Cast iron is a class of iron–carbon alloys with a carbon content of more than 2% and silicon content around 1–3%.[1] Its usefulness derives from its relatively low melting temperature. The alloying elements determine the form in which its carbon appears: white cast iron has its carbon combined into the iron carbide compound cementite, which is very hard, but brittle, as it allows cracks to pass straight through; grey cast iron has graphite flakes which deflect a passing crack and initiate countless new cracks as the material breaks, and ductile cast iron has spherical graphite "nodules" which stop the crack from further progressing.
If you are looking for more details, kindly visit Hengfeng.
Carbon (C), ranging from 1.8 to 4 wt%, and silicon (Si), 1–3 wt%, are the main alloying elements of cast iron. Iron alloys with lower carbon content are known as steel.
Cast iron tends to be brittle, except for malleable cast irons. With its relatively low melting point, good fluidity, castability, excellent machinability, resistance to deformation and wear resistance, cast irons have become an engineering material with a wide range of applications and are used in pipes, machines and automotive industry parts, such as cylinder heads, cylinder blocks and gearbox cases. Some alloys are resistant to damage by oxidation. In general, cast iron is notoriously difficult to weld.
The earliest cast-iron artifacts date to the 8th century BC, and were discovered by archaeologists in what is now Jiangsu, China. Cast iron was used in ancient China to mass-produce weaponry for warfare, as well as agriculture and architecture.[2] During the 15th century AD, cast iron became utilized for cannons and shot in Burgundy, France, and in England during the Reformation. The amounts of cast iron used for cannons required large-scale production.[3] The first cast-iron bridge was built during the s by Abraham Darby III, and is known as the Iron Bridge in Shropshire, England. Cast iron was also used in the construction of buildings.
Cast iron is made from pig iron, which is the product of melting iron ore in a blast furnace. Cast iron can be made directly from the molten pig iron or by re-melting pig iron,[4] often along with substantial quantities of iron, steel, limestone, carbon (coke) and taking various steps to remove undesirable contaminants. Phosphorus and sulfur may be burnt out of the molten iron, but this also burns out the carbon, which must be replaced. Depending on the application, carbon and silicon content are adjusted to the desired levels, which may be anywhere from 2–3.5% and 1–3%, respectively. If desired, other elements are then added to the melt before the final form is produced by casting.[citation needed]
Cast iron is sometimes melted in a special type of blast furnace known as a cupola, but in modern applications, it is more often melted in electric induction furnaces or electric arc furnaces.[5] After melting is complete, the molten cast iron is poured into a holding furnace or ladle.[citation needed]
Cast iron's properties are changed by adding various alloying elements, or alloyants. Next to carbon, silicon is the most important alloyant because it forces carbon out of solution. A low percentage of silicon allows carbon to remain in solution, forming iron carbide and producing white cast iron. A high percentage of silicon forces carbon out of solution, forming graphite and producing grey cast iron. Other alloying agents, manganese, chromium, molybdenum, titanium, and vanadium counteract silicon, and promote the retention of carbon and the formation of those carbides. Nickel and copper increase strength and machinability, but do not change the amount of graphite formed. Carbon as graphite produces a softer iron, reduces shrinkage, lowers strength, and decreases density. Sulfur, largely a contaminant when present, forms iron sulfide, which prevents the formation of graphite and increases hardness. Sulfur makes molten cast iron viscous, which causes defects. To counter the effects of sulfur, manganese is added, because the two form into manganese sulfide instead of iron sulfide. The manganese sulfide is lighter than the melt, so it tends to float out of the melt and into the slag. The amount of manganese required to neutralize sulfur is 1.7 × sulfur content + 0.3%. If more than this amount of manganese is added, then manganese carbide forms, which increases hardness and chilling, except in grey iron, where up to 1% of manganese increases strength and density.[6]
