This article will take an in-depth look at iron castings.
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This chapter will cover the definition of iron castings, their production methods, and the various casting processes involved.
Iron casting is a durable material made by combining iron with carbon, which allows it to be easily molded. It contains a higher carbon content compared to steel, usually ranging from 2% to 4.3%.
Cast iron also includes varying levels of silicon, manganese, and minor impurities such as sulfur and phosphorus. The production process begins with the reduction of iron ore in a blast furnace, resulting in the formation of liquid cast iron. This molten iron is poured into molds and solidified into crude ingots known as pigs. These pigs are then remelted with scrap metal and other alloying elements in cupola furnaces, and recast into molds to create a range of products.
Cast iron is produced from pig iron, which is derived by melting iron ore in a blast furnace. It can be obtained directly from molten pig iron or through the re-melting of pig iron. This process often involves the addition of other significant quantities of iron, steel, limestone, and coke (a form of carbon). Various steps are then undertaken to remove undesirable contaminants from the mixture.
Contaminants like sulfur and phosphorus can be burned off from molten iron, but this process also risks removing the necessary carbon, which must then be replenished. The carbon and silicon levels in the molten iron are adjusted based on the intended application of the final product, typically ranging from 2% to 3.5% for carbon and 1% to 3% for silicon. Additional desired elements are added before the final casting is made.
Casting involves pouring liquid metal into a mold, usually through a crucible, which contains a negative impression of the desired shape. The metal flows into the mold through a channel called a sprue. Once the metal and mold cool, the solidified casting is removed from the mold.
Casting is commonly used to create complex shapes that would be challenging or cost-prohibitive to produce using other methods. Traditional casting techniques include lost wax casting, which can be further categorized into centrifugal casting and vacuum-assisted direct pour casting. Other methods include plaster mold casting and sand casting.
Cast iron is sometimes melted in a specialized type of blast furnace called a cupola. It can also be melted using electric induction furnaces or electric arc furnaces. Once the melting process is complete, the molten cast iron is transferred to a holding furnace or ladle for further processing.
Iron castings are known for their hardness, which provides resistance to abrasion and indentation. They also exhibit toughness, allowing them to absorb energy. These castings possess a degree of elasticity, meaning they can return to their original dimensions after deformation. Additionally, iron castings are ductile, capable of deforming without fracturing.
Malleability is another characteristic of iron castings, enabling them to withstand compression without rupturing. Mechanical properties such as tensile strength and fatigue strength are also notable. Tensile strength measures the maximum longitudinal stress a material can endure before breaking, while fatigue strength indicates the maximum stress a material can endure for a specific number of cycles before failing.
There are two main types of casting: expendable and non-expendable. These categories are further divided based on the mold material, which can be sand or metal, and the pouring method, such as gravity, vacuum, or low pressure.
Expendable mold casting is a broad classification that includes various types such as sand, plastic, plaster, shell, and investment moldings. This method utilizes temporary molds that are not reusable.
Sand casting is cost-effective for producing smaller batches compared to permanent mold casting. This method allows for the creation of products at a lower cost and is suitable for small-scale operations. Castings can be as small as fitting 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 is used to create large, symmetrical objects such as cannons and church bells. The process involves mixing clay and sand with straw or dung to form loam. A model of the object, known as a chemise, is created from this friable material. A mold is then built around the chemise by covering it with loam, which is subsequently dried or baked. After removing the chemise, the mold is placed upright in a pit in front of the furnace for pouring molten metal. Once the casting is complete, the mold is broken away.
Plaster mold casting is similar to sand casting but uses plaster instead of sand for the mold material. This method typically takes less than a week to prepare and can produce 1 to 10 units per hour per mold. It accommodates items ranging from 30 grams to 45 kilograms, providing good surface finishes and close tolerances. Plaster casting is often more economical for complex parts due to the low cost of plaster and its ability to produce near-net-shape castings. However, it is limited to low melting point non-ferrous metals such as copper, zinc, and magnesium.
Shell molding shares similarities with casting, but it uses a hardened shell of sand to form the molding cavity rather than a flask filled with sand. The sand used in this process is finer and mixed with a resin, which hardens when heated by the pattern to create the shell. This results in a finer surface finish compared to traditional sand casting. Shell molding is particularly suited for small to medium-sized, complex items, offering greater precision and a smoother finish.
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 ideal for producing high-performance components in net shapes from various metals and alloys. Although this process can be more expensive than other casting methods, it is particularly suited for small castings. One of the key advantages of investment casting is that products typically require little to no additional finishing work.
Non-expendable mold casting differs from expendable casting in that the mold does not need to be remade after each production cycle. This process encompasses various methods, including permanent mold casting, die casting, centrifugal casting, and continuous casting. Non-expendable casting typically results in improved part quality and near-net-shape accuracy.
