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This article will take an in-depth look at iron castings.
The article will bring more detail on topics such as:
This chapter will discuss what iron castings are, iron castings production, and casting processes.
An iron casting is a hard product obtained from combining iron with carbon. This can be readily cast in a mold, and it contains a higher proportion of carbon compared to steel. The proportion typically ranges from 2 to 4.3 percent.
Cast iron also contains varying amounts of silicon, manganese, and some traces of impurities, which are sulfur and phosphorus. Iron ore is reduced in a blast furnace, leading to the formation of cast iron. The liquid iron is cast, poured, then hardened into ingots of crude which are referred to as pigs. These pigs are subsequently re-melted along with scrap and other alloying elements in furnaces of cupola, then recast into molds to produce a variety of products.
Cast iron is made from pig iron and is the product of melting iron ore in a blast furnace. In other ways, cast iron can be derived directly from the molten pig iron or by the process of re-melting pig iron. This process is done often along with other substantial quantities of iron, steel, limestone, and carbon which is coke. Various steps then taken to remove undesirable contaminants which will be contained.
Contaminants such as sulfur and phosphorus can be burnt out from the molten iron, but this has a disadvantage as it can also burn out the required carbon which must be replaced. Carbon and silicon contents are adjusted depending on the application of the end product to the desired levels. These levels may be anywhere between 2 percent to 3.5 percent and 1 percent to 3 percent respectively. Other desired elements must be added to the melt before the final product is produced by casting.
This process of casting refers to when a liquid metal is delivered into a mold, usually by a crucible in most cases, that contains a negative compression of the intended shape. The metal is poured into the mold through a spree, which is a hollow channel. The metal and the mold are then cooled and the metal part which is the casting is then extracted.
Casting is most often used for making shapes that would be difficult in nature or uneconomical to make using other methods of production. In this casting venture, traditional casting techniques include lost wax casting. This type may further be divided into centrifugal casting and vacuum assist direct pour casting, there is also platter mold casting and sand casting included.
Cast iron is sometimes melted in a special type of blast furnace, and is known as the cupola. Cast iron is often melted in electric induction furnaces or electric arc furnaces. When the melting process is complete, the molten cast iron is then poured into a holding furnace or ladle.
Iron casting is hard which allows for the materials to have resistance to abrasion and indention. Toughness is included in the iron castings, as they can absorb energy. The iron castings have an ability to return to their original dimensions after they have been deformed, and thus they have elasticity as their mechanical property. In relation to elasticity, the iron castings are ductile in nature, as they can deform and not fracture.
Malleability is experienced in the characteristic properties, as this allows the component to undergo compression and avoid rupturing. Tensile strength and fatigue strength are some of the mechanical properties of iron casting. Tensile strength refers to the greatest longitudinal stress a material can bear without tearing apart whereas fatigue strength refers to the highest stress that a material, in this case, an iron casting can withstand for a given number of cycles without it breaking.
The two main types of casting are expendable and non-expendable casting. This is then further divided down by the mold material which may be sand or metal and the pouring method used such as gravity, vacuum, and low pressure.
Expendable mold casting is a classification which is generic in nature and this type of class includes sand, plastic, plaster, shell, and investment moldings. This method uses temporary molds, which are not reusable.
In sand casting, there is an allowance for smaller batches at a reasonable cost than permanent mold casting. Not only does this method allow one to create products at a low cost, but it also works in small size operations. Casting can be small enough to fit in the palm of a hand.
Sand casting typically allows most metals to be cast, though this depends on the type of sand used for the molds. For production at a high output rate, casting requires a lead time of days or even weeks. This production rate can be 1 to 20 pieces/hr-mold. Green sand contains a black color which has almost no part weight limit, whereas dry sand has a practical mass limit. The sand is bonded together using clay, chemical binders, or oils that are polymerized, such as motor oil. Sand is advantageous as it can be recycled many times and requires little maintenance.
