What is Cast Iron?

Cast iron, an alloy of iron that contains 2 to 4 percent carbon, along with varying amounts of silicon and manganese and traces of impurities such as sulfur and phosphorus. It is made by reducing iron ore in a blast furnace. The melting temperature is usually ranging from 1.150C to 1.200C. The liquid iron is cast, or poured and hardened, into crude ingots called pigs, and the pigs are subsequently re-melted along with scrap and alloying elements in cupola furnaces and recast into molds for producing a variety of products. Iron alloys with less carbon content are known as steel.

A bridge made of cast iron (Iron Bridge) /England

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 (declining usage), cylinder blocks and gearbox cases (declining usage). It is resistant to destruction and weakening by oxidation (rust).

History of cast iron:

The earliest cast iron artefacts date to the 5th century BC, and were discovered by archaeologists in what is now Jiangsu in China. This is based on an analysis of the artifact’s microstructures. 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 5th century BC and poured into moulds to make ploughshares and pots as well as weapons and pagodas. 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.

Cast iron cookware has excellent heat retention properties and can be produced and formed with a relatively low level of technology. Types of cast iron cookware include panini presses, waffle irons, crepe makers, Dutch ovens, frying pans, deep fryers, woks, flattop grills and griddles and pan supports. Enameled cast iron is cast iron that has a vitreous enamel glaze. The enamel coating over the cast iron prevents rusting, eliminates the need to season the metal, and allows for more thorough cleaning. Furthermore, pigments used in the enameling process can produce vibrant colors.

The Chinese produced cast iron as early as the 5th century bc, and it was produced sporadically in Europe by the 14th century. It was introduced into England about 1500; the first ironworks in America were established on the James River, Virginia, in 1619. During the 18th and 19th centuries, cast iron was a cheaper engineering material than wrought iron because it did not require intensive refining and working with hammers, but it was more brittle and inferior in tensile strength. Nevertheless, its load-bearing strength made it the first important structural metal, and it was used in some of the earliest skyscrapers. In the 20th century, steel replaced cast iron in construction, but cast iron continues to have many industrial applications.

 

6th century B.C. Iron manufacture furnace drawn on Greek vase
6th century B.C. Iron manufacture furnace drawn on Greek vase

 

 Chill structure of Cast iron

 

It is believed that in 1700 B.C., the Hittite empire has first obtained iron by deoxidizing iron ore. It is assumed that they were producing iron products by using a batch method furnace which deoxidizes iron sand by charcoal and extracting iron, which is close to spongy pure iron that could be collected at the bottom of a furnace, to heat forge it. It is indicated in one of the epic poetries of Homeros of around 800 B.C. that iron was very much valuable. Probably, its value was way higher than gold.

Additionally, since it is also noted about a blacksmith, it could also be imagined that the hardening technology was already established by the era when Homeros lived. An odd fact is that in Europe, iron production by forging continued till 14th century while casting to pour in melted metal in a mould did not come into practice until after 14th century. The fact that weapons used in then Europe were knives and sables was maybe due to this difference in history of iron. The casting technology, which produces product by pouring melted metal into a mould, was first developed in China, approximately 7th century B.C. It is generally thought that the Chinese bronze ware technology achieved an extremely high temperature by utilizing bellows and from this line of technology, it is assumed that pig iron was developed by melting iron with high carbon content. An iron with high carbon content has low melting temperature of about 1150 degrees C as well as good flow property and these have enabled the casting of iron. The components of cast iron of around 4th century B.C. in China were about: C2.5-4.3%、Si0.1-0.2%、Mn0.01-0.2%、P0.1-0.5% 、S0.01-0.1%. Due to the low Si, this sort of structure is called as chill which does not have graphite and has hard and brittle characteristics. This brittle characteristic probably led to a factor for iron casting technology not being introduced to Europe until 14th century. It was not until 1779, when the Iron Bridge was constructed in England, that cast iron with non-chill structure and high Si content was found.

From 7th century B.C. to 18th century at the time of Industrial Revolution, cast iron was made hard and brittle. Thus, no wonder iron with stickiness which could be hardened by hardening used for forging products was more valued. However, by increasing Si content, the property of cast iron has shifted largely. This non-chill cast iron came to become an important material to support the industry. All sorts of casting products as bridge and cannon begun to appear on manufacturing line. For modern cast iron, there are two significant developments. One of it is the manufacturing method of spheroidal graphite cast iron


High strength flake graphite

based on inoculation technology which was developed by G. F. Meehan and O. Smalley in 1940s. By this technology, it became possible to produce high strength cast iron with tensile strength more than 300N/mm2 stably.

 

The second is spheroidal graphite cast iron which was discovered by Morrogh in England back in 1947. By this discovery, the manufacturing of cast iron with even higher strength of more than 800N/mm2 realized. By 1945 when Japan was about to experience the end of war, its cast iron technology was already greatly advanced. Japan took in the technology of post war foreign countries, improved it and established the highest level of casting technology in the world. Furthermore, from here on, the Japanese casting technology grew in tandem with the rapid progress of automobile industry.