The Shop > Metal Stuff
Oil fired crucible furnace
vtsteam:
hmmmm, just recalled that powdered charcoal probably won't work as a gas producing facing the way plumbago does, because it actually needs the impurities of coal to produce the momentary gassing that gives it the characteristic smooth finish. Charcoal is too pure carbon. I think I recall that from some conversation here -- somewhere -- maybe with ironman.....
But charcoal still might work in a steel conversion to cast iron experiment where pure carbon is wanted. And for plumbago substitution, I wonder if store bought charcoal briquets, powdered are impure enough to make the gas layer. Well, the only way to find out is to experiment! :zap: :zap: :zap:
awemawson:
I understand that finely powdered bituminous coal was added to the greensand in iron foundries to create this gas boundary layer to achieve a smooth casting.
vtsteam:
Yes, Andrew, "seacoal".
I actually have some coal that I could powder, but it's anthracite, the common coal here. I don't think it will do the same thing as bituminous coal. It will not work in a cupola either, Or I should say, it has been used historically only when it was discovered that it absolutely required a heated blast to work, unlike coke. At one time I wanted to experiment to see if a hot blast would work with anthracite on my "sawed off" cupola (now the re-configured oil furnace) but never did. Charcoal was tried, but found to be not energy dense enough on the very small scale of my short and narrow cupola.
awemawson:
I thought that 'Seacoal' was just washed up coal found on the beach originating where eroding cliffs have an exposed coal seam. So in North East England it is found in significant quantities and used to be 'gleaned' to fuel cottage hearths.
So presumably if this is the case 'seacoal' can be any sort of coal, bituminous or not :scratch:
(Another source of seacoal was from the myriad of colliers that plied up and down the East coast of England for centuries with coal from the Newcastle coal fields to London - any that went overboad due to accident or sinking could be washed up on the beach.)
(Hence the expression 'taking coals to Newcastle' ie a pointless action)
vtsteam:
Well, Andrew here's a lengthy reference from from American Foundryman, (January- February 1951), which we can take with a grain of seacoal, as usual re. foundry opinions and beliefs. Personally I wouldn't be surprised if seacoal got it's name from the floaties you mention, being an obvious relationship. Unless of course we really do have good old "Dud" Dudley to thank for invention of the name :)
--- Quote ---SEACOAL
History
The use of "coal dust" in molding sand originated in England. It was later introduced to the North American foundrymen as "seacoal." One particular English or Welsh coal seam appeared to be the most satisfactory for foundry purposes and became the standard with which other coals were compared. Some authors believe that since the coal seam was near the Welsh coast and the mines were deep under the ground, extending under the sea, the term "seacoal" emphasized both its origin and its sea transportation to the U.S.A. However, the author has pursued the term "seacoal" for many years and believes that it originated from Lord Dudley's nomenclature in his patent applications in 1620. Lord Dudley applied for a patent in behalf of his son, "Dud" Dudley, for "melting oron ewre (ore) and of making the same into caste workes or barrs with "SEA-COLES" or "pit-coles" in furnaces with bellowes. " These "sea-coles" were described as being low in ash, sulphur and impurities for making the metal. The foundryman thought the same coal should be used in making sand molds. After a long and tiring search by the author, an old English file produced this answer after many years of investigation.
Characteristics
Seacoal (coal dust) is a highly volatile bituminous coal, ground to various degrees of fineness by pulverizing mills which is then graded by screening or by air classification methods. It is mixed and mulled with molding sand in various proportions. When it, or the products of its destructive distillation come into direct contact with molten iron, seacoal seems to improve the casting finish. Where molds are cast which contain seacoal, the sand peels more freely from the castings and the cleaning of the castings are more easily accomplished. Seacoal is one of the least expensive materials used in molding sand to give these results. Seacoal has been termed a "facing" in the foundry as a result of this property.
Effect
The effect of seacoal is so well known that it is accepted as a requirement in the pr3oduction of gray iron, ductile (nodular or S.G.) iron and malleable castings. The many advantages it offers have perpetuated its foundry use.
