This chapter will cover the definition of iron castings, their production methods, and the various casting processes involved.
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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 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.
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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.
This set of guidelines provides general information on the characteristics and common uses of cast iron and identifies typical problems associated with the material. See also: 'Checklist for Inspecting Cast Iron Failures'.
Cast iron is one of the oldest ferrous metals used in construction and outdoor ornament. It is primarily composed of iron (Fe), carbon (C) and silicon (Si), but may also contain traces of sulphur (S), manganese (Mn) and phosphorus (P). It has a relatively high carbon content of 2% to 5%. It is hard, brittle, nonmalleable (i.e. it cannot be bent, stretched or hammered into shape) and more fusible than steel. Its structure is crystalline and it fractures under excessive tensile loading with little prior distortion. Cast iron is, however, very good in compression. The composition of cast iron and the method of manufacture are critical in determining its characteristics.
The most common traditional form is grey cast iron. Common or grey cast iron is easily cast but it cannot be forged or worked mechanically, either hot or cold.
In grey cast iron, the carbon content is in the form of flakes distributed throughout the metal. In white cast iron, the carbon content is combined chemically as carbide of iron. White cast iron has superior tensile strength and malleability. It is also known as 'malleable' or 'spheroidal graphite' iron.
Cast iron is still manufactured by much the same process as it was produced historically. Iron ore is heated in a blast furnace with coke and limestone. This process 'deoxidizes' the ore and drives off impurities, producing molten iron. The molten iron is poured into molds of the desired shape and allowed to cool and crystallize.
Upon manufacture, cast iron develops a protective film or scale on the surface which makes it initially more resistant to corrosion than wrought iron or mild steel. Finishing may include bituminous coatings, waxes, paints, galvanizing and plating. In addition, there are a variety of treatments that can reduce rusting and corrosion caused by environmental factors. Factory preservative treatments are typically barrier coatings intended to prevent the castings from oxidizing (rusting) in the presence of humidity and oxygen in the air.
Margot Gayle, David W. Look, John G. Waite. Metals in America's Historic Buildings. Washington, DC: National Park Service, . (USGPO -332-360)
L. William Zahner. Architectural Metals: A Guide to Selection, Specification, and Performance. New York City: John Wiley & Sons, .
Cast iron is used in a wide variety of structural and decorative applications, because it is relatively inexpensive, durable and easily cast into a variety of shapes. Most of the typical uses include:
Historic markers and plaques
Hardware: hinges, latches
Columns, balusters
Stairs
Structural connectors in buildings and monuments
Decorative features
Fences
Tools and utensils
Ordnance
Stoves and firebacks
Piping
The basic cast iron material in all of these applications may appear to be the same, or very similar. However, the component size, composition, use, condition, relationship to adjacent materials, exposure and other factors may dictate that different treatments be used to correct similar problems. Any material in question should be evaluated as a part of a larger system and treatment plans should be based upon consideration of all relevant factors.
Cast iron is extremely strong and durable when used appropriately and protected from adverse exposure. It is much stronger in compression than in tension, therefore it is commonly found in columns, but not in structural beams. It is, however, highly susceptible to corrosion (rusting) when exposed to moisture and it has several typical problems which usually can be identified by visual inspection. The following sections will identify and discuss the most common problems encountered with cast iron. For general guidance on inspecting for cast iron failures, see -01-G.
The typical deterioration or corrosion process for cast iron is a one-step straight line process of oxidation (or rusting) which begins on exposure to air and moisture and will continue (unless interrupted) until the metal is gone. This process is described in the following section.
Rusting, or oxidation, is the most frequent and easily recognizable form of cast iron deterioration. Cast iron is highly susceptible to rusting when the humidity is higher than 65%. Iron (Fe) combines with oxygen (O) in the presence of water vapor (H2O) to become rust (Fe2O3). This process can take place at significantly different rates depending on the material composition, protective treatments applied and severity of exposure. If rusting occurs at a rapid rate, it can result in severe damage or total loss of a component in a short time; therefore, the presence of any rust on a cast iron artifact should alert the observer to the presence of a serious problem. Rusting can occur when the humidity is as low as 58% in the presence of certain pollutants, especially sulfur dioxide, ammonia sulfates or even the presence of body oils from touching. Reducing the humidity to 30% or below has been found to be effective in preventing rusting, however this is not a practical solution for outdoor cast iron.
