General Properties of Iron-Carbon Alloys
a) Irons
Irons are very soft and exhibit low strength, yet they have high plastic workability. Their corrosion resistance is good, and this property is further enhanced by galvanizing and enameling processes. Iron pipes and sheets are galvanized by immersing them in a molten zinc bath. On the other hand, bathtubs and similar components are enameled by coating them with a ceramic film. In practice, galvanized pipes are very widely used in water and heating installations, while galvanized sheets are extensively used in structures.
б) Steels
In steels, as the carbon content increases, the amount of the hard and brittle iron carbide phase increases alongside the soft ferrite phase in the internal structure; deformation becomes more difficult, thus hardness and strength increase, while ductility decreases.
The modulus of elasticity of steels does not depend on carbon; low strength mild steels and high strength steels have the same modulus of elasticity, averaging 210,000 N/mm2.
The representative shapes of the stress strain diagrams of steels depending on the carbon content are shown in the figure below. In low carbon steels, the yield plateau is clearly observed; although strength is low, toughness is high. In high carbon steels, strength is high, but toughness is low. The temperature level at which the forming process is applied in steels affects the mechanical properties. Steels formed above the recrystallization temperature (hot rolling) are called naturally hard steels. The variation of the mechanical properties of hot worked naturally hard steels depending on the carbon content is given above. The recrystallization temperature in steels is around 600°C. Steels formed below this temperature are called cold worked steels. Their hardness and strength can vary greatly depending on the forming ratio. Low carbon steels have low hardness and strength, are easily machined, and easily welded. They cannot be hardened by quenching; their hardness and strength are increased by cold forming. They are the cheapest steels used for general purposes.
Medium carbon steels have high strength and toughness, and they can be processed by plastic forming. After forming, their hardness and strength can be increased several fold by quenching. These steels are generally used in applications where strength is required, such as structures, machine parts, steam boilers, and railway tracks.
Although high carbon steels have high hardness and strength, their ductility and toughness are low, and they are difficult to form. Their weldability is poor; localized hardening and embrittlement occur during welding, and cracking may take place. These steels are generally used as tool, die, and spring steels. If held for a long time at around 700°C prior to forming, the lamellar and network iron carbide phase takes a spherical shape. After this spheroidizing process, the steel consisting of spherical iron carbide grains dispersed within a soft ferrite phase is easily machined and subsequently hardened by quenching.
в) Cast Irons
White Cast Irons: In white cast irons, the main phase, iron carbide, is very hard and brittle; it cannot be machined by cutting or plastic forming. In practice, it is used where wear resistance is more important than strength. It is suitable for the production of wear-prone parts such as grinding balls in grinding mills in the cement industry, slurry pumps, excavation machinery, and similar areas. On the other hand, malleable cast iron is obtained from white cast iron. If annealed above 800°C for a long time, the unstable iron carbide compound decomposes into ferrite and fine particles of graphite. This structure, consisting of fine graphite clusters within a soft ferrite phase, is called malleable cast iron. The tensile strength of malleable cast iron is around 380 N/mm2 and its ductility is at the level of 20%. In practice, parts with complex shapes are first produced as white cast iron, then converted into malleable cast iron by annealing.
Gray Cast Irons: They are easy to produce because their melting temperature is much lower than that of steel; they fill the mold extremely well and shrink very little during solidification. Although brittle, they are soft and can be easily machined by cutting. Although their tensile strength is low, their compressive strength is superior to structural steels. They are considered the cheapest metal material. They are used in the production of parts not subjected to high stress, such as machine beds and drainage pipes.
Ductile (Nodular Graphite) Cast Irons: If approximately 1% magnesium (Mg) is added to liquid metal just before casting, the graphite structure disperses as spheres within the ferrite phase. This metal containing spherical graphite grains in a soft ferrite phase is called ductile cast iron or nodular cast iron. Its strength and ductility are very close to structural steel. It is particularly suitable for the production of high strength machine parts with complex geometries by casting.