Graphite in iron

Both carbides, Fe3C and Fe2C, are thus endothermic compounds at temperatures above 700° C., and the diminished solubility of graphite in iron above 2220° C. is due to dissociation of the diferro carbide. Below 2220° C. the reaction... 

In these conditions the cooled casting deposits pure carbon in the state of graphite the temperature at which this deposit is made, at which, consequently, the melted casting may be regarded as a saturated solution of graphite in iron, depends on the proportion of carbon in the liquid mixture it is the lower as x is smaller when x is reduced to the value 0.043 this temperature descends to 1130. The locus of points, which have for absciss values of x and for ordinates the temperatures, is the curve (Fig. 102), solubility curve of graphite in melted iron. 

Tur] Tmkdogan, E.T., Hancock, R.A., Herhtz, S.I., Dentan, J., Thermodynamics of Carbon Dissolved in Iron Alloys. Part V Solubihty of Graphite in Iron-Manganese, Iron-Cobalt, and Iron-Nickel Melts , J. Iron Steel Inst., London, 183, 69-72 (1956) (Experimental, Phase Relations, Thermodyn., 17)... 

A metallurgical term describing the formation of graphite in iron or steel, usually from decomposition of iron carbide at elevated temperatures. Not recommended as a term to describe graphitic corrosion. 

ASTM. 2006a. Standard test method for evaluating the microstructure of graphite in iron castings. ASTM A247-06. West Conshohocken, PA ASTM International. 

Carbon, present in iron or remaining after inadequate degreasing, can form CO or CO2. Carbon particles may occur in the chlorate if graphite anodes were used in the production process. Additionally, barium peroxide contains carbonate as a contaminant. 

Magnesium ferrosihcon alloys react vigorously when added to molten iron. As the magnesium vaporizes and cools, it reacts with residual surface tension modifiers such as sulfur and oxygen and greatly increases the surface tension of the molten iron. The dissolved graphite in the molten iron nucleates and grows into a spheroidal shape because of the increased surface tension of the molten iron. 

In metallurgical practice, sodium uses include preparation of powdered metals removal of antimony, tin, and sulfur from lead modification of the stmcture of siHcon—aluminum alloys appHcation of diffusion alloy coatings to substrate metals (162,163) cleaning and desulfurizing alloy steels via NaH (164) nodularization of graphite in cast iron deoxidation of molten metals heat treatment and the coating of steel using aluminum or zinc. 

Tellurium also improves the properties of electrical steels by aiding in the magnetic anisotropy, malleable cast iron (77), and spheroidal (graphitic) cast irons (see also Metal surface TiiEATiffiNTs). 

When the layer of graphite and corrosion products is impervious to the solution, corrosion wdl cease or slow down. If the layer is porous, corrosion will progress by galvanic behavior between graphite and iron. The rate of this attack will be approximately that for the maximum penetration of steel by pitting. The layer of graphite formed may also be effective in reducing the g vanic action between cast iron and more noble alloys such as bronze used for valve trim and impellers in pumps. 

Most of the surface is covered with a black corrosion product that is thicker in relatively low-flow areas near the hub. This layer of soft corrosion product can be shaved from corroded surfaces. Microstructural examinations revealed flakes of graphite embedded in iron oxide near the surfaces. 

Most commercial cast irons contain 2.5-4% carbon, and it is the occurrence of some of this carbon as free graphite in the matrix that is the characteristic feature of thin material. About 0.8-0.9% carbon is in a bound form as cementite (iron carbide). 

Limitations on the use of cast irons are similar to those for steel, since in many environments most cast iron has poor corrosion resistance. Most grades are also susceptible to graphitization (the loss of iron, leaving a weak structure of graphite) in acidic environments below a pH of approximately 5.5. This attack occurs in soils. 

Scaling results for the flake graphite cast irons a, b, d and e in Fig. 7.21 show that the weight increase related to surface area due to scaling rises as the quantity of graphite increases with increase in carbon content (iron a), and as the graphite becomes more widely dispersed in the matrix (iron d). Apart from carbon content, changes in the base metal composition and... 

Magnetite anodes exhibit a relatively low consumption rate when compared with other anode materials, namely graphite, silicon iron and lead and can be used in seawater, fresh water and soils. This low consumption rate enables a light-weight anode construction to be utilised. For example, the anode described by Linder is 800 mm in length 60 mm in diameter, 10 mm wall thickness and 6 kg in weight. 

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