microstructure of low carbon steel

 

Low Carbon Steel ( Fe 0.18wt%C )

This 0.18wt% carbon steel has been normalised (slowly cooled from high temperature).
Above about 850°C, the microstructure consists of austenite. This tranforms to ferrite as the steel cools. The amount of ferrite increases as the temperature decreases, while the amount of austenite decreases. The solubility of carbon is much lower in the ferrite than in the austenite, so the carbon concentration of the austenite increases as the temperature decreases.
At approximately 730°C the remaining austenite, which now has a carbon concentration of about 0.8wt%, transforms for pearlite. This is the eutectoid of ferrite and iron carbide, Fe₃C. The iron carbide is also known as cementite.

At low magnifications, the pearlite is the dark phase, and the light phase is the ferrite. The amount of pearlite in this steel is quite low due to the low carbon content.
Compare this steel with the 0.3wt%C steel and the 0.8wt%C steel.
At higher magnifications, the lamellar eutectoid structure of the pearlite can be observed. This structure is due to the simultaneous formation of ferrite and Fe3C from the austenite. The spacing of the lamellae increases with slower cooling rates.
This material is commonly called Mild Steel. The carbon content of mild steels is typically 0.1 to 0.2wt% carbon. They have moderate strength and high ductility. They are easily machined, formed and welded.
The surface hardness of low carbon steels can be increased by carburisation.

Low Carbon Steel ( Fe 0.1wt%C )

0.1wt% carbon steel.
This low carbon mild steel has been normalised and has a very similar microstructure to the 0.18wt% carbon steel.
Typical applications include panels, nails and wire. It has high ductility and low strength in the normalised condition. The strength can be increased by work hardening.




link :
http://pwatlas.mt.umist.ac.uk/internetmicroscope/micrographs/microstructures/more-metals/steel/low-carbon-steel/fe-0.18-normalised.html
http://pwatlas.mt.umist.ac.uk/internetmicroscope/micrographs/microstructures/more-metals/steel/low-carbon-steel/fe-0.1-normalised.html