Iron, the commonest and most useful metal, is always used commercially in the alloyed form, as its properties can be varied as to hardness, ductility, flexibility, tensile strength, chemical resistance, and other properties by the choice, amounts, and combination of alloying elements. Gray cast iron is distinguished by a relatively high amount of carbon, approximately 3%, which imparts to it the characteristic hardness, castability, wear resistance, and machinability displayed by no other metal.
Gray cast iron is unique in its high content of carbon, and in the form of a large portion of this carbon as a separate phase of graphite. The strength, wear resistance, brittleness or conversely toughness, and machinability are all controlled to a large and primary extent by the graphitic carbon content. Graphite in gray iron appears in several forms well-known to the foundry metallurgist, of which the so-called type A, a flake, is preferred, in a pearlitic iron matrix. If the carbon is present as iron carbide, or cementite, the metal will be what is known as white iron, hard, brittle, and unmachinable. If the carbon is present in the correct proportion as graphite in the pearlitic matrix, it will display the characteristic gray color and good machinability of gray iron.
(This treatment ignores the effects of the other alloying elements and heat treatment and will be limited to the effects of silicon and carbon upon the properties of gray cast iron, in order to simplify its complex subject matter.)
When gray iron is melted in a cupola over a bed of hot coke, it gains some carbon content from the coke, which may be varied by adjusting the coke-iron ratio, the air blast, by additives such as silicon, and by the slag chemistry.
When it is poured into the molds to produce parts, the utility of these parts is affected by the cooling rate, and the rate of precipitation from solution of the various forms of iron. An iron melt which hardens too quickly will have an excess of iron carbide and have the characteristics of white iron, hard, brittle, poorly machinable, and relatively strong.
If the iron has an excess of carbon as graphite with the metal predominantly in the form of primary ferrite from a too slow cooling rate, the metal will have low tensile strength and be too soft to be commercially useful.
The amount and shape, size, and distribution of graphite present in a gray cast iron are usually controlled by the addition of an inoculant to the metal in the cupola, the ladle, or the mold which furnishes seeds for formation of crystals of graphite. Inoculants commonly used are silicon in various forms, such as ferrosilicon or silicon carbide, and graphite itself. Other metals used include chromium, manganese, calcium, titanium, zirconium, aluminum, barium and strontium.
Some of the elements function as alloying elements as well, in particular molybdenum, chromium, and manganese. Aluminum and the alkaline earths are the most effective non- graphitic inoculants.
Silicon is the principal element used as an inoculant, controlling graphite formation, allowing the formation of the pearlitic iron matrix over a wider temperature range, and thus decreasing the chill depth of the cast metal.
The chill depth test is usually conducted by casting a graduated wedge-shaped test piece under specific conditions, and measuring the extent of the white iron from the tip of the wedge. Since the thinner portion cools faster, the tip will be of white iron or iron carbide, which will crystallize earliest, and is light colored, hard, brittle and unmachinable in normal operation. The extent of the chill depth controls principally the thickness of the casting which can be made from a particular melt, a melt with a low chill depth enabling a relatively thinner casting to be poured without the formation of white iron. A thick cross-sectioned casting is made with iron with a greater chill depth to avoid the formation of excess graphite and ferrite. The desired metal consists of graphite flakes in a matrix of pearlitic iron, which is stabilized over a widely varying cooling rate.
Past practice in this area has shown the use of silicon carbide as an added ingredient in the cupola charge or to the ladle by U.S. 2,020,171 and U.S. 2,119,521 tc Brown.
The use of silicon carbide in briquette form is shown by U.S. 2,497,745 to Stohr; U.S. 2,527,829 to Leitten; U.S. 3,051,564 to Drenning; and U.S. 3,666,445 to Stone et al. U.S. 4,015,977 to Crawford claims briquettes of petroleum coke with refractory oxides or a derivative which will yield a metal oxide.
A clear explanation of the use of silicon carbide in gray iron melts is given by Moore, U.S. 3,764,298, showing desirable and undesirable grain structures and chill wedges with small additions of silicon carbide to the metal.
In accordance with the invention there is provided a coke suitable for use as fuel in a foundry cupola to produce gray iron, characterized by the fact that it contains in its structure an effective amount of silicon carbide which is blended with the coal or blend of coals used to produce said coke prior to coking said coal or blend of coals.
The invention further provides a method of producing gray iron for castings in a cupola in which the improved coke of the invention is used as a fuel.
The silicon carbide used in the practice of this invention is preferably a conventional silicon carbide which is a by-product of the Acheson graphite process. When baked carbon electrodes are packed with resistor coke and then covered with a coke-silica mixture and electrically heated to transform the amorphous carbon to crystalline graphite, some o. the silica reacts with carbon forming silicon carbide according to the following equation:
- Si02 + 2C÷ SiC + C02.
The commercial grade preferably used in this invention contains approximately 50% to 60% graphite and 20-25% silicon carbide with the remainder a mixture of silicon dioxide and other metallic oxides.