Permanent mold casting is a metal casting technique that utilizes reusable molds, typically made of metal. The mold is generally filled using gravity, although gas pressure or vacuum methods may also be employed in some cases. A variation known as slush casting is used to produce hollow castings. Common metals used in this process 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 an advanced variation of die casting designed to reduce or eliminate residual porosity commonly found in standard die castings. This method uses a feed material with higher viscosity, which is partially liquid and partially solid, rather than entirely liquid metal. A modified die casting machine is employed to inject the semi-solid slurry into hardened steel dies. The high viscosity of the semi-solid metal and controlled die filling conditions ensure that the metal fills the die turbulently, effectively minimizing 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 an advanced casting process designed for high-volume production of metal sections with a uniform cross-section. In this method, molten metal is poured into a water-cooled mold that is open at both ends. This allows a solid metal skin to form around the still-liquid core, gradually solidifying from the outside in. Once the metal has completely solidified, the strand is continuously drawn from the mold.
The solidified strand is then cut to predetermined lengths using either mechanical shears or traveling oxyacetylene torches. Continuous casting helps reduce costs associated with the ongoing production of standard products and improves their quality. The process can produce various cast sizes, including strips, billets, and slabs, each with distinct dimensions.
Numerous manufacturers provide machinery for producing iron castings. Below are five notable brands in the United States and Canada, each with a specific model showcasing their distinct features and capabilities:
Features: The Hunter HLM Series features high-pressure molding technology that ensures accurate and consistent mold quality. It employs both hydraulic and pneumatic systems to deliver uniform mold hardness, stable mold conditions, and precise flask squeeze. The series is recognized for its dependable performance, user-friendly interface, and advanced control systems designed for streamlined production.
Features: The Sinto FBO Series introduces flaskless molding technology, eliminating the need for traditional flasks. It utilizes an innovative sand filling method that compacts sand directly onto the pattern, enhancing productivity and flexibility. This approach results in quicker mold production and cost efficiency due to its streamlined operation.
Features: The DISAMATIC D3 is designed for high-volume production with a vertical molding approach combined with horizontal parting. This machine provides exceptional accuracy, consistent density, and minimal mismatch. It boasts rapid cycle times, low operating costs, and excellent dimensional precision, making it ideal for large-scale foundry operations.
Features: The Roberts Sinto FDNX Series offers advanced flaskless molding capabilities. It integrates technologies like aerated sand filling and adjustable mold height to produce high-quality molds. The series provides flexibility in mold dimensions, high-density molds, and reduced operational expenses, making it suitable for a range of casting requirements.
Features: Buhler Prince delivers a variety of casting solutions, including both horizontal and vertical high-pressure molding machines. These machines feature hydraulic systems with precise control for achieving high mold hardness, consistent sand compaction, and accurate pattern reproduction. Buhler Prince equipment is noted for its robust design, longevity, and capability to handle intricate casting shapes.
For the most current information on these models and their features, it is advisable to visit the manufacturers' websites or contact them directly.
This chapter will explore the different types of iron castings.
Gray cast iron is distinguished by its graphite microstructure, which can lead to fractures and gives the material its gray color. It is the most widely used type of cast iron and is commonly used based on weight. Typically, gray cast iron contains 2.5% to 4% carbon, 1% to 3% silicon, with the remainder being iron.
This type of cast iron exhibits lower tensile strength and shock resistance compared to steel. However, its compressive strength is comparable to that of low and medium carbon steels.
The mechanical properties of gray cast iron are influenced by the shape and size of the graphite flakes within its microstructure.
This type of iron features fractured surfaces that appear white due to the presence of iron carbide, also known as cementite. In white cast iron, carbon solidifies as cementite rather than graphite because of lower silicon content and a rapid cooling rate. This results in large cementite particles forming within the iron.
As iron carbide forms, it extracts carbon from the original melt, shifting the composition closer to a eutectic mixture. The remaining phase consists of austenite with lower carbon content, which transforms into martensite upon cooling.
The eutectic carbides present in white cast iron are too large to offer the advantages of precipitation hardening. In some steels, much smaller cementite precipitates can enhance plastic deformation resistance by obstructing dislocation movement through the pure iron ferrite matrix. These small precipitates improve the overall hardness of the cast iron due to their own hardness and volume fraction. Consequently, the bulk hardness of the cast iron can be estimated using a rule of mixtures.
While white cast iron offers excellent hardness, it does so at the expense of toughness. Generally, white cast iron is classified as a cement type due to its high carbide content. It is too brittle for structural applications but is used as a wear surface in slurry pumps due to its hardness, abrasion resistance, and cost-effectiveness.
Cooling thick castings quickly enough to solidify them as white cast iron can be challenging. However, rapid cooling can be employed to solidify a thin outer layer of white cast iron, while the core cools more slowly, resulting in gray cast iron. This combination, known as a chilled casting, provides a hard exterior with a tougher interior.
High-chromium white iron alloys allow for the sand casting of large components, such as a 10-tonne impeller. Chromium reduces the required cooling rate to form carbides in thicker sections and enhances the abrasion resistance of the cast iron.
Malleable cast iron starts as white iron casting and undergoes heat treatment at approximately 950°C for one to two days. After heating, it is cooled over the same period.
During this heating and cooling process, the carbon in iron carbide converts into graphite, along with the formation of ferrite and additional carbon. This gradual process allows surface tension to convert graphite from flakes into spheroidal particles.