Nickel is one of the most common alloying elements, because it refines the pearlite and graphite structures, improves toughness, and evens out hardness differences between section thicknesses. Chromium is added in small amounts to reduce free graphite, produce chill, and because it is a powerful carbide stabilizer; nickel is often added in conjunction. A small amount of tin can be added as a substitute for 0.5% chromium. Copper is added in the ladle or in the furnace, on the order of 0.5–2.5%, to decrease chill, refine graphite, and increase fluidity. Molybdenum is added on the order of 0.3–1% to increase chill and refine the graphite and pearlite structure; it is often added in conjunction with nickel, copper, and chromium to form high strength irons. Titanium is added as a degasser and deoxidizer, but it also increases fluidity. Vanadium at 0.15–0.5% is added to cast iron to stabilize cementite, increase hardness, and increase resistance to wear and heat. Zirconium at 0.1–0.3% helps to form graphite, deoxidize, and increase fluidity.[6]
In malleable iron melts, bismuth is added at 0.002–0.01% to increase how much silicon can be added. In white iron, boron is added to aid in the production of malleable iron; it also reduces the coarsening effect of bismuth.[6]
Grey cast iron is characterised by its graphitic microstructure, which causes fractures of the material to have a grey appearance. It is the most commonly used cast iron and the most widely used cast material based on weight. Most cast irons have a chemical composition of 2.5–4.0% carbon, 1–3% silicon, and the remainder iron. Grey cast iron has less tensile strength and shock resistance than steel, but its compressive strength is comparable to low- and medium-carbon steel. These mechanical properties are controlled by the size and shape of the graphite flakes present in the microstructure and can be characterised according to the guidelines given by the ASTM.[7]
White cast iron displays white fractured surfaces due to the presence of an iron carbide precipitate called cementite. With a lower silicon content (graphitizing agent) and faster cooling rate, the carbon in white cast iron precipitates out of the melt as the metastable phase cementite, Fe3C, rather than graphite. The cementite which precipitates from the melt forms as relatively large particles. As the iron carbide precipitates out, it withdraws carbon from the original melt, moving the mixture toward one that is closer to eutectic, and the remaining phase is the lower iron-carbon austenite (which on cooling might transform to martensite). These eutectic carbides are much too large to provide the benefit of what is called precipitation hardening (as in some steels, where much smaller cementite precipitates might inhibit plastic deformation by impeding the movement of dislocations through the pure iron ferrite matrix). Rather, they increase the bulk hardness of the cast iron simply by virtue of their own very high hardness and their substantial volume fraction, such that the bulk hardness can be approximated by a rule of mixtures. In any case, they offer hardness at the expense of toughness. Since carbide makes up a large fraction of the material, white cast iron could reasonably be classified as a cermet. White iron is too brittle for use in many structural components, but with good hardness and abrasion resistance and relatively low cost, it finds use in such applications as the wear surfaces (impeller and volute) of slurry pumps, shell liners and lifter bars in ball mills and autogenous grinding mills, balls and rings in coal pulverisers.