In carrying cut the invention, an effective amount of the composition consisting principally of graphite and silicon carbide, is added to the blend of coals used in making foundry coke. The addition is preferably from 0.2 to 2.5% by weight silicon and from 0.5 to 6% graphite based on the amount of coal or blend of coals. The mix is pulverized and coked in a by-product coke oven (see: Making Efficient Use of Coke in the Cupola, American Coke and Coal Chemicals Institute, Washington, D.C.).
The resulting coke has superior physical and chemical properties. Its superior hot strength gives improved operation in the cupola; aids in maintaining the physical integrity of the coke in the cupola, avoiding breakdown into smaller particles and consequent plugging which increases the back pressure of the air draft necessary to maintain smooth operation of the cupola. This in turn contributes to operation with less fuel and consequent savings.
The silicon carbide decomposes in the hot metal, releasing exothermic heat and lowering the overall coke combustion.
When the silicon carbide is blended into the coal mix, preferably in combination with graphite powder, and consequently pulverized and coked, it is dispersed much more uniformly and homogeneously within the coke particles and is more uniformly and readily available to the liquid iron at the coke-iron interface. This availability aids in promoting the reactions of decomposition of the silicon carbide and its reactions with the iron.
The availability of the silicon carbide in the coke also aids in simplifying the operation of the cupola in lessening the need for additional inoculants, reducing labor needed and the possibility of weighing and adding errors.
The graphite, and silicon from the silicon carbide, act as inoculants for deposition of graphite in the desired pearlitic matrix on cooling and hardening of the metal when cast, thus controlling the grain structure, hardness, strength and machinability of the cast metal, enabling the founder to produce thinner cross-section castings economically and profitably.
In a preferred procedure, from 1-10% of a commercial grade of impure silicon carbide containing graphitic carbon is added to the mix of coking coals in a physical blend, the mix pulverized and coked in a conventional by-product coke oven.
The coke produced in the above fashion is then used as a replacement for the regular metallurgical coke in a gray iron foundry cupola.
The invention will be further described with reference to the following specific Examples.
To 95 parts by weight of a mixture of coking and non-coking coals 5 parts of commercial silicon carbide was added.
The silicon carbide used had the following approximate analysis:
This mixture was blended, pulverized, loaded into a by-product coke oven and coked during a 26-1/2 hour cycle. The coke produced had the following analysis by various samples:
This coke was used in a gray iron cupola in a jobbing foundry with a daily melt of approximately 70 tons of gray iron, with the following results reported:
- 1 - Approximately 5-10% less coke was required for melting.
- 2 - Silicon gain in the metal was approximately 0.10% at a 6 to 1 coke ratio (wt. iron to coke).
- 3 - Back pressure in the cupola was reported to be less variable than in the past.
- 4 - Carbon pickup in the iron increased considerably at normal coking levels.
- 5 - Melting rates and metal temperature were equal to or slightly higher than with regular coke.
Nos. 3, 4 and 5 above were qualitative determinations only and were not quantitatively determined.
The reduction in back pressure was the result of a higher hot strength by the coke, which maintained its physical integrity while burning, and for that reason offered less resistance to the air flow.
The fact that the melt rate and metal temperature were equal to or slightly higher than with regular coke verified that the silicon carbide reacted in the melt, releasing heat. The reactions are:
Ten carloads of coke were made as in Example 1 with 5% of the same type silicon carbide in the blend. The coke produced had a composite analysis as follows:
The above coke was used in a four day run in a 90" diameter, water-walled, refractoryless front slagging cupola with water cooled projecting tuyeres, and a carbon lined wall. Typical operating data for this cupola during this run was:
- 1 - Bed height - 60" above centerline of tuyeres. Bed coke weight - 9,000 lbs. Limestone - 500 lbs.
- 2 - Stack holding capacity - 10-12 x 6,000 lb. charges.
- 3 - Typical cupola charges:
The iron produced with the coke containing SiC had the following analysis as compared to iron produced with regular coke:
- Typical Metal Composition:
In this test, there was an overall reduction in coke use of 6.2%. The running coke charge, not including booster charges or bed coke was reduced from 700 lbs. to 650 lbs. or 7.1%.
These reductions in charged coke did not reduce carbon gain or pickup by the iron.
Silicon melting loss or oxidation loss was reduced 18.9%. Silicon pickup in the iron was 0.07%.
There was a reduction in hardness and in chill depth apparent in this test, indicating the effectiveness of the graphite and silicon carbide as inoculants.
From the above data, it can readily be observed that the use of this coke results in improvement of operation of a cupola by lowering the consumption of coke needed to melt the iron, or conversely, increasing the production rate, and lessening the amount of the expensive ferrosilicon alloy needed.
Back pressure in the above run was also reduced and more uniform than in previous runs, indicating that this coke broke down less in the cupola and had higher hot strength than regular coke.