Loam molding has been used to produce objects that are large and symmetrical, such as cannons and church bells. Loam is a product of the mixture of clay and sand with straw or dung. The product model is formed in a friable material, which is the chemise. A mold is then formed around the chemise by covering it with loam and then fried or baked and followed by the removal of the chemise. The mold is then held upright in a pit in front of the furnace so that the molten metal can be poured. Upon completion, the mold is broken off.
Plaster mold casting has a similar nature to sand casting, except that plaster is used instead of sand as a mold material. This form usually takes less than a week to prepare and has a production rate of 1 to 10 units/hr-mold, with items as heavy as 45 kg and as small as 30g having a good surface finish and close tolerances. Plaster casting is cheaper than other molding processes for complex parts, since plaster is cheap and can produce near net shape castings. Its major disadvantage is that it can only be used with low melting point non-ferrous materials such as copper, zinc, and magnesium.
Shell molding is also similar to casting, though the molding cavity is formed by a hardened shell of sand instead of a flask filled with sand. The sand in this process is finer than the sand used in sand casting, and it is mixed with a resin so that it can be heated by the pattern and hardened into a shell. Because of the resin and the finer sand, the end product will have a finer surface finish. This process is ideal for complex items which have a small to medium size, and are more precise than sand casting.
Investment casting is also known as lost wax casting, and it is best for ensuring the production of high quality components with the main benefits of accuracy, integrity, versatility, and repeatability. In this process, the pattern is surrounded by a refractory material. The wax patterns require extreme care, for they are not strong enough to take up forces which are encountered during the mold making. One of its advantages is that the wax contained in the investment casting can be used again and again.
Investment casting is suitable for repeatable production of net shape components from a combination of different metals and alloys of high performance. This process can be an expensive process as compared to other casting processes, and it is generally used for small castings. Products obtained from this casting process require a little to no rework, which is an added advantage.
Non-expendable mold casting is differentiated from the expendable type of casting because the mold doesn‘t need to be reformed after each production cycle. This process includes different methods which are permanent, die, centrifugal, and continuous casting. This type of casting produces an improvement in parts produced and delivers near net shape results.
Permanent mold casting is a metal casting process that uses reusable molds, which are usually made from metal. Gravity I generally used to fill in the mold, but in some cases gas pressure or a vacuum can also be used. Hollow castings are produced on a variation of the typical gravity casting process called slush casting. Casting metals that are common include aluminum, copper alloys, and others.
The die casting process forces molten metal under high pressure into mold cavities and these are machined into dies. Non-ferrous metals are used in the making of most die castings, specifically zinc, copper, and aluminum based alloys, although ferrous based alloys are also possible. This type of casting process is especially suited for uses where many small to medium-sized parts are needed with detail that is good and having a fine quality consisting of a right dimensionality.
Semi-solid metal casting is a modified type of die casting process that reduces or cancels out the residual porosity which is present in most die castings. The semi-solid process of metal casting uses a feed material of higher viscosity which is partially liquid and also partially solid, instead of using liquid metal as the feed material. To inject the semi-solid slurry into usable hardened steel dies, a modified dies casting machine is used. To ensure that the semi-solid metal fills the die in a turbulent manner, the high viscosity of the semi-solid metal and the use of controlled die filling conditions cater for it. This helps in eliminating high porosity.
Centrifugal casting is a process where molten metal is poured in the mold, and it is then allowed to solidify while the mold will be rotating. Into the center of the mold, metal is poured at the axis of rotation. The liquid metal is thrown out towards the periphery, all due to the help of the inertial force. This process is both gravity and pressure independent because it creates its own force-feed. This is done by the use of a temporary sand mold held in a spinning chamber. The time of lead varies with the application, but semi and true centrifugal processing permits 30 to 50 pieces/hr-mold to be produced. This contains a practical limit for batch processing of about 9000 kg in total mass with a limit per item of about 2.3 kg to 4.5 kg.
