What Is A Malleable Iron? Types Of Malleable Iron
What Is A Malleable Iron?
Malleable iron is a type of cast iron that has been heat-treated to make it more flexible and less brittle. It is made by casting white iron, which is a metastable carbide in a pearlitic matrix, and then annealing it to change its structure.
The process causes carbon to form small, spherical aggregates of graphite, leaving a matrix of ferrite or pearlite. There are three main types of malleable iron: blackheart, whiteheart, and pearlitic. Malleable iron has a long history, dating back to ancient China where it was used as early as the 4th century BCE. However, by the Tang Dynasty, its use had declined.
In the 17th century, it was mentioned in England in a patent, and in the 18th century, French scientist Réaumur conducted extensive research on it. In the United States, malleable iron production began in 1826 with the opening of a foundry by Seth Boyden.
Pig iron is a form of iron that is made by melting iron ore, charcoal, and limestone together under intense pressure. The result is a high-carbon iron material that is quite brittle and not suitable for use without further processing.
One way to improve the properties of pig iron is through a process called annealing. This involves heating the iron to a high temperature and then allowing it to cool very slowly over a period of about 24 hours. This controlled cooling process rearranges the chemical structure of the metal making it more malleable and less brittle.
Types Of Malleable Iron
There are two main types of malleable iron: whiteheart and blackheart. Whiteheart iron is lighter in color and has a different chemical composition than blackheart iron. It is made up of a surface of pure ferrite with a core of pearlite, temper carbon, and ferrite.
Blackheart iron, on the other hand, is mostly ferrite. A third type of malleable iron is called pearlitc malleable iron which is a type of blackheart iron that includes pearlite.
The International Organization for Standardization (ISO) has published a grading system for whiteheart and blackheart malleable iron, known as ISO 5922-2005. This system assigns a letter, two numbers for tensile strength, and two numbers for minimum elongation or ductility of the iron sample. This grading system is used to help determine the quality and suitability of the iron for different uses.
Advantages Of Malleable Iron
Malleable iron is a type of iron that exhibits good ductility, similar to other irons with carbon formed into spherical or nodular shapes. Despite being sometimes considered an “old” or “dead” material, malleable iron still has a place in the design engineer’s toolbox.
It is a good choice for small castings or castings with thin cross sections, as other nodular irons can be difficult to produce in these applications due to the formation of carbides from rapid cooling. Additionally, malleable iron exhibits better fracture toughness properties in low temperature environments than other nodular irons, due to its lower silicon content.
However, care must be taken in the annealing process to ensure that the iron casting will solidify with an entirely white iron cross section, as thicker sections will cool slowly and allow some primary graphite to form, lowering the fracture strength.
Some countermeasures can be applied to enhance the formation of the all-white structure, but malleable iron foundries often avoid producing heavy sections.
After the casting and heat treatment processes, malleable iron can be shaped through cold working, such as stamping for straightening, bending or coining operations, due to its desirable property of being less strain rate sensitive than other materials.
Applications Of Malleable Iron
Malleable iron is a type of iron that is known for its good tensile strength and ability to flex without breaking. It is commonly used for small castings such as electrical fittings, hand tools, pipe fittings, washers, brackets, fence fittings, power line hardware, farm equipment, mining hardware and machine parts.
In the past, heavy-duty bench vises were made from cast steel. However, due to its poor castability, the use of cast steel has decreased significantly and high-quality cast vises, as well as other tools and machine elements are now mostly made of malleable iron.
