Iron forms a vital part of our lives and is, in fact, the backbone of many industries. Iron is used in many ways. It’s in buildings, infrastructure, vehicles, and equipment. The types of iron are diverse and have special properties. Knowing the different types of iron, like pig iron, wrought iron, and cast iron, helps us choose wisely. This article will explore the types of irons and their properties.
What is Iron?
Iron is a chemical element. Its symbol is Fe, from the Latin “ferrum.” It has a crystalline structure. Pure iron is a silvery-white, bright metal. It is dense, with a density of 7.86 g/cm³. It has a melting point of 1539°C. Iron is electrically and thermally conductive.
Iron is one of the most abundant and versatile metals on Earth. It makes up about 5.1% of the Earth’s crust, ranking fourth in abundance after oxygen, silicon, and aluminum. In nature, free iron can be found only in meteorites, and on Earth, iron exists primarily in compound form.
The main iron ores are hematite and magnetite. Hematite has 50-60% iron. Magnetite has over 60% iron. Other important iron ores are limonite and siderite.
Different Types of Iron and Properties
3 types of iron include pig iron, wrought iron, and cast iron.
Pig iron
Pig iron is a crude product from the smelting of iron ore in a blast furnace. It is an alloy of iron and carbon with a carbon content greater than 2%. Industrial pig iron has a carbon content of 2%-4%. It also contains C, Si, Mn, S, and P. It is hard and wear-resistant but has no toughness and almost no plasticity. So, it can’t be forged. Usually, it is dark gray or black, and the surface is rough with possible pores and impurities on it. According to the different forms of carbon in pig iron, it can be divided into basic pig iron and foundry pig iron.
Basic pig iron: It is characterized by having white color cross-sections. It has a high carbon content and includes silicon, manganese, sulfur, and phosphorus. Basic pig iron is hard and brittle and is used mainly for steelmaking.
Foundry pig iron: Its fracture is gray. Carbon, silicon, and impurities are high. Sulfur and phosphorus are low. Foundry pig iron has good cutting and casting properties. But, it has poor impact resistance and cannot be forged.
Wrought iron
What is wrought iron? Wrought iron is made by heating pig iron to a certain temperature. Then, it is melted to remove impurities and alloying elements. This makes it purer and softer. The carbon content of wrought iron is usually lower than 0.05%.
Wrought iron is ductile, elastic, and tensile, weldable, corrosion-resistant, and forgeable. It can be heated and worked into various shapes. The more it is processed, the greater its strength. It is suitable for tensile or compressive components.
Cast iron
Cast iron is an alloy containing iron, carbon, and silicon. It has 2%-4% carbon and 1%-3% silicon. Cast iron is made from pig iron, mainly foundry pig iron. It involves remelting pig iron by adding a mix of iron, steel, limestone, and carbon. Cast iron and pig iron differ in that cast iron is made by secondary processing.
Cast iron types could be classified by different standards. GB/T 5612-2018 lists 5 cast iron types: grey, ductile, vermicular, malleable, and white.
Gray cast iron
This is the most common type of cast iron. Most carbon in this type of cast iron is in the form of free-flake graphite. And its fracture is gray.
Gray cast iron contains flake graphite. Graphite has a low density, low strength, and low hardness. It has almost no plasticity or toughness. They are like many small gaps in the matrix. They reduce the bearing area and increase the crack source. So the gray cast iron has low strength and poor toughness.
Gray cast iron can be classified into three types. This depends on the matrix structure. They are ferrite-based, ferrite + pearlite-based and pearlite-based grey cast iron.
Ductile iron
Ductile cast iron has a small amount of magnesium and ferrosilicon. This changes the structure of its graphite. The graphite forms spherical nodules instead of flakes. This improves the material’s ductility, toughness, and tensile strength.
Ductile iron can be further improved in performance by heat treatment and alloying. For example, the treated ductile iron can upgrade toughness. It has an elongation of 24% and a tensile strength of 1400MPa. Ductile iron has good castability, wear resistance, and shock absorption. It’s low-cost, too. So, it is used to make auto parts like rear covers, connecting rods, and crankshafts.
Vermicular iron
Vermicular graphite cast iron is a cast iron. It is mixed with a magnesium-titanium alloy. This makes the graphite form a distinctive, worm-like shape. Many elements, like magnesium and cerium, affect the graphite in vermicular graphite cast iron. Low magnesium content will cause the graphite to be more like gray cast iron. High magnesium content will make it more like ductile iron.
Vermicular graphite cast iron is stronger than gray cast iron. It has higher tensile and yield strengths. They are close to those of ductile iron. Its impact toughness and elongation are between those of gray cast iron and ductile iron. Vermicular graphite iron has good wear resistance. It’s over twice that of gray cast iron.
Malleable iron
Malleable cast iron is a type of cast iron. It is made by heat-treating white cast iron. This changes its microstructure and improves its ductility and malleability. This process, called “annealing,” involves heating the white cast iron to a set temperature. Then, it is slowly cooled. This makes the graphite form a floccule shape.
In general, malleable cast iron is better in terms of toughness, strength, and ductility than other cast irons, such as gray or white cast iron. It has good wear resistance and machinability. So, it is useful where strength and slight deformation are needed.
Malleable cast iron has different metallographic structures. It can be divided into three types: blackheart, pearlitic, and whiteheart Malleable cast iron.
White iron
White cast iron is a type of cast iron that has a white, brittle fracture surface. It contains cementite in its microstructure. The carbon in white cast iron is present in the form of cementite or iron carbide, unlike in gray cast iron, which has graphite flakes. This gives it a smoother, lighter-colored fracture surface.
It has good hardness and wear resistance but is not easily machinable due to brittleness. In order to improve its properties, white cast iron is often further treated. White cast iron is rarely used directly and is only used for a few parts that require wear resistance but not impact.
Cast vs Wrought Iron: Understanding Their Difference
Both cast and wrought irons are iron-carbon alloys. But, they differ in composition and properties.
Composition
Cast iron is an alloy of iron and carbon with a carbon content of more than 2%. Some kinds of cast iron can contain carbon in as high a percentage as 6.67%. Other alloying elements in cast iron include silicon, manganese, phosphorus, and sulfur.
Wrought iron, on the other hand, is a relatively pure form of iron obtained by further refining pig iron. Its carbon content is extremely low, usually less than 0.05%. It is produced by melting pig iron and removing impurities.
Physical Properties
Cast iron has a high carbon content, so it’s hard and brittle with little plasticity. It possesses low toughness and is fragile, though this also adds to its high compressive strength and hardness. Cast iron has medium thermal conductivity. Its thickness lets it store heat during heating.
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Wrought iron has low carbon content, hence it is soft and ductile. It has lower strength and hardness but is easy to forge and weld. Wrought iron has excellent thermal conductivity. It heats up fast. This is useful for making cookware.
Price
As wrought iron needs more refining processes to remove impurities, it costs more and the price is relatively expensive. On the other hand, cast iron has a simpler refining process and therefore is cheaper in terms of cost.
Conclusion
China CNC machining manufacturers use various types of iron. It’s vital to choose the right one to meet specific performance needs. Both types of iron have unique properties. Cast iron is hard and wear-resistant. Wrought iron is ductile. Each is best for different uses. Knowing the properties of different types of iron is key. This includes tensile strength and machinability. It will help you make the right decision. It will ensure your project’s success and longevity.
At SogaWorks, we specialize in rapid prototyping and on-demand manufacturing. Supported by the strong network of high-vetted manufacturing plants, we deliver custom CNC machining services with a broad range of materials including different types of iron, stainless steel, aluminum, and more.