The spheroidal graphite particles in malleable cast iron are relatively small and spaced farther apart due to their low aspect ratio. These particles have a lower cross-section, which helps in reducing crack propagation and stress concentration compared to the flakes found in gray cast iron. With their blunt boundaries, spheroidal graphite particles help alleviate stress concentration issues, resulting in properties more akin to mild steel.
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 content in malleable cast iron typically ranges from 3% to 4%, while silicon levels are between 1.8% and 2.8%. Small quantities of magnesium (0.02% to 0.1%) and cerium (0.02% to 0.04%) are added to these alloys. These elements help slow the growth rate of graphite precipitation by bonding to the edges of graphite flakes.
With careful control of these elements and precise timing during the casting process, carbon can segregate into spheroidal particles as the material solidifies. This results in castings that resemble malleable cast iron but can include larger sections.
The properties of cast iron can be modified by incorporating various alloying elements. Silicon plays a significant role in controlling the carbon content within cast iron. When silicon is present in low amounts, it does not fully remove carbon from the solution, resulting in the formation of iron carbide and the creation of white cast iron.
In contrast, higher silicon levels effectively push carbon out of the solution, leading to the formation of graphite and the production of gray cast iron. Other alloying elements, such as manganese, chromium, titanium, and vanadium, also influence the characteristics of cast iron by counteracting silicon's effects, retaining carbon, and promoting carbide formation. Nickel and copper enhance the strength and machinability of cast iron but do not alter the carbon content.
Graphite within cast iron makes the material softer, which can reduce shrinkage, lower strength, and decrease density. Sulfur, often considered a contaminant, forms iron sulfide, which inhibits graphite formation and increases hardness.
Sulfur also increases the viscosity of molten cast iron, leading to defects. To address this issue, manganese is added to the melt. Manganese reacts with sulfur to form manganese sulfide, which is lighter than the molten metal and floats to the surface, where it is removed as slag.
To effectively neutralize the effects of sulfur, approximately 1.7 times the sulfur content in manganese, plus an additional 0.3 percent, is required. Exceeding this manganese amount can lead to the formation of manganese carbide, which enhances hardness and chilling effects. However, in gray iron, manganese levels up to 1 percent can improve strength and density. Nickel is widely used as it refines the pearlite and graphite structures, enhancing toughness and balancing hardness across different section thicknesses.
Chromium, added in small quantities, serves to reduce free graphite and induce chilling due to its strong carbide-stabilizing properties. It can also work synergistically with nickel. Additionally, a minor amount of tin can be used as a substitute for chromium. Copper, typically added in amounts ranging from 0.5 percent to 2.5 percent, helps reduce chilling, refine graphite, and improve fluidity. Molybdenum, at concentrations of 0.3 percent to 1 percent, also contributes to increased chilling, graphite refinement, and improved pearlite structure.
Titanium is included to act as a degasser and deoxidizer, enhancing fluidity. Vanadium, usually added in amounts from 0.15 percent to 0.5 percent, stabilizes cementite, boosting hardness and resistance to wear and heat.
Zirconium aids in graphite formation and is added in proportions of about 0.1 percent to 0.3 percent. It also assists in deoxidization and increases fluidity. For malleable iron, bismuth is added in small amounts (0.002 percent to 0.01 percent) to allow for greater silicon addition. In white iron, boron is introduced to enhance malleability and mitigate the coarsening effects of bismuth.
This chapter will explore the various applications and advantages of iron castings.
Cast iron is widely utilized across various engineering fields, including mechanical engineering, construction, woodworking, and more.
Applications of cast iron include the production of decorative items such as gates, lampposts, and ornamental iron columns. Additionally, cast iron is employed in the construction of compression members.
Cast iron finds extensive use in various fields, including the production of water pipes, gas pipes, sewers, sanitary fittings, and manhole covers. It is also utilized in manufacturing rail chains and carriage wheels. Gray iron, with its wear-resistant properties, is commonly used for producing engine blocks, cylinder heads, manifolds, enclosures, gas burner gear blanks, and housings.
White cast iron, known for its brittleness, is ideal for applications requiring high resistance to wear and abrasion. It is commonly used in shot blasting equipment, nozzles, mill linings, rolling mills, slurry pump housings, crushers, and railroad brake shoes. Its brittleness is attributed to the chilling process used during its production.
Ductile iron castings offer versatility due to their various grades and are appreciated for their machinability, fatigue resistance, yield strength, and wear resistance. They are widely used in manufacturing steering knuckles, hydraulic components, crankshafts, heavy-duty gears, automotive suspension parts, and automobile door hinges.
Malleable iron comes in various grades and is known for its ability to retain and store lubricants and non-abrasive wear particles. Its porous surface can trap abrasive debris, making it suitable for heavy-duty applications. This includes use in bearing surfaces, chain formations, connecting rods, sprockets, drivetrain components, axle parts, and railroad rolling stock.
Iron castings are widely used across various fields due to their beneficial properties, including strength and ductility. They are available in large quantities and are cost-effective. Gray cast iron, in particular, offers good machinability, making it ideal for applications like cylinder heads. These castings are reinforced with alloys, enhancing their resistance to wear.
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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