steels phase diagrams

Question 1

What are some different phases and microconstituents found in steel?

Try to give their characterisitics and crystal structure

Answer 1

• Austenite (γ): FCC lattice, stable at high temp
• Ferrite (α): BCC lattice, stable at low temp
• Cementite (Fe3C): intermetallic of fixed composition, hard brittle phase
• Pearlite: microconstituent of ferrite and cementite, lamellar, eutectoid mixture.

Question 2

What is the difference between a phase and a microconstituent?

Answer 2

• Phase: Completely distinguishable, uniform physical and chemical characteristics
• Microconstituent: It is identifiable, but can consist of more than one phase. But looks like one thing under a microscope.

Question 3

Describe the three main features of this steel phase diagram.

Hint: the * and ** are two of the points, look at the %wt too!

Answer 3

• At the 6.7%wt C, only Cementite forms after so is vertical and is the cut-off point
• Eutectic point at * the top were L-> two solid phases
• Eutectoid point at lower **, where solid-> two solid phases

Question 4

Define and describe a eutectiod.

Answer 4

• General def: Eutectoid is like eutectic but instead you go from a solid to two different solids, (rather than a eutectic which goes from liquid to two solids).
• Eutectoid described : We hit the solibility line so no more carbon can fit and has to be removed, (such as removing carbon from the ferite phase). This process is a diffusion based process. The atoms need to defuse and need to move to the minimum distance possible (i.e. the grain boundaries). Atoms start to grow into the austenitic grain, froming the lamellar microstructure of ferrite and cementite.
• This is essentailly pearlite.

Question 5

Describe and try to draw how a hypoeutectoid mircostructure will form, using example below.

Answer 5

Note that the austenite just converts to pearlite.

Question 6

Describe and try to draw how a hypereutectoid mircostructure will form, using example below.

Answer 6

Question 7

What is meant by the proeutectoid?

Answer 7

Like the primary phase, its the inital phase that formed before the austenite is converted into pearlite.

Question 8

How do you use the lever rule to find the % of primary or proeutectoid formed and eutectoid formed on a steel phase diagram?

Answer 8

• Primary eutectoid %: oppos / total
• eutectoid % : oppos / total

Question 9

How does the wt% of carbon in steel effect the mechanical properties of steel?

Think of the different phases and their properties.

Answer 9