Factors
Several principal factors must be considered in the selection of seacoal (coal dust). The Volatile Combustible Matter, Fixed Carbon, Ash Content, Sulphur, and Moisture Content of the coal source measures its quality. Cost and reliability of the producer are the most important factors when purchasing the coal. Other factors to be considered in the selection of a specific grade of seacoal are: 1) The grain fineness of the base sand, coupled with its sand grain distribution. 2) Type and amount of bonding material used. 3) Permeability of the molding sand desired. 4) Weight of the casting to be produced. 5) Desired surface finish of the casting. 6) System of gating the pattern. 7) Metal pouring temperature and length of pouring time. 8) Density of the rammed sand mass around the casting. 9) Length of time the molds stand after pouring. 10) Type and temperature of the metal poured.
There are many additional variables to be considered, as seacoal affects most of the properties of the molding sand. Investigators claim that seacoal principally acts as a reducing agent, thus it prevents sand from adhering, or burning-on to the casting. Others dispute this claim.
Functions of Seacoal as a Sand Additive
Coal and coke in the molding sand burn, thus consuming the oxygen in the mold cavity to provide a reducing atmosphere as the mold is initially poured. Heat is absorbed by the mold from the molten metal. The temperature is increased to the "coking" temperature range of the seacoal present. During "coking," uncondensable reducing gases are produced, such as, hydrogen, methane, ethane, tars, light oils, and others which are distilled-off. The tar fraction plus the light oils burn and then contribute a carbon film or "sooting" action on the mold as a "bonus" factor, acting as an inhibiting agent. The volatile matter in seacoal consumes some of the oxygen immediately in the mold cavity as the metal ignites the seacoal upon entering the mold. This sooting prevents cutting action of the flowing metal against the molding sand and inhibits fusion of the silica sand grains. The gas content from the seacoal tends to form a cushion at the surface of the mold-metal interface, and the metal lies more evenly and quietly. Metal penetration is reduced by the formation of the gas cushion which is aided by the coal's coking action. The gas pressure fills all the mold's voids between the sand grains giving the desired smooth casting surface but still permitting the venting of noxious gases. A certain amount of mold gas pressure is desired to help prevent metal penetration.
Sizing of The Seacoal
It is believed that seacoal should approximate the same size as the base sand grain in order to not alter the permeability of the molding sand, but this is only an opinion. Seacoal on losing its volatile matter becomes coke, a fixed carbon. Some coals on coking swell to nearly three times their original volume, which may be detrimental to the surface of the castings. The coking theory has not been fully defined or proven, but the author believes it is a functioning part of the seacoal in helping to overcome apparent metal shrinkage. Some foundrymen claim that fines (those less than the U.S. No. 200 Standard Sieve) should be removed from commercial seacoal. It is the author's opinion that to narrow the range of particle sizes might exhibit a lag in the formation of the seacoal's gas cushion which it generates on heating. The reducing atmosphere which seacoal develops should form instantaneously, and removal of the fines may act as a disadvantage in certain cases. To cite an example, one cubic foot of coal in the solid state burns rather slowly when thrown into a flame; whereas, if it is ground to fine particle sizes, the coal would instantaneously ignite, if subjected to the flame combustion. Time is a factor to be considered. Foundries find it detrimental to remove too many fines from foundry sands, as metal penetration and rough casting surfaces can result. Since sand surfaces in the drag side of the casting is subjected to metal weight, the metal tends to force easier into the interstices of the sand grains, as the sand voids increase in size. Washing and cutting of the metal may also result around the gates from too-open sands as they are more brittle. Open sands are also difficult to patch, or work. A carbon film is highly desirable in most cases to improve the finish of the castings.
Amounts Added
Too much seacoal increases the temper water demand due to increased surface area of the mixture. This causes rougher castings. Excess seacoal creates an evolution of gas which may cause blows, or porosity in the castings. Molding sand which is too rich in seacoal may promote defects called, "Map of Ireland," "Fins" or "Veining." Seacoal in moderate amounts is very beneficial, but as with the case of any other raw material used in the foundry, too much is detrimental. Seacoal depreciates hot compression strength at 1250°F. and 2000°F. (1010° C. and 1093° C.) rather effectively.
