.4 —3.7%, of which 2.5 —3.0% is in graphite form. The base material should be mostly pearlitic, since free cementite in the form of larger crystals is responsible for bubble formation by oxidation. The quality of enamels is adversely affected by sulphur. 

Gray Iron. This most common type of cast iron is characterised by flake graphite and requires a high CE to ensure a sufficient graphitization potential which is also increased by A1 addition. Gray irons may be moderately alloyed e.g., by 0.2-0.6wt%Cr, 0.2-1 wt% Mo, and 0.1-0.2wt% V which promote the formation of alloy carbides and pearlite. Upon plastic deformation the flake form of graphite promotes early internal crack... 

Malleable Irons. The melt treatment of malleable cast irons involves Mg, Ca, Bi, or Te additions. But malleable irons have an as-cast stmcture consisting of FcbC in a pearlitic matrix. By heat treatment in the range of 800-970 the cementite phase is transformed into graphite (TG). The cooling is controlled in such a way as to promote pearlite formation, ferrite formation, or a mixture of the two. 

Waste water pumps made of grey cast iron (EN-GJL-250, 0.6025) have surface roughnesses between 12 and 50 pm (Ra, arithmetic mean roughness) and are thus ideal substrates for adhesive biofilms. Biocorrosion is accelerated on these materials, in contrast to polishable spherical graphite cast iron and cast ferrosilicon. Bronze pumps have improved mean roughness values of approx. 10 pm Ra, and pumps made of cast high-alloy CrNiMo can be polished to values of 5 pm Ra. However, the formation of cavities with a risk of biocorrosion cannot be excluded in this case [14]. 

It is proposed that the major nucleation mechanism in cast iron doping, known as inoculation, is the formation of sulfide species upon the addition of strong sulfide formers such as calcium, barium, cerium, or strontium. These sulfides possess lattice parameters very similar to the graphite crystal structure, serving as substrates for... 

It is also known as ductile iron in USA and is produced by treating a high carbon equivalent liquid iron with manganese or cerium to prevent the formation of graphite flakes. The structure at room temperature comprises a graphite spheres in a matrix of ferrite. Ductility is increased by heating to 900°C followed by a slow quench. Typical properties of cast irons are given in Table 9.12. 

Chromium is frequently added alone and in combination with nickel and/or silicon to increase the corrosion resistance of cast irons. As with nickel, small additions of chromium are used to refine graphite and matrix microstructures. These refinements enhance the corrosion resistance of cast irons in seawater and weak acids. Chromium additions of 15 to 30% improve the corrosion resistance of cast irons to oxidizing acids. Chromium increases the corrosion resistance of cast iron by the formation of protective oxides on the surfaces of castings. The oxides formed will resist oxidizing acids but will be of little benefit under reducing conditions. 

Theoretical anode consumption rates are at 9 kg A-i y-i. For cast iron (containing graphite) consumption rates may be lower than theoretical due to the formation of carbon-rich surface films. Full utilization of the anode is rarely achieved in practice due to preferential dissolution in certain areas. Fundamentally, these anodes are not prone to failure at a particular level of current density. For long anode... 

---