It is difficult to cool thick castings fast enough to solidify the melt as white cast iron all the way through. However, rapid cooling can be used to solidify a shell of white cast iron, after which the remainder cools more slowly to form a core of grey cast iron. The resulting casting, called a chilled casting, has the benefits of a hard surface with a somewhat tougher interior.[citation needed]
High-chromium white iron alloys allow massive castings (for example, a 10-tonne impeller) to be sand cast, as the chromium reduces cooling rate required to produce carbides through the greater thicknesses of material. Chromium also produces carbides with impressive abrasion resistance.[8] These high-chromium alloys attribute their superior hardness to the presence of chromium carbides. The main form of these carbides are the eutectic or primary M7C3 carbides, where "M" represents iron or chromium and can vary depending on the alloy's composition. The eutectic carbides form as bundles of hollow hexagonal rods and grow perpendicular to the hexagonal basal plane. The hardness of these carbides are within the range of -HV.[9]
Malleable iron starts as a white iron casting that is then heat treated for a day or two at about 950 °C (1,740 °F) and then cooled over a day or two. As a result, the carbon in iron carbide transforms into graphite and ferrite plus carbon. The slow process allows the surface tension to form the graphite into spheroidal particles rather than flakes. Due to their lower aspect ratio, the spheroids are relatively short and far from one another, and have a lower cross section vis-a-vis a propagating crack or phonon. They also have blunt boundaries, as opposed to flakes, which alleviates the stress concentration problems found in grey cast iron. In general, the properties of malleable cast iron are more like those of mild steel. There is a limit to how large a part can be cast in malleable iron, as it is made from white cast iron.[citation needed]
Developed in , nodular or ductile cast iron has its graphite in the form of very tiny nodules with the graphite in the form of concentric layers forming the nodules. As a result, the properties of ductile cast iron are that of a spongy steel without the stress concentration effects that flakes of graphite would produce. The carbon percentage present is 3-4% and percentage of silicon is 1.8-2.8%.Tiny amounts of 0.02 to 0.1% magnesium, and only 0.02 to 0.04% cerium added to these alloys slow the growth of graphite precipitates by bonding to the edges of the graphite planes. Along with careful control of other elements and timing, this allows the carbon to separate as spheroidal particles as the material solidifies. The properties are similar to malleable iron, but parts can be cast with larger sections.[citation needed]
Cast iron and wrought iron can be produced unintentionally when smelting copper using iron ore as a flux.[11]: 47–48
The earliest cast-iron artifacts date to the 8th century BC, and were discovered by archaeologists in what is now modern Luhe County, Jiangsu in China during the Warring States period. This is based on an analysis of the artifact's microstructures.[2]
Because cast iron is comparatively brittle, it is not suitable for purposes where a sharp edge or flexibility is required. It is strong under compression, but not under tension. Cast iron was invented in China in the 8th century BC and poured into molds to make ploughshares and pots as well as weapons and pagodas.[12] Although steel was more desirable, cast iron was cheaper and thus was more commonly used for implements in ancient China, while wrought iron or steel was used for weapons.[2] The Chinese developed a method of annealing cast iron by keeping hot castings in an oxidizing atmosphere for a week or longer in order to burn off some carbon near the surface in order to keep the surface layer from being too brittle.[13]: 43
Deep within the Congo region of the Central African forest, blacksmiths invented sophisticated furnaces capable of high temperatures over years ago. There are countless examples of welding, soldering, and cast iron created in crucibles and poured into molds. These techniques were employed for the use of composite tools and weapons with cast iron or steel blades and soft, flexible wrought iron interiors. Iron wire was also produced. Numerous testimonies were made by early European missionaries of the Luba people pouring cast iron into molds to make hoes.[14] Metallographic analysis of Luba artifacts also indicates the use of cast iron.[15]
The technology of cast iron was transferred to the West from China.[16] Al-Qazvini in the 13th century and other travellers subsequently noted an iron industry in the Alburz Mountains to the south of the Caspian Sea. This is close to the silk route, thus the use of cast-iron technology being derived from China is conceivable.[16] Upon its introduction to the West in the 15th century it was used for cannon and shot. Henry VIII (reigned –) initiated the casting of cannon in England. Soon, English iron workers using blast furnaces developed the technique of producing cast-iron cannons, which, while heavier than the prevailing bronze cannons, were much cheaper and enabled England to arm her navy better.
Cast-iron pots were made at many English blast furnaces at the time. In , Abraham Darby patented a new method of making pots (and kettles) thinner and hence cheaper than those made by traditional methods. This meant that his Coalbrookdale furnaces became dominant as suppliers of pots, an activity in which they were joined in the s and s by a small number of other coke-fired blast furnaces.
