The Iron-Iron Carbide Phase Diagram

Heat Treatment of Steel

Features

Peritectic L + d = g

Eutectic L = g + Fe3C

Eutectoid g = a + Fe3C

Phases present L

Reactions

dBcc structureParamagnetic

g austeniteFcc structureNon-magneticductile

a ferriteBcc structureFerromagneticFairly ductile

Fe3C cementiteOrthorhombicHardbrittle

Max. solubility of C in ferrite=0.022%

Max. solubility of C in austenite=2.11%

Microstructures

a g

SteelsSteel is an interstitial solid solution of carbon in iron.

Theoretically steel has a maximum of 2.11% carbon.

In practice, the amount of carbon rarely exceeds 0.8%

Classification/NomenclatureLow carbon steels up to 0.2%C

Medium carbon steels 0.2-0.4%C

High carbon steels >0.4% C

AISI 1020: Last two numbers indicateAmount of carbon :0.2%C10 indicates plain carbon steelAISI 4340: 0.4%C43 indicates alloy steel

Microstructural changes in steel on cooling for different compositions

Eutectoid steel

Hypoeutectoid steel

Hypereutectoid steel