Continuous casting is a refined process of the casting work where there is a high production volume of metal sections with a constant cross-section. The molten metal is poured into a water cooled mold that is open on the ends. It allows a skin of solid metal to form over the still liquid center, thus gradually solidifying the metal from the outside to the inside. When the process of solidification is completed, the strand is drawn from the mold in a continuous manner.
Already determined lengths of the strand can be then cut off by using either mechanical shears or traveling oxyacetylene torches. This method of casting is used to lower costs which are linked with the continuous production of a standard product, and also due to the increased quality of the standard product. In the continuous casting process, cast sizes can range from a strip to billets and then to slabs, each having different dimensions.
There are many manufacturers of machines available to produce iron castings. Here are five brands of machines used to produce iron castings in the United States and Canada, along with a specific model and their unique abilities, features, or characteristics:
Unique Features: The Hunter HLM Series is a line of high-pressure molding machines that offer precise and repeatable mold quality. They utilize a combination of hydraulic and pneumatic technologies to achieve tight flask squeeze, uniform mold hardness, and excellent mold stability. These machines are known for their reliability, ease of use, and advanced control systems for efficient production.
Unique Features: The Sinto FBO Series is a range of flaskless molding machines that eliminate the need for flask and flask handling. They use a unique sand filling system, where sand is compacted and squeezed directly onto the pattern without the need for a flask. These machines offer high productivity, flexibility, and cost-effectiveness due to their ability to produce molds quickly and efficiently.
Unique Features: The DISAMATIC D3 is a vertical molding machine designed for high-volume production. It combines vertical parting with horizontal molding, ensuring superior accuracy, uniform density, and minimal mismatch. The DISAMATIC D3 offers fast cycle times, low operating costs, and excellent dimensional stability, making it suitable for large-scale foundries.
Unique Features: The Roberts Sinto FDNX Series is another line of flaskless molding machines that provide efficient and reliable molding solutions. These machines incorporate advanced technologies such as aeration sand filling and adjustable mold height to achieve excellent mold quality. The FDNX Series offers flexibility in mold size, high mold density, and reduced operating costs, making it suitable for various casting applications.
Unique Features: Buhler Prince offers a range of casting solutions, including horizontal and vertical high-pressure molding machines. These machines utilize hydraulic systems with precise control to achieve high mold hardness, consistent sand compaction, and accurate pattern reproduction. Buhler Prince machines are known for their robust construction, durability, and the ability to handle complex casting geometries.
Please note that specific models and their features may evolve over time, so it's always recommended to refer to the respective manufacturers' websites or contact them directly for the most up-to-date information on their products.
This chapter will discuss the various types of iron castings.
The characteristic of gray cast iron is the graphic microstructure, which is able to cause fractures to the material and have a gray appearance. This one is the most commonly used type of cast iron and also commonly used cast material based on weight. A majority of the gray cast irons have a chemical decomposition of 2.5 percent to 4 percent carbon, 1 percent to 3 percent silicone and the rest is a composition of iron.
This type of cast iron has less tensile strength and less resistance to shock compared to steel. Its compressive strength is comparable to low and medium carbon steel.
All these mechanical properties are controlled by the graphite flake‘s shape and the size of the graphite flakes, which are present in the microstructure of the gray cast iron.
This type of iron has fractured surfaces which are white due to the presence of an iron carbide precipitate named cementite. The carbon that is contained in white cast iron precipitates out of the melt as met stable phase cementite rather than as graphite. This is achieved with lower silicon content as the graphitizing agent and a faster supplied cooling rate. After this precipitation, the cementite forms as large particles.
During the precipitation of the iron carbide, the precipitate draws carbon from the original melt, thus moving the mixture toward one that is closer to eutectic. The remaining phase is lowering iron to carbon austenite, which transforms to martensite once cooled.
