Cast iron Flashcards
What is the carbon content of cast iron?
Between 2.1-6.7% carbon.
What is a metastable phase diagram?
If one of the following conditions is NOT met, the metastable diagram is followed:
- cast iron (carbon content > 2.1%)
- enough Si%
- slow cooling rates
The metastable diagram si shown in SOLID LINES.
What is the stable phase diagram?
If all of the following requirements are fulfilled the diagram is stable:
- cast iron (C content > 2.1%)
- enough Si
- slow cooling rates
The stable diagram is shown in DASHED LINES.
What diagram (stable or metastable) is used for white cats iron?
Metastable (solid lines) - Fe + Fe3C
What diagram (stable or metastable) is used for grey cast iron?
Stable (dashed lines) - Fe + C
What are some mechanical properties of white cast iron?
Very hard and brittle, unmachinable. Tempering is required to use this material.
Application: rollers for rolling mils
What is the development of microstructure for white cast iron?
Before eutectic point - austenite dendrite form + ledeburite
After eutectic point - cementite plates + ledeburite.
What is the final microstructure of white cast iron after crossing the eutectoid tie line?
Before the eutectic point - all the austenite transforms into perlite, dendrites, and the austenite layers in the ledeburite (the microstructure will show dendrites and a eutectic structure).
After the eutectic point - same structure but with cementite plates, austenite still transforms into perlite after crossing the eutectoid tie line.
How does tempering influence the microstructure of white cast iron?
Tempering helps regain some ductility in the material.
The cementite decomposes into graphite nests and ferrite.
What dare some mechanical properties of gray cast iron?
Gray cast iron contains high amounts of carbon and silicon.
It is weak and brittle in extension because of the graphite lamellae which act like cracks.
During the compression test the graphite lamellae don’t have such a big influence.
The graphite lamellae influence a lot the mechanical properties.
What is the development of the microstructure of gray cast iron?
Because gray cast iron follows the stable phase diagram (solid lines) this leads to the formation of austenite grains and graphite lamellae after crossing the eutectic line.
The final microstructure will be ferrite + graphite lamellae (ferritic lamellar cats iron).
How does the cooling rate influence the development of gray cast iron microstructure?
Fast cooling rates
- material switches to the metastable phase diagram
- diffusion to grow graphite lamellae is not rapid enough
- after the eutectic tie line the cementite grain boundaries start to form
- the final microstructure will be pearlite grains with cementite grain boundaries - PEARLITIC GRAY CAST IRON.
What is the difference between slow cooling rates and fast cooling rates in the microstructure of gray cast iron?
At slow cooling rates, the material follows the stable phase diagram, while at fast cooling rates, it switches to the metastable phase diagram.
The final structure for slow cooling rates is ferritic lamellar gray cast iron.
The final structure for fast cooling is perlitic gray cast iron.
What are some mechanical properties of nodular cast iron?
Thanks to the added Mg, nodular cats iron is more ductile than other cast irons.
It also presents a higher tensile strength, yield strength and elongation.
What phase diagram (stable or metastable) does nodular cast iron follow?
Stable - 3.5% C + enough Si + slow cooling + Mg
