What are the types of metallurgical iron? Information about iron types, their properties. What is cast iron?
TYPES OF METALLURGICAL IRON
Pig iron, which is the product of the blast furnace, is an impure form of iron containing iron, 3% to 4% carbon, 2% to 3% silicon, 0.5% to 2.0% manganese, and about 0.04% of both sulfur and phosphorus. The name “pig iron” originated in the early days of iron ore reduction when the total output of the blast furnace was sand-cast into “pigs”—a mass of iron roughly resembling a reclining pig. Now most iron is received from the blast furnace and transported in insulated railroad tank cars directly to refining furnaces, such as the open hearth furnace and, to a declining extent, the Bessemer converter, for purifying into steel.
The pig iron that is scheduled for foundry use in the cast iron industry is cast into pigs that are roughly 50.8 X 22.8 X 10.1 cm (20 x 9 X 4 inches) in size. The oldest method of pig-casting is done in sand beds that consist of a main runner called a “sow,” connected by smaller runners to smaller depressions called “pigs.” The sand-casting of pig iron has been largely superseded by pig-casting machines, which utilize a conveyor belt carrying sheet molds beneath the spout of the blast furnace or pouring ladle. Machine-cast pigs are much cleaner than sand-cast pigs and have no adhering sand to contaminate later remelt processes.
Pig iron is classified by chemical composition into three grades. Basic pig iron that is used for steelmaking is low in silicon (1.5% maximum) to prevent attack of the refractory linings of refining furnaces and to control slag formation. Basic pig iron must be low in sulfur (0.04%) since sulfur is an active impurity in steel and is not eliminated in the refining furnaces. Phosphorus normally is held to less than 1 % and manganese to a range of 1 to 2%. Carbon content varies from 3.5 to 4.4%.
Foundry pig iron includes all the types that are used for the production of iron castings. Containing from 0.5% to 3.5% silicon, up to 0.05% sulfur, 0.035% to 0.9% phosphorus, 0.4% to 1.25% manganese, and 3.0% to 4.5% carbon, the specific composition of each type of cast iron will be discussed in the sections describing the several cast irons.
Ferroalloys are alloys of pig iron, each rich in one specific element; they are used as additives in the steel and iron industries. For example, ferromanganese is pig iron containing from 74% to 82% manganese ore. Ferrosilicon, with a silicon content of 5% to 17%, and ferrophosphorus, with 15% to 24% phosphorus, are also used as additives to control or alter the properties of iron and steel.
Sponge iron is used chiefly as a melting base for high-purity electric furnace alloy steels, as an absorber in certain chemical processes, and as a source of high purity iron in the electronics industry. It is produced by the Krupp-Renn process in which solid ore is reduced to solid iron. Sponge iron is relatively impure, but due to its method of manufacture most of the impurities, except sulfur, are not reduced and are compounded with the slag rather than the iron, as in the case of blast furnace iron. The melting of sponge iron releases many of the impurities in the form of slag, leaving a relatively pure form of iron, running as high as 99.93% pure.
Hydrogen reduction of iron oxide is an extremely complex process involving actual reduction, vacuum melting, and further purification; it produces iron that is similar to sponge iron but with total metallic impurities as low as 0.001%. Hydrogen-reduced iron, because of the high cost and low yield, is used exclusively in research activities.
Carbonyl iron is used for preparing magnetic fluids, for manufacture of high-frequency cores in the electronics industry, and as a research material.
Iron carbonyl, usually in the form of iron pentacarbonyl, Fe(CO)5, yields exceptionally pure iron when thermally decomposed into iron and carbon monoxide. It is prepared by reacting finely divided hydrogen-reduced iron with carbon monoxide in the presence of a catalyst, such as ammonia, under pressure, and heated. The material that is formed is volatile, and it decomposes at 200°C.
Electrolytic iron, another high-purity form of iron, is produced by the electrolytic deposition of iron from compounds such as ferric chloride. Depending on the type of electrodes used, the current density, and the electrolyte itself, iron of purity in excess of 99.9% is produced.
Ingot iron was developed in about 1905 and is considered to be the purest form of iron produced on a tonnage scale. It is used chiefly in sheet form that is usually galvanized or enameled; it is also used in applications requiring great ductility and corrosion-resistance. Culvert tubes, guttering, and refrigerator and other appliance shells account for a large percentage of ingot iron production.