These contained eutectic carbides are too large to provide the benefit of precipitation hardening. In some steels there might be much smaller cementite precipitates that might carry the deformation of plastic by impeding the movement of dislocations through the pure iron ferrite matrix. They have an advantage as they increase the bulk hardness of the cast iron simply because of their own hardness and volume fraction. This results in the bulk hardness being able to be approximated by a rule of mixtures.
This hardness is offered at the expense of toughness in any case. White cast iron can be generally classified as a cement, since carbide makes up a larger fraction of the material. White iron is too brittle to be used in structural components, but because of its good hardness, resistance to abrasion, and low cost, it can be used as the wear surface of slurry pumps.
It is hard to cool thick castings at a faster rate which is enough to solidify the melt as white cast iron, however rapid cooling can be put to use in order to solidify a hell of white cast iron and after this a remainder of it will be cool at a slower pace thus forming a core of gray cast iron. This resulting cast is called a chilled casting, and it contains the benefits of having a hard surface but with a tougher interior.
High chromium white iron alloys had an ability of allowing massive casting of about a 10 tonne impeller to be sand cast. This is due to the fact that the chromium reduces the cooling rate required to produce carbides through the greater thicknesses of material. Carbides with an excellent abrasion resistance are also produced by chromium elements.
Malleable cast iron begins as a white iron casting, then heat treated at temperatures of about 950°C for two or a single day, and then it is cooled for the same period of time.
The carbon in iron carbide then transforms into graphite and ferrite plus carbon due to this heating and cooling process. This is a low process, but it enables the surface tension to transform the graphite into spheroidal particles rather than flakes.
The spheroids are relatively short and further away from each other due to their low aspect ratio. They also contain a lower cross-section, propagating crack and a photon. As opposed to flakes, they contain blunt boundaries which partake in the alleviating of the stress concentration problems which are found in gray cast iron. All in all, the properties included in the malleable cast iron are more like those of steel which is mild in nature.
Sometimes referred to as nodular cast iron, this cast iron has its graphite in the form of very tiny nodules, with the graphite having the form of layers which are concentric and thus forming the nodules. Due to this, the properties of ductile cast iron are that of a spongy steel which has no stress concentration effects produced by the flakes of the graphite.
The carbon concentration amount contained is around 3 percent to 4 percent, and that of silicon is around 1.8 percent to 2.8 percent. Small amounts of 0.02 percent to 0.1 percent of magnesium, and only 0.02 percent to 0.04 percent cerium when added to these alloys slows down the rate at which graphite precipitation grows through bonding to the graphite lanes edges.
Carbon can have a chance of separating as spheroidal particles as the material solidifies, due to the careful control of other elements and proper timing during the process. The resulting particles are similar to malleable cast iron, but parts can be cast with sections that are larger.
The properties of cast iron are changed and added in various alloying elements or alloyants in the cast iron. In line with carbon is the element silicon because it has an ability to force carbon out of the solution. A smaller percentage of silicon can not be able to fully achieve this as it allows carbon to remain in the solution, thus forming iron carbide and also producing white cast iron.
A bigger percentage or concentration of silicon is able to force the carbon out of the solution and then form graphite and also produce gray cast iron. Other alloying agents not noted include manganese, chromium, titanium and then vanadium. These counteract silicon, they also promote the retention of carbon and thus also the formation of carbides. Nickel and the element copper have an advantage as they increase strength and machinability, but they do not then be able to change the amount of carbon formed.
The carbon that is in the form of graphite results in a softer iron, thus reducing the effect of shrinkage, lowering the strength and decreasing the contained density. Sulfur is mostly a contaminant when contained, and it forms iron sulfide which prevents the formation of graphite and also which increases hardness.
The disadvantage imposed by sulfur is that it makes molten cast iron viscous, which causes defects. To cater for and eliminate the effects of sulfur, manganese is added to the solution. This is done because when the two are combined they form manganese sulfide instead of iron sulfide. Resulting manganese sulfide is lighter than the melt and tends to float out of the melt and get into the slag.
