Class 7 Phase Diagrams And Microstructure Iron Carbon
Class 7 Phase Diagrams And Microstructure
Iron Carbon (Fe 3 C) Phase Diagram Primary solid solubility in a is 0. 022 wt% C, for g is 2. 14 wt% C. Eutectoid reaction is important for steels. g> a + Fe 3 C at 727 C. At 726 C , a 99. 978%Fe 0. 022%C Fe 3 C 93. 3%C 6. 7%C Fe 3 C is a compound a Metastable phase, not an equilibrium phase, but stable unless thermally changed. Eutectoid composition Is Fe 99. 24%, C 0. 76%
Eutectoid Composition At 727 C Eutecoid Reaction occurrs. g > a + Fe 3 C Above 727 C all g, equiaxed. Below 727 C, a + Fe 3 C Amounts of a and Fe 3 C given by Inverse Lever Arm Rule. Amount of a = 6. 7 – 0. 76/ 6. 7 -0. 022 = 5. 94/6. 678 = 0. 89 = 89% a Amount of Fe 3 C = 0. 76 - 0. 022/ 6. 7 -0. 022 = 0. 738 / 6. 678 = 0. 11 = 11% Fe 3 C a composition is 99. 978%Fe, 0. 022 C Fe 3 C composition if 93. 3%Fe, 6. 7%C
Eutectoid Formation a Carbon moves from a g Fe 3 C Iron moves from carbide Growth Direction
Eutectoid Alternate ferrite and cementite plate structure in Eutectoid steel.
Hypereutecoid Structures Hypereutecoid – more carbon then eutectoid >0. 76% Start in single phase austenite, g, range, grains of g. Cool into two phase region, then some g transforms to Fe 3 C on the existing g grain boundaries. Called “Pro Eutectoid” cementite. g composition follows phase boundary to the eutectiod composition, so at 727 C the remaining g transforms to pearlite, the mixture of a + Fe 3 C. Cementite exists as Pro eutectoid and eutectoid forms.
Hypoeutectoid Steels Hypoeutecoid steels – less carbon than eutectoid < 0. 76% Start in single phase austenite, g, range, grains of g. Cool into two phase region, then some g transforms to a on the existing g grain boundaries. Called “Pro Eutectiod” ferrite. g composition follows phase boundary to the eutectiod composition, so at 727 C the remaining g transforms to pearlite, the mixture of a + Fe 3 C. Ferrite exists as Pro eutectoid and eutectoid forms. ( Pearlite has narrow plates in this case of a steel with 0. 08%C)
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