Actually, ingot iron is technically not iron but very low (approximately 0.012%) carbon steel made by the open-hearth process. Ingot iron differs from open-hearth steel of the same carbon content in that the processing is much more rigidly controlled and purification is continued, in terms of both time and component reactions, to a much greater extent than for steel. The resulting iron has a total content of the usual impurities—carbon, manganese, silicon, sulfur, and phosphorus—of less than 0.06 %.
Wrought iron is the oldest form of iron known; it is mentioned in the Bible, and wrought iron implements 5,000 years old have been found in one of the pyramids of Egypt. It is used mainly in the manufacture of pipe, in heavy industries, such as railroad and shipbuilding, and in the automotive and farm implement industries.
Wrought iron consists of high-purity iron with finely divided and dispersed fibers of iron silicate slag as a physical mixture; it has been produced by a wide variety of processes. The earliest wrought iron was made by heating iron ore, limestone, and charcoal until the iron ore was reduced and the resultant pasty mass could be worked, or wrought, into the desired shape with slag as a part of the structure. Various refinements of the direct processing of wrought iron from iron ore were used until the blast furnace evolved from the early attempts to actually melt the iron by adding stacks to the primitive furnaces.
The indirect method of producing wrought iron from remelted pig iron introduced the puddling process, in which pig iron was melted in a hearth-type furnace using coal as the fuel. As the pig iron melted, the hot gases from the burning fuel oxidized carbon from the iron, which had an initial carbon content of 3.5% to 4%. Silicon from the pig iron also was oxidized, and the resultant slag formed over the melting pigs. As melting continued to completion, roll scale (magnetic iron oxide) was added to oxidize most of the carbon, silicon, sulfur, phosphorus, and manganese from the fluid iron; the oxides then became part of the slag.
During the oxidation of the impurities, the molten iron was agitated manually by an operator using a long steel rod. As purification continued, the melting point of the iron climbed until the operator, or puddler, was no longer able to agitate the pasty iron interspersed with fluid slag. At that point, the spongelike ball of iron-slag mixture was removed from the furnace, the excess slag squeezed out, and the ball rolled into slabs. In an attempt to ensure uniformity, the slabs were sheared into short lengths, piled, and reheated to welding temperature, and then rerolled into the desired shapes.
Wrought iron, in lots weighing approximately 540 kg (1,200 pounds), continued to be produced by variations of the puddling processes, both manual and mechanical, until about 1930, when the Aston-Byers process was developed. This process, which is currently in use, utilizes a somewhat different approach by separating the steps used in hand or mechanical puddling. Pig iron is melted, desulfurized, and then refined in a Bessemer converter to nearly pure iron. The molten iron, at about 1510°C, is then poured at a controlled rate into an oscillating ladle containing an iron silicate liquid slag; the slag is held at a temperature of about 1320°C. As the hotter metal contacts the slag, it releases any dissolved gases, shatters into fragments due to the sudden solidification, and settles to the bottom of the ladle where it forms a spongelike ball of iron interspersed throughout with the slag, which is still in liquid form. These balls, weighing from 3 to 10 tons, are then squeezed and rolled as in the earlier processes.
The composition of a typical wrought iron includes 0.08% carbon, 0.03% manganese, 0.18% silicon, 0.11% phosphorus, 0.015% sulfur, and about 2.85% slag by weight. In a good wrought iron sample, there should be at least 250,000 slag fibers per square inch. Due to its structure, wrought iron possesses excellent fatigue and corrosion resistance because of the chemically inert and discontinuous slag content.
Cast iron is a general term that describes a series of iron-carbon-silicon alloys, which are produced by pouring the molten alloy into molds. In contrast with steel, in which carbon is chemically combined with the iron, cast irons contain more carbon than can be retained in solution. By varying the carbon and silicon content of the alloy, several types of cast iron are produced, each with distinctive properties and uses. The production of a cast iron object, regardless of the type of iron, involves several steps. See also Casting.
A replica, or pattern of the object to be cast is made, using wood or metal and sizing the pattern to allow for shrinking of the molten cast iron as it solidifies. The mold is formed by packing a moist sand-clay mixture around the pattern in a suitable container, or flask. The pattern is removed from the mold, and a molten iron alloy is poured into the cavity created by the pattern. Heat treatment of the raw casting to alter its properties may be necessary, depending on the type of structure required.
