Phase Diagrams Flashcards
Under what conditions do phase diagrams apply? Explain.
Phase diagrams only apply in near equilibrium conditions. Therefore assume very slow changes in temperature as this allows time for carbon atoms to diffuse through the solid material.
State the Eutectoid Reaction.
Y (gamma) —-> a (alpha) + Fe3C
What temperature does the eutectoid reaction occur at?
723 Degrees C
What are the three main types of steel and give the carbon content of each.
- Eutectoid - %C=0.76
- Hypoeutectoid - %C<0.76
- Hypereutectoid - %C>0.76
How much pearlite does a eutectoid steel have?
A eutectoid steel has 100% pearlite.
Explain the mircostructual changes which occur in a hypoeutectoid reaction.
In the Austenite region, the alloy is a solid solution of FFC iron and (x<0.76% C). As the temperature decreases, ferrite starts to form at the grain boundaires (a+Y region). Then the Eutectoid reactions occurs. Below 723C, all remaining austenite transforms to pearlite.
Explain the microstructural changes which occurs in a hypereutectoid reaction.
In the austenite region, the alloy is a solid solution of FCC iron and (x>0.76% C). As the temperature lowers, cementie starts to form at the grain boundaries (Y+Fe3C region). The amount of Fe3C increases as we approach the eutectoid reaction. After the reaction, all remaining austenite turns to pearlite.
Pearlite may be regarded as a composite material, state the properties of its components.
Pearlite is made of alternate layers of ferrite and cementite (Fe3C).
- Ferrite is soft and ductile
- Cementite is hard and brittle
What is the composition of the three types of steel?
Hypoeutectoid - Ferrite + Pearlite
Eutectoid - 100% Pearlite
Hypereutectoid - Pearlite + Cementite
Name the three main diagrams used to detail phase transformation in the iron-iron carbide system.
- The iron-iron carbide phase diagram (binary)
- The isothermal transformation diagram
- The continuous cooling transformation (CCT) diagram
