Phases of steel Flashcards
What does it mean with polymorphism of iron?
Means that several elements and alloys under different temperatures and pressure can exist in more than one crystalline form.
IOW: Iron is a material that can occur in several different forms due to different values of the temperature and pressure.
Which are the three main phases of Iron due to the temperature?
Alpha-iron: Also called Ferrite.
- It is a solid form stable at room temperature below 912 and atmospheric pressure.
- BCC.
- The carbon content is low, max value is 0.036% at 727 degree C.
Gamma-Iron: Also called Austenite
- Stable at higher temperatures, 912-1394
- FCC
- Carbon has a higher solubility, max 2.1% at 1148 degree C.
Delta-Iron:
- Stable at very narrow temperature range, from 1394 - 1536
- BCC
- Carbon solubility is low again, max 0.1 %
At what temperature is the melting point of iron with zero carbon content?
1538 if there is no carbon content
What is a phase diagram?
The phase diagram shows how iron changes due to the amount of carbon in it and due to changes of temperature.
It only shows equilibrium phases.
What happens if we add carbon into the different phases ferrite, austenite and delta-ferrite?
New phases will occur, ex:
Ferrite + carbon = “Ferrite + Cementite”
Austenite + carbon = “Austenite + Cemenite”
Delta-ferrite + carbon = Liquid phase
Pearlite: not a phase, but occurs when austenite is cooled down slowly and forms ferrite+cementite.
Martensite: Metastable phase, formed from rapid cooling of austenite, it is not an equilibrium phase.
What is the difference in the material properties of the different phases?
Ferrite: Soft and ductile, making it the basis for many steels.
Austenite: More ductile and can dissolve more carbon than α-iron, making it crucial for austenitic steels.
The presence of a cementite phase can have a significant strengthening effect which is part of the reason steel is much stronger than pure iron.
Which are the critical points in the phase diagram?
Some special transofrmations in the phase diagrams are:
* 1495 & C = 0.18% is called Peritectic point → transformation from liquid to solid.
* 1148 & C=4.3% is called Eutectic point → liquid transforms into two solid phases
* 727 & C= 0.8% is called Eutectoid point → a single solid phase transforms into two solid phases when cooling
Some critical points are:
* A1: Eutectoid transformation line: Austenite - Perlite
* A3: Limit existence of alpha-ferrite: Austenite - Ferrite
* Acm: Solubility limit of carbon in austenite: austenite - cementite
What is martensite?
Martensite is an example of a non-equilibrium phase
It is produced through quenching - which does not allow diffusion to happen → Called diffusionless transformation
What is hypoeutectoid steel, eutectoid steel, and hyper eutectoid steel?
Classifications of carbon steels based on their carbon content.
* Hypoeutectoid: Typically between 0.02% and 0.76% carbon. Mix of ferrite and perlite
* Eutectoid steel: Exactly 0.76% carbon. (avrundas till 0.8%). 100% perlite
* Hyper eutectoid: between 0.76% and 2.11% carbon. Mix of perlite and cementite.
List the compostition of iron-carbon alloys and classification
- Iron: When the carbon content is zero or so small that it does not interfere with the physical characteristics
- Steel: is an iron carbon alloy with carbon content included in the range 0%, or slightly less, to 2,1%.
- Cast iron: is an iron carbon alloy with carbon content included in the range 2,1% - 4,5%. Cast irons are characterised by the presence of graphitic carbon (in this case the stable Fe-C phase diagram is the reference one).