The casting industry, one of the earliest of the basic industries, owes its success and longevity to several factors. There is almost no limit to the intricacy of items that can be cast. The process can be adjusted to any production rate from a single, nonrepeating order to a fully automated and computerized scale. The industry is extremely versatile, producing castings weighing from a few ounces to hundreds of tons. By adjusting the chemistry of the alloys, castings can be made that vary from glass hard and brittle to soft and ductile.
Compared to other forming methods, such as forging and machining, the casting industry usually enjoys a favorable position because of the low cost of raw materials and lower capital investment. Because of the higher carbon and silicon content, cast irons begin to melt at 1130°C, and most grades are completely molten at about 1300°C. For this reason, refractory linings of the furnaces have a much longer life. For the same reason, cast iron has much greater fluidity than molten pure iron, resulting in greater ease of pouring complicated shapes.
Cast irons are normally produced by melting pig iron with selected scrap iron in the cupola furnace; coke, with limestone and other fluxes, is used for fuel. The cupola furnace is basically the same in design as the blast furnace, but it is about one fourth as large. By controlling the chemistry of the molten iron—chiefly the silicon and carbon contents—several types of cast irons are possible.
Gray Cast Iron.
Gray cast iron accounts for the greatest percentage of cast iron manufactured in the United States. Actually, gray cast iron is not one alloy but a series of alloys whose composition varies within fairly broad limits; all alloys of the series possess similar mechanical properties. A typical gray cast iron may contain 3.2% carbon, 2.2% silicon, 0.65% manganese, 0.15% phosphorus, and 0.10% sulfur. As the molten alloy cools to about 1250° C, grains begin to form from the liquid. These grains are a solid solution of iron and carbon that is called austenite. As the temperature continues to fall, more austenite forms; simultaneously, carbon, in the form of graphite flakes, precipitates from the liquid alloy.
Freezing continues until the alloy is completely solid at about 1130°C. This temperature is referred to as the eutectic isotherm, since the final portion of liquid solidifies with no appreciable drop in temperature. Below 1130°C, if the cooling rate has been very slow, the alloy consists of austenite grains intermixed with graphite flakes. As the slow cooling continues, carbon, in the form of graphite, is rejected from the austenite. At 723°C, the austenite, which is unstable at this temperature, transforms isothermally into graphite and ferrite, provided the alloy is cooled extremely slowly. No further changes take place as the alloy is cooled to room temperature.
The final structure is called ferritic gray cast iron; this is the softest type of gray cast iron and has a yield strength of 15,000 to 25,000 psi and practically no ductility because of the discontinuous structure caused by the presence of the graphite flakes. The formation of the graphite flakes as the alloy solidified was due to the presence of the silicon and the relatively high carbon content, which resulted in more carbon than could be retained in solution with the iron.
If the same alloy is remelted and cooled at a slightly faster rate, the resultant structure will differ from that of the ferritic gray cast iron. As the alloy is cooled through 723°C, the austenite will not have sufficient time to completely decompose into ferrite and graphite. Instead, it will transform into a mechanical mixture of iron and iron carbide, FesC. This reaction is termed the eutectoid reaction and the mixture is called pearlitic gray cast iron. This mixture is even less ductile than ferritic gray cast iron since the pearlitic structure is harder than ferrite, and flake graphite is present as in the ferritic type. The tensile strength ranges from 20,000 to 40,000 psi.
Gray cast iron is easily machined, and due to its graphite content, it is considered to be a relatively “dead” material—that is, it absorbs vibrations; it is therefore widely used in the manufacture of machine tools, automobile engine blocks, and a great variety of items requiring an alloy with rigidity, good compressive strength, and ease of casting and machining.
Chilled Cast Iron.
Chilled cast iron is formed by the rapid cooling of the newly solidified gray iron casting. The rapid cooling does not allow enough time for the various transformations to occur, and the end product is completely structured as iron carbide. Chilled cast iron, which is brittle and as hard as glass, is used for abrasive applications, such as the outer surface on some types of railroad car wheels, brake shoes, rock crushers, and similar applications.
White Cast Iron.
White cast iron is produced by lowering both the carbon and silicon contents of the alloy. With less carbon to form graphite and less silicon to act as the graphite promoter, the final constituent consists entirely of iron carbide after cooling. A typical white cast iron composition is 2.5% carbon, 1.0% silicon, 4.5% manganese, 0.18% phosphorus, and 0.16% sulfur. Metallographic examination of the white cast iron reveals no graphite and varying but low amounts of pearlite, with the predominant constituent showing as iron carbide. White cast iron, as in the case of chilled cast iron, is used for abrasive applications, but its chief use is transformation into malleable cast iron.
