2090 – 03 – Understanding the Effects of Carbon Content

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Steel in the ferrite phase isn’t able to hold as much carbon in the austenite phase. At the start of manufacturing, steel is in the austenite region at higher temperatures. At an elevated temperature, solid steel that has less than 2.11% carbon is able to retain carbon — even in a dissolved state. Austenite steel’s crystal structure has gaps that store carbon.
FCC structure changes to BCC structure when the steel eventually cools down. The ferrite steel’s BCC crystal structure has fewer gaps that can store carbon. And so, when the temperature increases, the carbon that was stored in the crystal structure becomes a precipitation of cementite. This process is called precipitation because, just like the way temperature dictates whether it snows or rains, temperature controls the amount of carbon that forms in steel.