Introduction to Cast Iron Castings

Posted on Aug 24, 2020

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Introduction to Cast Iron Castings

Cast iron has excellent fluidity due to the combination of high carbon and silicon content.The presence of iron in everyday life began around 1200 BC, covering a wide range of applications, from agricultural implements to weapons of war. Blacksmiths became a key profession, working with iron castings to change its properties and give the material a tool. There would be a forge in every village and town that produced sickles, plowshares, nails, swords, candlesticks, and more.

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. There will be another milestone for metals - the industrial revolution changed the way metals were produced and transformed into products including iron.

 

Cast iron

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

Although both steel and cast iron contain trace amounts of carbon and appear similar, there are significant differences between the two metals. Steel contains less than 2% carbon, which enables 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, and therefore has more than one microcrystalline structure present in the material.

This combination of high carbon content and the presence of silicon gives cast iron excellent castability. Various types of cast iron are produced using various heat treatment and processing techniques, including gray, white, malleable, ductile and compact graphite.

 

Cast iron bridge

Cast iron construction details are made by smelting the metal and pouring it into a mold.

 

gray iron

Gray cast iron is characterized by the flaky shape of graphite particles in the metal. When the metal is fractured, the fracture occurs along the graphite flakes, giving it a gray color to the surface of the fractured metal. The name gray iron comes from this characteristic.

It is possible to control the size and structure of the graphite flake matrix during production by adjusting the cooling rate and composition. Gray cast iron is not as ductile as other forms of cast iron and its tensile strength is also lower. However, it is a better heat conductor and has a higher level of vibration damping. It has a damping capacity of 20-25 times that of steel and is superior to 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 cast iron products with the largest volume.

Our hardscape products are made of gray cast iron. Vibration damping and wear resistance are properties that make it a suitable material for many street applications. Raw gray cast iron also produces a patina that protects it from damaging corrosion even on the outside.

:: Read more : Gray Iron Properties and Grades Explained

 

White iron

With the appropriate carbon content and high cooling rate, the carbon atoms combine with iron to form iron carbide. This means that there are little or no free graphite particles in the solidified material. When white iron is sheared, the cracked surface appears white due to the lack of graphite. The microcrystalline structure of cementite is hard and brittle with high compressive strength and good abrasion resistance. In some specialized applications, it is desirable that there should be white iron on the surface of the product. This can be achieved by using a good heat conductor to make the mold parts. This will quickly remove heat from the molten metal from that specific area, while the rest of the casting will cool down more slowly.

One of the most popular types of white iron is Ni-Hard Iron. The addition of chromium and nickel alloys gives this product excellent performance in low impact sliding abrasion applications.

White iron and hard ni-iron fall within the alloy classification in ASTM A532; "Standard Specification for Abrasion-Resistant Cast Iron".

 

Cast iron

White cast iron can be further processed into malleable cast iron by a heat treatment process. An extensive heating and cooling program breaks down the iron carbide particles, releasing free graphite particles into iron. Due to the different cooling rates and the addition of alloys, ductile iron with a microcrystalline structure is obtained.

 

Ductile iron (Ductile iron)

Ductile iron, or ductile iron, obtains its special properties by adding magnesium to the alloy. The presence of magnesium causes the graphite to form a spherical shape as opposed to the gray iron flakes. Composition control is very important in the production process. Small amounts of impurities such as sulfur and oxygen react with magnesium, affecting the shape of the graphite particles. Different grades of ductile iron are made by manipulating the microcrystalline structure around a graphite sphere. This is achieved by either the casting process or the heat treatment as the next processing step.

Because ductile iron deforms on impact rather than breaks into pieces, we use this material to make our cast iron posts. The impact profile of nodular iron makes it a good cast iron for bollards close to traffic.

:: Read more : How to choose the right grade of Ductile Cast Iron

 

Cast iron with compact graphite

Compact graphite iron has a graphite structure and related properties that are a mixture of gray and white iron. The microcrystalline structure is formed around blunt graphite flakes that are attached to each other. An alloy, such as titanium, serves to inhibit the formation of spheroidal graphite. Compact graphite cast iron has higher tensile strength and better ductility compared to gray cast iron. The structure and microcrystalline properties can be adjusted by heat treatment or the addition of other alloys.

 

MECHANICAL PROPERTIES OF CAST IRON

The mechanical properties of a material indicate how it reacts under certain conditions, which helps to determine its suitability for different applications. Specifications are set by organizations such as the American Society for Testing and Materials (ASTM) so that users can purchase materials with confidence that they meet the requirements for their application. This is the most commonly used cast gray iron specification is ASTM A48.

To qualify castings according to their specifications, the standard way is to cast a test bar along with technical castings. ASTM tests are then applied to this test rod and the results are used to qualify 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 to be welded, otherwise, cracks and damage may occur.

 

Cast iron welding

When welding, it is extremely important that the weld meets or exceeds the mechanical properties of the material to prevent cracks and failures.

A few common mechanical properties of cast iron include:

●     Hardness – material’s resistance to abrasion and indentation

●     Toughness – material’s ability to absorb energy

●     Ductility – material’s ability to deform without fracture

●     Elasticity – material’s ability to return to its original dimensions after it has been deformed

●     Malleability – material’s ability to deform under compression without rupturing

●     Tensile strength – the greatest longitudinal stress a material can bear without tearing apart

●     Fatigue strength – the highest stress that a material can withstand for a given number of cycles without breaking

 

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