Types of Microstructure of Steel

This article demonstrates the different types of microstructure of steel such as: Ferrite, Austenite, Cementite, Pearlite and Martensite.

Steel can be classified on the basis of various factors. One of them is microstructure. Read through this guide as we illustrated the classification of steel by its microstructure.

Microstructure of Steel: Introduction

Steel microstructure is a function of presented carbon content. As we illustrated in the previous article, microstructure of steel is modified through the heat treatment (hot and cold forming) of steel. Heating and cooling operations are used to manipulate chemical composition and other characteristics to shift the microstructure from one form to another. The microstructure of steel cannot be seen with the human eye but can made visible by magnification over 25 times using microscope.

Types of Microstructure of Steel

There are a variety of types of microstructure of steel such as:

1. Ferrite
2. Austenite
3. Martensite
4. Cementite
5. Pearlite

1. Ferrite

Ferrite is the steel microstructure having very low carbon content. In the microstructure of ferrite, carbon atoms are presented only in a small fraction of interstices of the iron lattice. This is a body centered cubic (BCC) type crystal structure that is soft and ductile, like pure iron. The quantity of carbon in the interstices is limited to 0.02% at 1,340 °F (725 °C) without affecting the ferrite microstructure, however at room temperature it drops to 0.006%.

2. Austenite

Austenite is another microstructure of steel which is generated when iron-based alloys are heated over 1500 °F (815 °C) but below to 1800 °F (982 °C). This microstructural phase can contain up to 2% carbon at 2,100 °F (1,150 °C). It is a face-centered cubic (FCC) type crystal structure. So the carbon molecules in austenite microstructure are more densely packed than those in ferrite that results in higher strength levels.

3. Martensite

The rapidly cooling of austenite (above 86 °F) limits the quantity of carbon atoms inside the iron lattice that diffuse from the crystal structure, resulting in martensite formation. It has a tetragonal crystalline structure that is body-centered. Martensite microstructural phase is associated with high level of hardness but usually undergoes further heat treatment to reduce it's distortions and improve toughness. Martensite microstructure has typically low-carbon content with about 12% chromium content.

4. Cementite

When carbon steel is heated to the austenite temperature (815 °C to 982 °C) and subsequently cooled in the presence of no alloy to maintain the austenite shape, the microstructure changes to ferrite. However, if the carbon level exceeds 0.006%, the excess carbon atoms bond with iron to create iron carbide (Fe₃C), that is known as cementite. Cementite does not form on its own since a portion of the material is ferrite. Cementite structure is hard but brittle and can strengthen steels.

5. Pearlite

Pearlite microstructure is a laminated composite made of alternating layers of ferrite and cementite. When the steel is progressively cooled, it generates a eutectic combination. In eutectic combination the two molten materials crystallize at the same time. In the eutectoid transformation, austenite is converted into intimately fixed solid phases called ferrite and cementite resulting in alternating layers within the microstructure.

These are the 5 types of microstructure of steel. Hope you have understood the steel structure and its types very well. To get similar information about steel and other construction materials, visit this website.

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