The approximate amount of manganese needed to cancel out the effects of sulfur is 1.7 units of sulfur content and an additional 0.3 percent added on top. The adding of more than this amount of manganese results in the formation of manganese carbide and this increases hardness and chilling except in gray iron where up to 1 percent of manganese can increase strength and the density contained. Nickel is one of the most general alloying elements because it has a tendency of refining the pearlite and the structure of the graphite, thus improving the toughness, and evens out the hardness difference between section thicknesses.
Chromium is added in small amounts to reduce free graphite and produce a chill. This is because chromium is a powerful carbide stabilizer, and in some cases it can work in conjunction with nickel. For chromium also, a small substitute amount of tin can be added. Copper is added in the ladle or furnace on the order of 0.5 percent to 2.5 percent to achieve a lowering chill, refining graphite, and the increment in fluidity. Molybdenum can also be added in the order of 0.3 percent to 1 percent so as to also increase the chill, refine the graphite, and refine the pearlite structure.
It is usually added working in line with nickel, copper, and chromium to produce high strength irons. The element titanium is added to work as a degasser and a deoxidizer, and increase fluidity. Proportions of 0.15 percent to 0.5 percent of the element vanadium are added to the cast iron and help in stabilizing cementite, to increase hardness and resist wear and heat effects.
Zirconium helps form graphite and is added in proportions of about 0.1 percent to 0.3 percent. This element also helps in deoxidizing and increasing fluidity. In malleable iron melts, to increase how much silicon can be added, bismuth is poured in a scale of 0.002 percent to 0.01 percent. In white iron, the element boron is added,which aids in the production of iron which is malleable, and it reduces the coarsening effect of the element bismuth.
This chapter will discuss the applications and benefits of iron castings.
Cast iron is common in the engineering world for applications such as mechanical engineering, construction sites, wood workshops, and others.
One of the applications is for casting ornaments such as gates, lampposts, and an iron column for a small coverage. Cast iron is also used for compression members.
Its applications can be seen in fields where they are used for production of water pipes, gas pipes, sewers, sanitary fittings, and manhole covers. Cast iron can be used for rail chains and carriage wheels. Gray iron can resist wear, which makes it used for producing blocks of engines, cylinder heads, manifolds, enclosures, gas burner gear blanks, and housings.
Because it is a brittle material, white cast iron is used for things that require resistance to wear and abrasions. These include shot blasting, nozzles, mill lining, rolling mills, slurry pump housing, crushers, and railroad brake shoes. White cast iron is brittle because of its chilling process used in its production.
Ductile iron castings have a wide range of applications because they can be broken down into different grades. This material can be easily machined; it contains good fatigue and better yield strength and an improved resistance to wear. Ductile iron castings are used for the production of steering knuckles, hydraulic components, crankshafts, heavy-duty gears, automotive suspension components, and automobile door hinges.
Malleable iron is also found in different grades. These types of castings have the ability to retain and store lubricants, wear particles that are non-abrasive, and have a porous surface which traps other abrasive debris. Due to these properties contained, malleable iron can be used for heavy-duty bearing surfaces, chain formation, for connecting rods, making sprockets, for production of drivetrain, axle components, and railroad rolling stock.
Iron castings are included in various types of fields because of their benefit, and some of these include its containment of good casting properties. Such properties include its advanced strength and its ductility. It is an advantage as it is also available in large quantities and also having a low cost to attain them. Gray cast has a good machinability and thus is proper for making cylinder heads and other applications. These iron castings contain a resistance to wear as the ferrous material is reinforced with several alloys.
Iron castings are generally the end result of the manufacturing process in which a material in a molten state is poured into a mold and then allowed to solidify. This solidified part is known as a casting, and these casting materials are mostly metals that cure after mixing two or more components together. Epoxy, clay, and plaster are usually used as examples for these. Iron casting is specifically used for making complex shapes that would be difficult to make using other methods of production.
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