Malleable Cast Iron.
Malleable cast iron is soft, ductile, as machineable as low-carbon steel, and has a tensile strength ranging from 60,000 to 100,000 psi. It is used in the automotive and railroad industries; it is also used in pipe fittings, electric motor housings, hardware, machine tools, hand tools, and many other items requiring easy formability, strength, and moderate ductility.
Malleable cast iron is derived from white cast iron by heat-treating processes. The production of malleable iron begins with the pouring of white-iron castings into sand molds. The composition of the white iron must be such that no free graphite forms during the solidification of the casting. After cooling, the hard, brittle white-iron castings are removed from the molds, cleaned, and packed into suitable furnaces for heat treatment. Depending on which of several malleabilizing cycles is used, the total cycle time may vary from 25 to 100 hours. Air furnaces or controlled atmosphere furnaces are normally used.
Entering the furnace as iron carbide, the white-iron castings are heated to approximately 925°C and held at that temperature for 10 to 20 hours, again depending upon which cycle is used. At 925°C, the iron carbide of the white iron slowly transforms into austenite (a solid solution of iron and carbon) and graphite by diffusion of the carbon. The major difference between malleable cast iron and gray cast iron lies in the time of formation and the shape of the graphite.
In gray iron, the graphite precipitated out rapidly into flake form upon solidification, resulting in the brittle type of iron. During the heating period of malleabilizing, the carbon atoms diffuse from the iron carbide and agglomerate into randomly spaced “rosettes” of graphite—often referred to as temper carbon. The temperature is then lowered to just above the temperature (723°C) where the austenite will transform into pearlite. The temperature of the cooling cycle is usually lowered to about 760°C, and the final cooling to below 723°C, is regulated so that 10 to 20 hours are required.
The slow cooling through the pearlite-forming temperature results in the decomposition of the austenite into ferrite and graphite, rather than pearlite. The final product is called ferritic malleable cast iron and has a tensile strength of about 55,000 psi and ductility measured as 15% elongation. Because of the roughly spheroidal shape of the graphite, as opposed to the flake shape in gray cast iron, the graphite does not act as stress risers (a stress riser is anything that causes a concentration of stresses in a small area, such as a scratch on a glass plate) as do the graphite flakes, resulting in the much greater ductility with no sacrifice in strength.
If a similar white-iron casting is carried through the same heating cycle as the one resulting in the ferritic structure, but permitted to cool more rapidly to below 723°C (with no arrest at 760°C), not enough time is available for the decomposition of the austenite into ferrite and graphite. Instead it transforms into pearlite. The final product in this case is pearlite with the rosette graphite, and it is called pearlite malleable cast iron. Since pearlite is a somewhat harder constituent than ferrite, the ductility is about one third that of the ferritic iron, while the strength ranges from 75,000 to 100,000 psi.
Nodular Cast Iron.
The most recently developed type of cast iron, which has been used in rapidly increasing amounts since about 1940, is nodular, or ductile, cast iron. It is slightly higher in both carbon and silicon and lower in sulfur than gray cast iron; the molten alloy solidifies directly into a malleable type casting with no necessity for heat treatment. Nodular cast iron is used in many of the same areas as malleable cast iron, but it probably will not supplant the conventional malleable iron for technical and economical reasons.
To produce a nodular iron casting, the mold is prepared in the same manner as for gray or malleable iron castings, and the alloy is melted in a cupola or other type of melting furnace. Immediately before casting the molten metal into the sand mold, the metal is inoculated with cerium or magnesium in alloy form. After pouring, as the casting nears the solidification temperature, graphite begins to precipitate from the liquids; but instead of forming into the usual flakes, it nucleates and grows into small nodules, which are more nearly spheroidal in shape than the rosette graphite in malleable cast iron. It is generally believed that the addition of the cerium or magnesium alloy to the molten metal causes the formation of insoluble oxide particles in the liquid, thus providing nuclei for the formation of the graphite nodules.
Nodular cast iron averages about 80,000 psi in tensile strength and 3% elongation in the as cast condition. If it is annealed, ductility is tripled, but the tensile strength is lowered to approximately 60,000 psi. Both gray and nodular cast irons can be quenched and tempered to increase their tensile strength. All types of east iron occur in alloy forms to enhance or suppress certain properties.