Application of the steam engine to power blast bellows (indirectly by pumping water to a waterwheel) in Britain, beginning in and increasing in the s, was a key factor in increasing the production of cast iron, which surged in the following decades. In addition to overcoming the limitation on water power, the steam-pumped-water powered blast gave higher furnace temperatures which allowed the use of higher lime ratios, enabling the conversion from charcoal (supplies of wood for which were inadequate) to coke.[17]: 122
The ironmasters of the Weald continued producing cast irons until the s, and armament was one of the main uses of irons after the Restoration.
The use of cast iron for structural purposes began in the late s, when Abraham Darby III built The Iron Bridge, although short beams had already been used, such as in the blast furnaces at Coalbrookdale. Other inventions followed, including one patented by Thomas Paine. Cast-iron bridges became commonplace as the Industrial Revolution gathered pace. Thomas Telford adopted the material for his bridge upstream at Buildwas, and then for Longdon-on-Tern Aqueduct, a canal trough aqueduct at Longdon-on-Tern on the Shrewsbury Canal. It was followed by the Chirk Aqueduct and the Pontcysyllte Aqueduct, both of which remain in use following the recent restorations.
The company is the world’s best Cast Iron Straight Ruler supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.
The best way of using cast iron for bridge construction was by using arches, so that all the material is in compression. Cast iron, again like masonry, is very strong in compression. Wrought iron, like most other kinds of iron and indeed like most metals in general, is strong in tension, and also tough – resistant to fracturing. The relationship between wrought iron and cast iron, for structural purposes, may be thought of as analogous to the relationship between wood and stone.
Cast-iron beam bridges were used widely by the early railways, such as the Water Street Bridge in at the Manchester terminus of the Liverpool and Manchester Railway, but problems with its use became all too apparent when a new bridge carrying the Chester and Holyhead Railway across the River Dee in Chester collapsed killing five people in May , less than a year after it was opened. The Dee bridge disaster was caused by excessive loading at the centre of the beam by a passing train, and many similar bridges had to be demolished and rebuilt, often in wrought iron. The bridge had been badly designed, being trussed with wrought iron straps, which were wrongly thought to reinforce the structure. The centres of the beams were put into bending, with the lower edge in tension, where cast iron, like masonry, is very weak.
Nevertheless, cast iron continued to be used in inappropriate structural ways, until the Tay Rail Bridge disaster of cast serious doubt on the use of the material. Crucial lugs for holding tie bars and struts in the Tay Bridge had been cast integral with the columns, and they failed in the early stages of the accident. In addition, the bolt holes were also cast and not drilled. Thus, because of casting's draft angle, the tension from the tie bars was placed on the hole's edge rather than being spread over the length of the hole. The replacement bridge was built in wrought iron and steel.
Further bridge collapses occurred, however, culminating in the Norwood Junction rail accident of . Thousands of cast-iron rail underbridges were eventually replaced by steel equivalents by owing to the widespread concern about cast iron under bridges on the rail network in Britain.
Cast-iron columns, pioneered in mill buildings, enabled architects to build multi-storey buildings without the enormously thick walls required for masonry buildings of any height. They also opened up floor spaces in factories, and sight lines in churches and auditoriums. By the mid 19th century, cast iron columns were common in warehouse and industrial buildings, combined with wrought or cast iron beams, eventually leading to the development of steel-framed skyscrapers. Cast iron was also used sometimes for decorative facades, especially in the United States, and the Soho district of New York has numerous examples. It was also used occasionally for complete prefabricated buildings, such as the historic Iron Building in Watervliet, New York.[citation needed]
Another important use was in textile mills. The air in the mills contained flammable fibres from the cotton, hemp, or wool being spun. As a result, textile mills had an alarming propensity to burn down. The solution was to build them completely of non-combustible materials, and it was found convenient to provide the building with an iron frame, largely of cast iron, replacing flammable wood. The first such building was at Ditherington in Shrewsbury, Shropshire.[18] Many other warehouses were built using cast-iron columns and beams, although faulty designs, flawed beams or overloading sometimes caused building collapses and structural failures.[citation needed]
During the Industrial Revolution, cast iron was also widely used for frame and other fixed parts of machinery, including spinning and later weaving machines in textile mills. Cast iron became widely used, and many towns had foundries producing industrial and agricultural machinery.[19]
Whilst every effort has been made to make this guide as informative and detailed as possible for owners of ornamental and architectural heritage ironwork this is not a definitive resource. This guide aims to provide an outline of the material, offer practical guidance on maintenance and highlight signs to look out for which may indicate signs of deterioration in cast iron. If you are uncertain about anything concerning your architectural cast ironwork, it is always advisable to get in touch with Dorothea Restorations first, who are more than happy to discuss best practice and a potential plan of action for cast iron in need of specialised care.