Rusting is such a common problem that it is quite easily recognizable. Rust (Ferrous Oxide, Fe2O3, and Ferric Oxide, Fe3O4) is an orange colored surface coating, ranging in texture from scaly to powdery. It is loosely bound and the outer layers will usually come off when rubbed by hand or brushed against. It is not a deposit on the surface. Rust is the result of the combination of the iron (Fe) with oxygen (O) in the air, in the presence of moisture. The presence of rust means that some original iron material has been converted to iron oxide and irreversibly lost from the cast iron piece.
The probability of rust occurring is generally dependent upon two factors:
The degree of protection (usually a protective coating) provided to keep moisture from contact with the metal, and
The degree of moisture present in the air.
Protective coatings used on iron include bituminous coatings (such as tars), waxes, paints and sophisticated metallic coatings. Effective coatings, well maintained, provide the most reliable protection against rust and corrosion of cast iron, however, there are a wide variety of coatings available, and these can be confusing to users not thoroughly versed in the technical data for each type.
Humidity is the second factor affecting the rate of oxidation (rusting) of iron. It is generally accepted that rusting cannot begin unless the relative humidity is at or above 65% (this figure can be lower, however, in the presence of pollutants). Relative humidity is, however, not the only factor to be considered. Once rusting has started, at least two other phenomena may occur:
Some rust or ferrous oxide can become hydrated, i.e. it can contain moisture within its chemical structure, thereby exposing the iron to additional moisture, and
The porous rust may act as a reservoir for liquid water, keeping it in contact with the iron and perpetuating the rusting process.
Both of these conditions are microscopic in nature and invisible to casual inspection. Maintenance staff and trained personnel, however, should be aware of the processes, and the potential for the processes to damage the cast iron. The presence of visible rust is the symptom indicating that a problem exists. Appropriate action should be taken to prevent rusting, and where it does occur, to correct it with an appropriate treatment. See individual repair or preventive maintenance procedures for specific guidance as needed.
Many other factors can affect both corrosion and the rate of corrosion. Sea water, salt air, cements, plasters, ashes, sulphur, soils and acids can accelerate the corrosion of iron. Corrosion rates can also be accelerated where the detailing of the cast iron provides pockets which can collect and hold moisture and corrosive agents. Preventive maintenance plans should consider detailing, such as crevices and recessed areas, in establishing routine inspection techniques and frequency of inspection.
Cast iron contains carbon, in the form of graphite, in its molecular structure. It is composed of a crystalline structure as are all metals; i.e. it is a heterogeneous mass of crystals of its major elements (Iron, Manganese, Carbon, Sulphur and Silicon). One condition which can occur in the presence of acid rain and/or sea water is 'graphitization.' The stable graphite crystals remain in place, but the less stable iron becomes converted to insoluble iron oxide (rust). The result is that the cast iron piece retains its shape and appearance but becomes weaker mechanically because of the loss of iron. Graphitization is not, however, a common problem. It generally will occur only after bare metal is left exposed for extended periods, or where failed joints allow the penetration of acidic rainwater to interior surfaces.
This corrosion process is galvanic, with the carbon present acting as the most noble (least corrosive) element and the iron acting as the least noble (most corrosive) element. The composition or microstructure of the iron affects the durability of the object because the rate of corrosion is dependent upon the amount and structure of the graphite present in the iron.
Barrier coatings are the most commonly used protective mechanisms for cast iron. Some type of coating (such as a wax, paint or metallic coating) should probably be considered an integral feature of cast iron in service. The absence of such a coating, or a failure in an existing coating should be corrected. Inspection should include a visual examination of all surfaces to determine if a coating exists, a fact which may be very apparent for opaque paints and coatings but substantially less apparent for clear lacquers, waxes or oils. Surfaces having the appearance of raw metal should be carefully examined for signs of rusting. Absence of a coating should be considered a major problem and corrective action should be undertaken. See individual repair or preventive maintenance procedures for specific guidance as needed.
Failure of a coating should also be identified and corrected. Coatings can wear away, crack, flake, blister, or peel away, indicating that the coating has failed and is no longer protecting the cast iron from moisture. Failed coatings can, in fact, trap moisture beneath the film and accelerate corrosion at certain points on the surface. Inspection of the surface should include a careful check for all of these types of coating failures. A record should be made of any coating failures observed so that corrective action may be taken.
Mechanical failures of cast iron are typically of two types and are relatively common problems.
