Introduction to casting iron: history, types, properties and uses

As a versatile metal, cast iron has many unique applications in commerce and industry.

 

The presence of iron in everyday life began around 1200 B.C. and included a wide range of uses from agricultural tools to weapons of war. Blacksmithing became an important occupation, using iron to alter its properties and shape the material into tools. Every village and town would have a blacksmith store where scythes, plows, nails, swords, candlesticks, etc. were produced.

 

The discovery of the value of iron led to the so-called Iron Age due to the dominance of this material in social and military applications. Another milestone for metals would follow - the Industrial Revolution changed the way metals were produced and processed, including iron.

 

Types of iron

There are two main types of iron produced: wrought iron and cast iron. Of these, cast iron includes its own family of metals.

 

Wrought iron

The first type of iron produced and processed by blacksmiths was wrought iron. It is actually pure elemental iron (Fe), which is heated in a furnace before being forged (worked) with a hammer on an anvil. Hammering iron expels most of the slag from the material and welds the iron particles together.

 

New uses for wrought iron were discovered during the accelerated industrial revolution and related construction activities. Its high tensile strength (resistance to fracture when subjected to tension) made it ideal for use in beams in large construction projects, such as bridges and high-rise buildings. However, wrought iron for this purpose was largely abandoned in the early 20th century when steel products were developed for construction applications where its properties were superior to those of iron.

 

Wrought iron is known for its decorative pieces. 15th and 16th-century churches contain finely wrought iron pieces made by skilled craftsmen. In the modern world, balustrades, doors and pews are still custom-made pieces made of wrought iron.

 

Cast Iron

Cast iron is produced by smelting an iron-carbon alloy with a carbon content of greater than 2%. After melting, the metal is poured into a mold. The main difference in production between wrought iron and cast iron is that cast iron cannot be worked with hammers and tools. There are also differences in composition - cast iron contains 2-4% carbon and other alloys, and 1-3% silicon, which enhances the casting properties of the molten metal. Small amounts of manganese and some impurities, such as sulfur and phosphorus, may also be present. The difference between wrought iron and cast iron can also be found in the details of a chemical structure and physical properties.

 

Although both steel and cast iron contain trace amounts of carbon and look similar, there are significant differences between the two metals. The carbon content of steel is less than 2%, which allows the final product to solidify in a single microcrystalline structure. The higher carbon content of cast iron means that it solidifies as a heterogeneous alloy, so there is more than one microcrystalline structure in the material.

 

It is the high carbon content and the presence of silicon that gives cast iron its excellent castability. Various types of cast iron are produced using different heat treatment and processing techniques, including gray cast iron, white cast iron, malleable cast iron, ductile iron, and compacted graphite cast iron.

 

Gray Iron

Gray iron is characterized by the flake shape of the graphite molecules in the metal. When the metal breaks, a fracture occurs along with the graphite flakes, which gives a gray color on the surface of the fractured metal. The name gray iron is derived from this property.

 

By adjusting the cooling rate and composition, the size and matrix structure of the graphite flakes can be controlled during production. Gray cast iron is less ductile than other forms of cast iron, and its tensile strength is lower. However, it is a better conductor of heat and has a higher level of vibration damping. It has 20-25 times the damping capacity of steel, which is better than all other cast irons. Gray cast iron is also easier to machine than other cast irons, and its wear resistance makes it one of the most productive cast iron products.

 

Our hard landscape products are made of gray iron. Vibration damping and wear resistance are characteristics that make it a suitable material for many street applications. Raw gray cast iron also produces a copper green color that keeps it safe from damaging corrosion, even outdoors.

 

White Iron

At the right carbon content and high cooling rate, carbon atoms combine with iron to form iron carbide. This means that there are virtually no free graphite molecules in the cured material. When white iron is sheared, the cross-section is white due to the absence of graphite. The carburized microcrystalline structure is hard and brittle, with high compressive strength and good wear resistance. In some special applications, it is desirable to have white iron on the surface of the product. This can be achieved by using a good heat conductor to create a part of the mold. This will absorb heat from that particular area quickly from the molten metal while the rest of the casting cools at a slower rate.

 

One of the most popular grades of white iron is nickel hard iron. The addition of chromium and nickel alloys gives this product excellent performance for low impact, sliding wear applications.

 

White iron and nickel hard iron belong to the alloy classification known as ASTM A532; Standard Specification for Wear Resistant Cast Iron.

 

Malleable Cast Iron

White iron can be further processed into malleable cast iron through a heat-treatment process. Extended procedures of heating and cooling result in the decomposition of the iron carbide molecules, releasing the free graphite molecules into the iron. Different cooling rates and alloy additions produce malleable cast iron with a microcrystalline structure.

