CHARACTERIZATION
There are at least seven types of carbon additives commonly used in iron and steel melting. They are generally characterized (or identified) by their origin, their chemistry, and their physical properties.
The origin is straightforward: Where did it come from? How much temperature has it been subjected to? How is it characterized? We can identify two graphitic types of carbon: natural graphite and synthetic (manufactured) graphite. For carbons (un-graphitized), there are calcined petroleum cokes (CPC), metallurgical coke (Met.Coke), electrically calcined anthracite (ECA), carbon electrodes, and silicon carbide (SiC). These constitute the bulk of carbon alloyed in iron and steel.
The chemical properties of carbon/graphite are: ash content, volatile matter, moisture content, and sulfur content. Historically, what's left is the available carbon for alloying. Also included would be gas contents: nitrogen, hydrogen, and oxygen. Some of the gas can be chemically reacted, yet a significant portion remains mixed or entrapped within the carbon/graphite particle.
Physical tests include sizing, density, and resistivity. Other more intricate tests are spectrographic analyses, and x-ray diffraction and crystallite sizing. The x-ray diffraction would be used mostly to differentiate various
graphites.
NATURAL GRAPHITE
Natural graphite is mined and of two types. One is graphite particles or flakes imbedded in rock. The rock is crushed, then the graphite is separated by flotation from the host rock. This graphite can be quite pure but has an attending high cost. It has been subjected to high geothermal temperature and pressure (often over 7000oF or 3900oC). The second type is vein graphite extracted from mines throughout the globe. An example would be Mexican graphite. It too, has been subjected to high geothermal temperatures and pressures. While vein graphite has lower costs, it also can contain high ash contents and gases
SYNTHETIC OR MANUFACTURED GRAPHITE
The next type of graphite is synthetic or man-made graphite, as graphitic electrodes, either as machinings from newly produced electrodes, or from scrap electrodes crushed and sized. The raw electrodes are made from calcined petroleum coke (roughly 85%) plus petroleum paste, iron oxide, and small amounts of other additives to enhance graphitization. This mixture is then carbonized at approximately 2000 to 2200oF (1100 - 1200oC) in a carbon furnace. They are then removed from the furnace, rough machined, re-furnaced into an Acheson-type furnace, and heated through electrical resistance to 4000 to 4500oF (2200-2500oC) for an extended period of time (from several days to weeks). This converts the carbon electrode into a graphite electrode.
Other graphitic plates and shapes are similarly treated and are used as anodes or cathodes, generally available as scrap products.
Another synthetic graphitic material is a desulfurized calcined petroleum coke that has been subjected to a high temperature thermal process, 4500 to 4800 degrees F (2500 - 2650 degrees C), that desulfurizes the CPC and converts most of the carbon to graphite
CARBONS (UNGRAPHITIZED)
Calcined petroleum coke (CPC) emanates as a by-product from oil refineries. Most of the CPC is utilized in the aluminum industry as anodes to reduce aluminum oxide to metallic aluminum. Several grades are available, based on chemistry. CPC has a high carbon content but it also contains measurable amounts of sulfur, gases (notably nitrogen and hydrogen), and sometimes metallics like Va or Ni. Sulfur can reach 5 to 6 %. However, iron and steel won't tolerate much more than 1.5 to 2.0 %. Nitrogen levels can also reach 2.0%, or 20,000 ppm. CPC has been coked at roughly 1800 to 2200oF. (1000 - 1200oC).
Also available is an ethylene coke made from high purity feed-stock with notably lower S (.10%) and N (.50%). The high purity of the precursor feed-stock yields a high purity coke suitable for carbon alloy additions in ferrous metals.
Metallurgical coke derives from foundry or blast furnace coke. Carbon additions are screened from the larger furnace coke and are perhaps the least expensive of carbon additions. Metallurgical coke contains some sulfur (.6 to .8%), a measurable amount of ash which impedes carbon recovery, and varying amounts of gas. It emanates from blend of coals, which are coked at 1600 to 2000o F. (900 - 1100oC.).
Electrically calcined anthracite (ECA) is made from anthracite coal and electrically calcined in special furnaces. It's end use is generally in ferro-alloy electrodes, often as a paste in Soderburg furnaces. The heat of its formation can vary, depending how close it passes to the furnace electrodes, or how fast it passes through the furnace.
Carbon electrodes originate similar to graphitic electrodes, except that they have not been graphitized. They are used primarily as ferro-alloy electrodes and have been subjected to 2000 to 2200oF. (1100 - 1200oC.) of temperature. Carbon electrodes are hard, often machined with diamond tooling. The carbonizing process removes only small percentages of sulfur and gases available in the precursor CPC. They are scrapped, crushed, and sized before usage as a carbon additive.
Silicon carbide can also be used as a carbon addition. Its chemistry is roughly 30% C and 60% Si and is known more for its deoxidizing potential. However, both its C and SI are available as alloy additions.