Cast iron may contain various imperfections due to the manufacturing process. These may occur due to air holes, interrupted pouring, uneven cooling (cold sheets), cracks and cinders. Where such imperfections occur, the piece may be weakened mechanically, sometimes severely. These manufacturing problems are not generally visible upon inspection; however, there are several non-destructive techniques of identifying these types of problems, such as the use of fluorescent fluids and ultraviolet lamps, or x-ray. These non- destructive techniques require specialized knowledge and equipment, and are not generally feasible for use by maintenance staff. They should be undertaken by specialists with experience.
Visible inspection may, however, enable detection of mechanical failures after the failure has occurred or begun to occur. Stress cracks in paint or metal may be symptomatic of this problem. Failures may begin as gradual separations which are visible upon inspection, and may be detected and corrected prior to a total, catastrophic failure of the piece. Linear cracks in paint film or metal should be investigated and/or monitored to determine if they are active. Non-destructive techniques may be used if symptoms exist, but the Regional Historic Preservation Officer (RHPO) should be consulted in the solicitation of professionals who are experienced in use of these techniques.
Larger cast iron pieces are generally systems composed of smaller castings, mechanically connected. This can even be the case for a simple baluster or historical marker. One of the most common failures that occurs with such systems is the failure of the connectors or joints. Loose, missing or broken screws, clamps or bolts may result in loose, failed or missing components. Visual inspection should include examination of cast iron pieces for sections which are loose and/or disoriented, and which have loose or missing screws or bolts. Further manipulation by hand, with probes, may indicate whether a casting is a discrete piece, mechanically attached, and whether or not it is in the early stages of working loose. It is especially important to detect connectors which are in danger of imminent failure if not corrected. Corrective action should be undertaken in either case, but the treatment plan should take into account the severity of the problem, consequences of failure and nature of the intervention required to correct the problem. See individual repair procedures for specific guidance as needed.
Another mechanical problem can be caused by inappropriate mechanical repairs to broken pieces. Some repairs may create openings that allow water penetration and 'pockets' that collect water, both of which can cause problems. Castings which have been filled with concrete are also a potential problem since they may promote 'crevice corrosion' due to entrapped water. Visual inspections should check for such conditions and where they exist, maintenance staff should plan to correct the problems and/or be vigilant for signs of deterioration.
Cast iron problems, especially corrosion problems, may be reduced or eliminated in cast iron that is an alloy of silicon, nickel, chromium and/or copper. For example, silicon is often present in cast iron to some degree, but it is not considered an alloy until the percentage exceeds the 3% upper range of non-alloy cast iron. Where silicon is present, a protective surface film develops during oxidation.
There are three main categories of cast iron alloys:
High silicon
High chromium
High nickel (frequently containing copper or chromium)
All of these alloys, plus copper alloys, have been tested and found to have increased corrosion resistance. The degree of increased resistance is dependent on many factors, primarily the alloying metal and the percentage of alloy relative to the carbon content of the cast iron. While a discussion of alloy durability and formulation is beyond the scope of this standard, users should be aware of the effect of alloying and consider the implications when ordering new cast iron replacement objects. Such consideration may involve experienced metallurgists, foundrymen, conservators, and historical architects.
The maintenance principles for cast iron are, in order of appearance:
Prevent rust and corrosion.
Paint and plug holes.
Maintain structural soundness.
keep it together with binding and bolts, welding, etc., and brace loose elements by resetting.
Recreate missing pieces using casting replacement parts (iron, aluminum, fiberglass, or epoxy), or wooden replacements, with appropriate composition and/or coatings to provide for color blending.
Cast iron requires continual maintenance. Check periodically for water collection spots and dry as necessary. Signs of corrosion are when rusty looking stain marks appear on the metal. If these areas are rubbed the metal surface is revealed as well as traces of perforation. Check for small chips in the coating surface and peeling of the coating surface.
Replace or repair as necessary if the damage is minimal missing and deteriorated pieces of metal prior to cleaning. If deteriorated condition is left unrepaired, perforation of the metal will occur and as a result structural failure.
Structural iron maintenance may require the services of a structural engineer when severe erosion or distortion occurs, to assist in the development of repair techniques when material loss is involved. For these repairs use only a professional iron worker. Before installation of new material verify the metal type and thickness. Prior to installation, remove all oil, dirt, and other debris from the surface. All surfaces shall be dry and free from frost.