 

Ductile Iron

Ductile iron or nodular cast iron obtains its special properties by the addition of magnesium to the alloy. The presence of magnesium causes the graphite to form spheres rather than the thin flakes of gray iron. Composition control is very important in the manufacturing process. Small amounts of impurities such as sulfur and oxygen react with magnesium and affect the shape of the graphite molecules. Different grades of ductile iron are formed by manipulating the microcrystalline structure around the graphite spheres. This is achieved through the casting process or heat treatment as a downstream processing step.

 

Because ductile iron deforms on impact, rather than breaking into pieces, we use this material to make cast iron bollards. The impact characteristics of ductile iron make it a good cast iron for bollards near vehicle traffic.

 

Compacted graphite iron

Compacted graphite iron has a graphite structure and related properties, and is a mixture of gray and white iron. The microcrystalline structure is formed around interconnected blunt sheets of graphite. Alloys, such as titanium, are used to inhibit the formation of spheroidal graphite. Compared to gray cast iron, compacted graphite cast iron has higher tensile strength and better ductility. The microcrystalline structure and properties can be adjusted by heat treatment or the addition of other alloys.

 

Mechanical Properties of Cast Iron

The mechanical properties of material indicate the way it responds under specific stresses, which helps determine its suitability for different applications. Specifications are developed by organizations such as the American Society for Testing and Materials (ASTM), so users can be confident that they are purchasing a material that meets the requirements of their application. The most commonly used specification for gray cast iron is ASTM A48.

 

To qualify a cast product to its specification, it is standard practice to cast a test bar along with an engineered casting. The ASTM test is then applied to that test bar and the results are used to determine the quality of the entire batch of castings.

 

Specifications are also important when welding cast iron parts together. The weld must meet or exceed the mechanical properties of the material being welded together - otherwise, fractures and failures may occur.

 

Some common mechanical properties of cast iron include.

Hardness - the wear resistance and indentation of the material

Toughness - the ability of the material to absorb energy

Ductility - The ability of a material to deform without fracture

Elasticity - The ability of a material to recover its original dimensions after deformation

Ductility - The ability of a material to deform under compression without breaking

Tensile strength - the maximum longitudinal stress that a material can withstand without tearing

Fatigue strength - the highest stress a material can withstand in a given number of cycles without fracture

 

Common Applications of Cast Iron

The various properties of different types of cast iron result in each type being suitable for a specific application.

 

Gray Iron Applications

One of the key properties of gray cast iron is its ability to resist wear even in situations where lubrication supplies are limited (such as the upper cylinder wall of an engine block). Gray cast iron is used in the manufacture of engine blocks and cylinder heads, manifolds, gas burners, gear blanks, housings and casings.

 

White Iron Applications

The cooling process used to make white iron produces a very wear-resistant, brittle material. Therefore, it is used in the manufacture of mill linings, shot blast nozzles, railroad gate tiles, slurry pump housings, mill rolls and crushers.

 

Nickel hard iron is used specifically for mixer paddles, augers and dies, ball mill liners, coal chutes and wire guides for wire drawing.

 

Ductile Iron Applications

Ductile iron itself can be divided into different grades, each with its own characteristic specifications and the most suitable applications. It is easy to machine, has good fatigue and yield strengths, and is also wear-resistant. However, its best-known characteristic is its ductility. Ductile iron is used in the manufacture of steering joints, plow heads, crankshafts, heavy gears, car and truck suspension components, hydraulic components, and automotive door hinges.

 

Malleable Iron Applications

Different grades of malleable iron correspond to different microcrystalline structures. Specific properties that make malleable iron attractive are its ability to retain and store lubricants, non-abrasive wear particles, and porous surfaces that trap other abrasive debris. Malleable iron is used for heavy bearing surfaces, chains, sprockets, connecting rods, driveline and axle components, railroad rolling stock, and agricultural and construction machinery.

 

Compacted Graphite Iron Applications

Compacted graphite iron is beginning to be widely known in commercial applications. The properties of gray and white cast iron combine to form a high strength and high thermal conductivity product - suitable for diesel engine blocks and frames, cylinder liners, train brake discs, exhaust manifolds, and gear plates in high-pressure pumps.

 

Machined and Finished

The hardness characteristics of cast iron require careful selection of machine tool materials. Coated carbide is effective in production machining environments, but new materials are being developed as technology advances.

 

The surface finish of cast iron products varies from application to application. A few common applications.

Electroplating

Hot dipping

Thermal spraying

Diffusion coating

Conversion coating

Vitreous Enamel

Liquid organic coatings

Dry powder organic coatings

 

Cast Iron and the Future

Iron has been an integral part of human society since its early use more than 3,000 years ago. Iron production has come a long way since blacksmiths made the iron from it over the centuries, to the invention of cast iron in the industrial age.

 

Since that time, wrought iron has become largely obsolete, except for decorative purposes. Cast iron, by contrast, is still progressing in terms of composition, microstructure and mechanical properties - continuing to make its mark on the modern world.

 

For more information on metal castings, or to request a quote for a custom metal project, please contact us.