Grades
Seacoal is sold by grade designations namely: A, B, C, D, D-½. A and B grades are coarser and are used for molding heavier castings, C to D-½ grades are recommended for lighter to medium castings and for giving more detailed surface finish. Foundry facing suppliers have attempted to standardize on the best three commercial grades, namely: B, C, or D grades. "Dustless" seacoal (treated) and standard non-treated grades are available through most foundry supply houses. "Dustless" seacoal is a ground coal which has received a secondary treatment of oil or waxes to minimize dust when it is handled in the foundry.
Analyses and Screen Tests of Seacoal
Seacoal Analysis
The analyses and screen tests (Table No. 25) are approximate. They may vary at least 5% on each sieve depending upon the suppliers choice of sieves, and arrangement of sieving with the customer.
The Properties of Seacoal
Properties of Seacoal
Summarizing
SEACOAL AFFECTS THE FOLLOWING PROPERTIES:
Green Compression Strength-Increases (generally, as long as the temper water doesn't increase).
Dry Compression Strength-Increases, as temper water increases. (6% by weight of seacoal increases dry compression strength approximately 35%, as temper water is also increased to give workability-moldability.)
Hot Compression Strength-Decreases, as seacoal furnishes a reducing mold cavity atmosphere.
Permeability-Generally decreases, due to the high "fines" content of the commercial grades of seacoal.
Flowability-Decreases, as the water demand increases in the molding mixture.
Temper Water Required-Increases by 10% of the weight of seacoal contained in the mixture. (Most of this water is held on the seacoal's surface, instead of being absorbed as in bonding agents. However, seacoal which is coked or formed into an ash sometimes acts as a sponge and absorbs an excessive amount of temper water. This build-up must be avoided by new clean sand additions, otherwise the molding sand becomes ashy and brittle.)
Mold Hardness-Increases, if the same effort or work force is applied. (Even though the mold hardness increases, the metal tends to lie more quietly in the mold when seacoal is present, than when it is not. Avoid high mold hardness on thin section castings which freeze rapidly.)
Deformation-Increases up to 6.5% by weight seacoal. The increase is dependent upon the seacoal's grain size. Temper water is generally increased. If the water does not increase slightly, the molding sands become brittle and break easily. Both wood flour and seacoal (coal dust) tend to make a smoother, softer and more moldable molding sand when used in regular amounts up to 4% to 5% seacoal, or 1% wood flour additions. Excessive amounts of seacoal and wood flour result in molding sands becoming low in resilience, they become brittle, they are difficult to handle and are poor in general practice.
Refractoriness and Sintering-Increases, which is possibly due to the carbon or carbon film developed during pouring of the metal.
Volume Changes in Molding Sand-Less expansion and contraction characteristics of the mold is beneficiated by the use of seacoal. Many of the common defects which are directly associated to the expansion and contraction of sand mixtures seem to disappear when seacoal is present. Seacoal is related to the clay content, moisture content, sand grain distribution and particle grain size. The beneficial limits of seacoal when added to the molding sand tend to vary under different working conditions.
Mold-Wall Movement-Effect of: ( 1) It is found that variations in mold materials influence metal solidification. Seacoal minimizes mold-wall movement. (2) As carbonaceous materials such as seacoal and wood flour are gradually increased in a molding mixture, within a definite limit, the piping of an iron riser of the casting decreases. (3) In comparison to western bentonite or fire clay, southern bentonite appears to lessen mold wall movement, but seacoal additions improve all three bond mixtures. (4) In experiments, wood flour decreases the piping tendency of the metal, which is further aided by seacoal additions. (5) As the temper water of the molding sand mixtures increases, the piping tendency of the metal increases, but seacoal additions help to overcome excessive additions of temper water. (6) It appears that a dense, hard rammed mold results in less movement of the mold-metal interface and less seacoal is required. (7) Oil bonded core sands produce sound castings, but so does seacoal when added to green sand mixtures. Metal exudes slightly from the riser instead of piping when there is a lack of mold-wall movement. Sufficiently rammed molds in green sand act similarly. (8) Mixtures of molding sands and bonding agents are very complex. It is illogical to make definite comparisons between different mixtures. Each base mixture should be considered on an individual basis, but 5% by weight of seacoal in gray iron, ductile (nodular or S. G.) iron and/ or malleable iron castings helps to hold castings closer to pattern size and to have lesser mold-wall movement."
--- End quote ---
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version