Ferrite is the softest and least strong constituent of steels.

Ferrite is a solid solution of carbon in iron with a body-centered cubic crystal structure. It is the purest form of iron and has a relatively low carbon content, typically less than 0.02%.

Here is a detailed explanation of why ferrite is the softest and least strong constituent of steels:

1. Composition:
- Ferrite is primarily composed of iron atoms with a few carbon atoms dissolved in it. The low carbon content makes it relatively soft and ductile compared to other constituents.
- Austenite, on the other hand, is a solid solution of carbon in iron with a face-centered cubic crystal structure. It has a higher carbon content than ferrite, typically between 0.02% and 2.11%. Austenite is stronger and harder than ferrite.
- Pearlite is a two-phase microstructure consisting of alternating layers of ferrite and cementite. It is stronger than ferrite but not as strong as martensite.
- Cementite, also known as iron carbide (Fe3C), is a hard and brittle compound of iron and carbon. It is the hardest and strongest constituent of steels.

2. Crystal Structure:
- Ferrite has a body-centered cubic (BCC) crystal structure, which allows for more slip planes and easier deformation. This results in its relatively soft and ductile nature.
- Austenite has a face-centered cubic (FCC) crystal structure, which provides less room for slip planes and makes it harder and stronger than ferrite.
- Pearlite consists of alternating layers of ferrite and cementite, combining the properties of both constituents. It is stronger than ferrite but not as strong as martensite.
- Cementite has an orthorhombic crystal structure and is the hardest and strongest constituent of steels.

3. Mechanical Properties:
- Due to its softness and ductility, ferrite is less strong and has lower hardness compared to other constituents of steels.
- Austenite is stronger and harder due to its FCC crystal structure and higher carbon content.
- Pearlite has improved strength compared to ferrite due to the presence of cementite in its microstructure.
- Cementite is the hardest and strongest constituent of steels, offering high strength but lower ductility.

In summary, ferrite is the softest and least strong constituent of steels due to its low carbon content, BCC crystal structure, and lower hardness and strength compared to other constituents like austenite, pearlite, and cementite.