Cast iron production innovations in the 18th century completely changed the face of British architecture and design. As coke replaced charcoal in the blast furnace and steam power developed in the late ’s, mass production of cast iron followed. Embellishments to buildings became affordable, fashionable and a typical feature of 19th-century architecture.
The industry peaked in the late ’s and the innovation and creative use of cast iron at the time led to its use in a range of designs, from kitchen products and ornamental architecture through to complete structures such as bandstands.
Traditional decorative architectural castings of this period were made using grey cast iron, derived from iron ore. Cast Iron constitutes around a 4% carbon content (higher than wrought iron which is <1% carbon) giving it a crystalline structure and making it brittle by nature. Where wrought iron is good in tension, cast iron is weak and excels in compressive strength (find out more here).
Unlike wrought iron, cast iron is neither ductile nor malleable and cannot be forged. Traditionally, reusable patterns were created from wood and used to press a shape into green sand. Molten iron was then poured into the void and left to cool.
The type of metal will influence both its maintenance and restoration methods. Therefore, it is important to be able to establish which metal type you are working with (cast, wrought ironwork or neither).
Since cast iron is formed by pouring its molten form into a mould, usually made of two halves, identification of cast iron is most easily done by finding a mould line. Cast objects are generally of larger sections than those produced in wrought iron by a blacksmith.
A mould can be used multiple times for particular casting designs. As such, cast ironwork tends to have identical elements with little or no variation and designs tend to have repeating sections.
Furthermore, cast iron can often be identified by the fixing methods used. Flanges, lap joints, mortise and tenon along with concealed bolts are common methods of joining castings together.
Castings may also have a manufacturer's name cast into them, which with a little research may aid identification but also start to build the historical context.
Before discussing caring for cast iron, there are some important safety notes that need attention.
Cast iron can be coated with lead-based paints; often with a 75% lead content. Lead is a very strong poison and there are serious health risks associated with it. Lead presence needs to be tested and then safety procedures must be followed when removing or disturbing lead-based paints in any way.
Balconies, verandas, canopies, window guards, etc are potentially very dangerous. Both from working at heights perspective, and the risk of them falling on somebody below if there is a serious structural issue. This kind of maintenance works MUST be carried out by an expert.
Regular Inspections
The most important thing you can do to look after architectural cast iron is to have it periodically inspected by a qualified professional. You’ll be able to form a conditional assessment and ascertain areas where your cast iron may require intervention, if at all.
Annual inspections should check for:
Keeping cast iron clean should be part of your routine care plan. It is very easy to over-clean cast iron, causing damage to decorative elements or destroying protective coatings. That said, insufficient cleaning will reduce the longevity of the ironwork and predispose it to corrosion and deterioration.
The best approach should really be decided on an individual case basis. Historical significance, your budget and performance requirements will of course vary. Larger scale projects such as bare metal restoration and serious corrosion should only be undertaken by a professional.
Minor patch repairs can normally be undertaken by the owner but it is essential to follow some basic procedures to ensure the paintwork doesn’t fail.
Cast Iron has a good resistance to corrosion. However without proper care cast iron will still come under threat.
