Class 6

1. Class 6 Phase Diagrams And Microstructure

2. Homeworks • If everyone agrees, homeworks will be in the lab on a bench in alphabetical groups. • Placed there this lunchtime so can be collected after 1.00pm • Exam • Multiple Choice – eg how does sodium and chlorine bond?

3. Microstructures and Composition At eutectic composition, and temp, amount of a = 97.5-61.9/97.5-19.2 = 35.6/78.3= 45% amount of b = 61.9-19.2/97.5-19.2 = 42.7/78.3= 55% For 70% Pb 30% Sn, a phase if formed in a+liquid range, called pro eutectica. Amount of proeutectic a and eutectic a+b is obtained from inverse lever arm rule Amount of pro eutectic a = 61.9-30.0/61.9-19.2 =31.9/42.7=0.75=75% Amount of eutectic a+b = 30.0-19.5/61.9-19.2 = 10.5/42.4 = 0.25 = 25% The composition of the a will be the same independent of whether it is pro eutectic or eutectic at 80.8%Pb 19.2% Sn.

4. Lead Tin Microstructures 38.1%Pb 61.9% Sn 90 %Pb 10%Sn 70% Pb 30%Sn 50%Pb 50%Sn

5. Lead Tin Microstructures 15%Pb 85%Sn Equiaxed Single Phase Grain Structure

6. Iron Carbon (Fe3C) Phase Diagram • Primary solid solubility • in a is 0.022wt% C, for • is 2.14 wt% C. Eutectoid reaction is important for steels. • > a + Fe3C at 727C. At 726C , a 99.978%Fe 0.022%C Fe3C 93.3%C 6.7%C Fe3C is a compound and a Metastable phase, not an equilibrium phase, but stable unless thermally changed. Eutectoid composition Is Fe 99.24%, C 0.76%

7. Eutectoid Composition At 727C Eutecoid Reaction occurrs. • > a + Fe3C Above 727C all g, equiaxed. Below 727C, a + Fe3C Amounts of a and Fe3 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 Fe3C = 0.76- 0.022/ 6.7-0.022 = 0.738 / 6.678 = 0.11 = 11% Fe3C a composition is 99.978%Fe, 0.022 C Fe3C composition if 93.3%Fe, 6.7%C

8. Eutectoid Formation a Carbon moves from a g Fe3C Iron moves from carbide Growth Direction

9. Eutectoid Alternate ferrite and cementite plate structure in Eutectoid steel.

10. Hypereutecoid Strucutres 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 Fe3C on the existing g grain boundaries. Called “Pro Eutectoid” cementite. g composition follows phase boundary to the eutectiod composition, so at 727C the remaining g transforms to pearlite, the mixture of a + Fe3C. Cementite exists as Pro eutectoid and eutectoid forms.

11. 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 727C the remaining g transforms to pearlite, the mixture of a + Fe3C. Ferrite exists as Pro eutectoid and eutectoid forms.( Pearlite has narrow plates in this case of a steel with 0.08%C)