CHEMISTRY OF CARBON ADDITIVES
Concerning chemistry of carbon, testing has historically provided an ash content whereby a measured sample is placed in a crucible, heated to 1700o
F. (950o C.) for several hours to burn off the carbon, after which the remaining ash content is weighed and calculated as loss of ignition. This is specified under ASTM C561-91. The color of the ash can also be noted. For instance, a reddish hue indicates the presence of Fe.
Volatile matter testing involves heating a sample of carbon or graphite to 1700oF. (950oC.) for 5 to 7 minutes, cooled, and reweighed to calculate the weight loss, yielding the % volatile content. ASTM D 3175-77 Revision 3105 (1990) is one of the specifications.
Moisture content involves a material sample dried in an oven at 230oF. (110oC.)for 2 hours, cooled, then the dried sample is reweighed to calculate % moisture loss. ATSM C562-85, Revision 2606 (1987) is the specification.
The % sulfur in carbon or graphite is generally provided by Leco furnace testing, burned with oxygen whereby the combusted gas is bubbled through a titrator which converts to % S. Specification are under Leco sulfur
testings.
Once the above tests have been performed, the customary reporting of available carbon can be determined: 100% minus % ash, minus % volatiles, minus % moisture, minus % sulfur, equals the carbon available. There is also a fixed carbon test which basically is 100%, minus % ash, minus % volatiles, minus % moisture, equals % fixed carbon; however, this does not accommodate any appreciable sulfur nor gas contents which of course are not available carbon.
The gases contained in carbon or graphite are generally measured by high temperature thermal conductivity using various Leco instruments. (Reference: nitrogen analyses comparison, Casting Industry Suppliers Assoc., 1990). Nitrogen can be determined by the Kjeldahl chemical digestion procedure (ASTM D 3179-89). Further, there is a chemiluminescence procedure which converts nitrogen to nitric oxide, then measured by a photo multiplier tube. Gases are generally reported in parts per million (ppm). It should be noted that this technology is undergoing alterations to improve the accuracy as time passes. Also note that the chemical Kjeldahl procedure involves lengthy time to fully dissolve the carbon or graphite sample (often a week) and is therefore, subjected to operator variation. Further note that significant gas contents are likewise weighed in the original test sample. 10,000 ppm of nitrogen, for instance, is 1.0 % nitrogen which is not part of carbon available for dissolution in iron or steel.
PHYSICAL TESTING OF CARBON/GRAPHITE ADDITIVES
Under physical testing, sizing is accomplished by screening out various fractions on a series of stacked screens, weighing each fraction, then dividing by the original sample weight to calculate the % retained on each screen. In general, sizing of carbon/graphite additives is ½ inch maximum, down to 100 mesh minimum. Additives can be procured in various sizings; for instance - 3/8"x 30 mesh, or 8 mesh x 60 mesh. Furnace additions generally are larger than ladle additions.
Density is another comparative measurement. Pure graphite has a density of 3.36 grams per cubic centimeter, whereas pure carbon is lower. There are cases where carbon densities have been over 3.40 gm/cc but this must be approached with caution: the carbon had a high ash content and there were metallics in the ash, increasing the density. To reiterate, density is simply a comparison.
Resistivity is measured in a Wheatstone or Kelvin bridge instrument whereby an electrical current is passed through a sample and the electrical resistance is measured. Resistance is the inverse proportion to electrical conductivity and can easily differentiate graphite from carbon.
In X-Ray Diffraction testing, a candidate sample is bombarded with x-rays which form a distinctive pattern of peaks on a graph. Pure graphite will create narrow, long vertical lines indicating a high degree of graphitization whereas less graphitized material will create lower, broader peaks of less graphitization. Carbon (ungraphitized) does not exhibit any peak, or little more than a bump in the pattern. Impurities can influence the peak pattern, flattening them out to a small degree.
In conjunction with x-ray diffraction, cystallite sizing of the graphite can be calculated, indicating in angstroms, how large the graphite flakes are. This has value in determining whether the graphite will inoculate irons. For instance, a crystallite sizing measurement of less than 100 angstroms indicates the sample is a carbon, which will not create an inoculating effect in irons. Synthetic graphite has been calculated at 300 to 500 angstroms and is known to suppress undercooling, reducing chill i.e., aiding in the inoculation effect. High purity graphite has been calculated at over 1000 angstroms. Graphitic materials are often added in conjunction with ferro-silicon based inoculants to create the desired effect.
Spectrographic Analyses have been run on carbon/graphite materials to determine various residual levels of the many elements carried in additives. Sometimes measured in percentage, some elements are reported in parts per million. The spectrograph has emerged in the last two decades with rapid measurements to further characterize carbon graphitic additives used to alloy iron and steel.
SUMMARY
The above chemical and physical tests of carbons and graphites properly characterize the additives, and judicious use of these results will determine which additive corresponds best to your needs. No attempt is made to compare costs. In general, melt shops should use these characterizations to procure the carbon alloy best suited to their application.
Thomas H. Witter
DIS Alumnus
April, 2001
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