The main ways in which cast iron is likely to deteriorate are:
The biggest threat to cast iron is exposure to water and oxygen, together causing oxidisation (rust). Cast iron is at its most susceptible to rusting when exposed to high humidity or where the detailing of a casting forms areas likely to trap water.
Sacrificial or galvanic corrosion pertains to the deterioration of one metal to another when in direct contact in the presence of water. One metal will be nobler and the other baser (in this case cast iron).
The base metal will sacrifice material to the nobler causing deterioration. Copper, nickel and brass are examples of metals that cast iron will sacrifice to. Zinc, on the other hand, is a heavier metal than iron, meaning it will corrode instead of iron – hence its use in paint primer as a protective coating.
Due to its composition, cast iron is particularly brittle. On impact, where wrought iron will bend, cast iron will shatter. Keep in mind however, that what might appear to be a fracture on first inspection might not be. Sometimes, original casting defects can appear to look like breaks but are purely cosmetic.
Sudden thermal shock causes strain in a material by unequal expansion, either in the same material by high thermal gradients, or in interfaces between materials with different coefficients of thermal expansion.
In short, when subjected to high heats, such as fire damage, cast iron’s strength and stiffness will decline.
Now we understand the ways in which cast iron deteriorates, it’s easy to start to see external factors that can contribute to and accelerate its decay.
Corrosion isn’t always immediately visible but there are certain warning signs which might indicate your cast iron requires attention
Dependent on its use, cast ironwork will deteriorate in a range of ways. The most common problems are outlined below with a brief indication of likely repair techniques.
Missing or broken cast iron finials, collars and husks
Since these embellishments act like water traps, they are particularly susceptible to corrosion over time.
Normally, these can be replaced by matching from stock of architectural patterns, which Dorothea holds a selection of. Alternatively, a new pattern can be made to fit with the existing design.
Railing Bar Fracture
Brittle and vulnerable to fracture, impact damage is quite common with cast iron railing bars. Where the fractured pieces have survived, it may be possible to repair them through pinning (dependent on section thickness) or an alternative cold repair technique. Failing this, a replica casting can be inserted.
Thin sections of cast iron e.g. frieze panels
Corrosion to already thin sections of cast iron can lead to significant loss of surface material, and eventually perforation. Dependent on whether this thinning compromises strength or structural integrity re-profiling with epoxy based fillers may be sufficient. In the worst case scenario, re casting is likely to be necessary.
Original casting flaws
Flaws from the casting process, appearing as holes or folds in the cast iron, can be commonplace, forming water traps and inevitably accelerating the corrosion process. Regular cleaning, filling and repainting to help prevent oxidisation of the cast iron are likely courses of action.
Cast Iron, whilst not impossible, can be particularly challenging to restore. Where repair works are required, this should be done with the intention of retaining as much of the original cast metal as possible.
Restoration will not necessarily mean restoring back to original condition but stabilising and preserving the cast ironwork dependent on its age, historical significance and character.
The use of inappropriate techniques, can damage your cast iron further and lead to further long term problems, so works should always be undertaken by a recognised expert.
As a general rule we do not recommended the welding of cast iron.
The sudden input of heat can cause thermal shock to the cast iron and create more cracks and more problems. In some cases items can be welded by using a process of pre and post heating of the iron but this is best done under controlled conditions in a workshop environment.
Cold repair techniques such as reinforcing the cast iron with plates, straps, metal stitching, threaded studs and/or bonding using epoxy resin are all particularly suitable for cast iron repairs. For more details have a look at our techinical cast iron resources here.
Where it is necessary for new sections or components of cast ironwork to be replaced, replication can often be achieved. Matching castings can be obtained from particular foundries who hold certain patterns. Where this is not possible, what remains of the system can be used as reference for a new pattern to be made.
Once repaired, protection of the cast iron is imperative to ensure the corrosion process has been stabilised. Zinc rich primer paints are often used to act as a protective layer.
Click below for guides on other architectural metalwork features:
If you want to learn more, please visit our website Leveling Jack.