A Study about CAST IRON - Ferrous Alloy
Cast iron is a ferrous alloy that contains components like iron, carbon and silicon. It is mainly composed of iron (more than 2 %) and silicon (above 0.1%). Cast iron constitutes large amounts of carbon when compared to steel. Based on the solidification paths, different components are formed from the ferrous alloy. The meta-stable path leads to iron carbide while stable path results in the formation of graphite. The differences in the eutectic constituents of iron leads to variations in the mechanical properties. This makes it suitable for different industrial applications.
Classification of Cast Iron:
I. Common classification:
1. White Iron
2. Grey Iron
3. Ductile Iron
4. Malleable Iron
During formation of White iron, the ferrous alloys follow a meta-stable solidification path. Due to this path, the resultant iron does not contain graphite. Meanwhile, the structure contains complex carbides. Large amounts of these iron carbides result in the formation of flakes and white crystalline fractures on the metal surface.
White iron is most commonly used in decorative applications, grinding mills, manufacture of kitchen equipment, ball bearings, etc.
The formation of Grey iron requires few basic structural necessities like absence of carbide, shape and placement of graphite. A stable solidification path of the ferrous ally results in Grey iron. The slow cooling process during its formation makes it more ductile. The stability in the solidification path results in the development of grey flakes on the graphite regions.
Grey cast irons are used in a wide range of applications such as gears, crankshafts, pipe fittings, hydraulic parts, machinery beds, etc.
Ductile irons are formed when Magnesium is induced into the ferrous alloy composition. This process is known as Inoculation where the resultant graphite forms spherulite. For this reason, ductile iron is otherwise known as Spheroidal graphite. As the name represents, Ductile irons exhibit better ductility when compared to grey cast iron.
Ductile iron can be used for manufacturing calipers, front wheel spindles, gear boxes, machine frames, etc.
When ferrous alloys solidify by heat treatment with white iron, nodular graphite called malleable iron is formed. These nodules are irregular in shape but exhibit high deformation causing it best in thinner casting applications. Apart from this, malleable iron is used in various applications that include vehicle components, pipe fittings, etc.
II. Micro-structural classification:
1. Graphite Shape:
III. Commercial Classification:
1. Common cast iron:
The iron with no constituents of alloys or consists of very less alloy are common cast irons. They are used for general applications.
2. Special cast iron:
Special cast iron contains high amounts of alloys in their composition. The presence of high- alloy elements results in various advantages in the micro-structural properties such as resistance to corrosion and wear.
Factors influencing the metallurgy of cast iron:
Several factors must be considered to yield the right type of cast iron for varying industrial applications.
1. Graphite content in a cast iron alters the hardness of the metal.
2. The amount of Silicon present in the micro-structure influences the critical temperature value of the metal
3. Addition of Carbon and Silicon causes the following changes:
a. Graphitization potential increases
b. Casting ability increases
c. Decreases cooling properties
d. Reduces strength of the metal
4. The absence of manganese in iron leads to the formation of iron sulphide
5. The size of graphite directly influences the strength of the iron
6. Addition of phosphorous hardens the metal
7. Nucleation: The addition of manganese and sulphur causes neutralization of both elements thereby increasing the solubility of cast iron
8. Addition of graphite and removal of carbon from the micro-structure alters carburization of the metal
9. The size of the metal is influenced by the cooling rate.
10. Size of the metal affects the sensitivity and strength
11. Abrasion resistance is increased by the increased presence of carbide
12. Chromium, Silicon and Nickel combines with iron and makes it more corrosion resistant
13. The stability of the micro-structure is maintained by the heat treatment of the metal
14. Effect of minor alloying elements:
a. Magnesium: Forms spheroidal graphite
b. Deleterious: Deteriorates graphite
c. Silicon & Aluminum: Increases graphitization potential, increases ferrite causing lower strength and hardness
d. Nickel, Copper: Increase pearlite content strengthening and hardening the metal
e. Chromium, Tungsten and Vanadium: Decreases graphitization potential
15. Liquid treatment:
a. Modification: Addition of Manganese causes the flakes in the metal to convert into spheroids
b. Inoculation: Induction of elements like Aluminum, Calcium and Manganese increase the nodule count in the structure
CAST IRON - Mechanical Properties
1. Tensile Strength - For iron casting tensile strength is the most important physical property.
2. Yield Strength - It’s another key essential to define the strength of cast iron.
3. Elongation - The higher elongation it has, can be pulled to longer, which means they have better ductility. Ductile iron has higher elongation when comparing with gray iron.
4. Hardness - For machining parts the harness should not be too high, which will then affect the quality of machining. At the same time too low will affect the application of the part. Often it is demanded for Rockwell hardness testing to make the standards are fulfilled.