Download Iron-Carbon Phase Diagram: Properties and Transformations of Steels and Cast Irons and more Study notes Engineering in PDF only on Docsity!
Iron-Carbon Phase Diagram Its defined as:- A map of the temperature at which different phase changes occur on very slow heating and cooling in relation to Carbon content. is Isothermal and continuous cooling transformation diagrams for plain carbon and alloy steels.
Iron- Carbon diagram shows 1- the type of alloys formed under very slow cooling. 2- Proper or ( suitable ) heat-treatment temperature. 3- the properties of steels and cast irons can be depend on changed by heat-treatment.
In their simplest form, steels are alloys of Iron (Fe) and Carbon (C).
Notes: - This graph, which is known as iron- carbon equilibrium diagram has the following important points: -
1- The percentage of carbon is between 0% and 6.67%. Because:- 6.67 % carbon is maximum ratio of carbon can be dissolved in iron.
after 6.67 % carbon , all metallic alloys are non-important alloys in engineering applications.
2- Iron contains from (0% to 1.7% C ) known as steel.
3- Steel contains below approximately ( 0% to 0.83 % C) known as hypoeutectoid steel consist of primary ferrite and pearlite phases.
4 - Eutectoid steel (carbon content 0.83%) entirely consists of pearlite.
5- steel contains from (0.83% to above 1.7 % C) known as hypereutectoid steel consist of primary cementite and pearlite phases.
6- Iron contains carbon more than 1.8 % C known as cast iron.
There are two iron-carbon equilibrium diagrams:
1- stable iron-graphite Fe-Gr. The stable condition usually takes a very long time to develop.
2- metastable iron-cementite Fe-Fe 3 C
While The metastable diagram is of more interest or important.
The Iron–Iron Carbide (Fe–Fe 3 C) Phase Diagram
The iron at room temperature to , was stable and called as (Alpha iron) or ( α- ferrite ), and have a body center cubic structure ( BCC) , is ductile but not very strong..
After 912 C^0 to 1395 C^0 , the structure transform to a face center cubic structure (FCC) and form new phase called as ( Camma iron ) or (γ-Austenite ).
From 1395 C^0 to 1539 C^0 , the structure transform to a body center cubic (BCC) and form new phase called as ( Delta iron ) or (δ-ferrite ).
1539 C^0 is melting point of iron , and after this degree the iron transform to liquid phase.
Cooling Curve of pure iron :-
The diagram shows three horizontal lines which indicate isothermal reactions (on cooling / heating process ):
1-First horizontal line is about at 1493 °C, where peritectic reaction takes place: Liquid + ↔ austenite
2-Second horizontal line is at about (1130°C to 1147C) , where eutectic reaction takes place: liquid ↔ austenite + cementite
3-Third horizontal line is at 723°C, where eutectoid reaction takes place: austenite ↔ pearlite (mixture of ferrite & cementite)
Delta region of Fe-Fe carbide diagram
Liquid + ↔ austenite
Phases in Iron Carbon Phase Diagram
1- α -ferrite interstitial solid solution of Carbon in BCC iron (Fe). Stable form of iron at room temperature to 912 C. The maximum solubility of Carbon is 0.022 wt% at 727°C. Transforms to FCC γ-austenite phase at 912 °C. it dissolves only 0.008 % C at room temperature..
Properties it is ductile, highly magnetic and it has a low tensile strength of approximately 2800 Kg/cm^2. Its soft phase. 2- γ -austenite
interstitial solid solution of Carbon in FCC Fe. The maximum solubility of Carbon is 2.14 wt % at 1147°C. Transforms to BCC δ-ferrite at 1395 °C. Is not stable below the eutectic temperature (727 ° C) unless cooled rapidly. It is stable above 727°C. This phase plays an important role in the phase transformations of steels. High formability, most of heat treatments begin with this single phase_._ It is normally not stable at room temperature. But, under certain conditions it is possible to obtain austenite at room temperature.
FCC structure Properties it is generally soft, ductile, non- magnetic and it is denser than ferrite. Summary of austenite transformations
1- Austenite…… slow cooling ………… Pearlite (α + Fe 3 C). 2- Austenite…… moderate cooling……. Bainite (α + Fe 3 C). 3- Austenite ……rapid quenching………martensite (BCT phase).
3- δ -ferrite solid solution of Carbon in BCC Fe The same structure as α-ferrite. Stable only at high Temperature , above 1395 °C. Melts at 1539 °C. Maximum carbon solubility: 0.09-0.10 wt.%. BCC structure Paramagnetic
4- Cementite( Fe 3 C)
This intermetallic compound is metastable, It is a product which contains 6.67% carbon and 93.33% iron by weight. It is found in steel containing over 0.8% carbon when it cools. The amount of cementite increase with increasing the percentage of carbon in iron. is very hard, brittle intermetallic compound of iron & carbon , can strengthen steels , with chemical formula Fe 3 C. as Fe 3 C, contains 6.67 % C.
0.76 wt% Carbon , at 727 °C
γ (0.76 wt% C) ↔ α (0.022 wt% C) + Fe 3 C
in Eutectoid reaction , the two-phase mixture (ferrite & cementite). They are steels.
2- Eutectic reactions :
4.30 wt% Carbon , at 1147 °C
L ↔ γ + Fe 3 C In Eutectic reaction , alloys called cast irons.
Eutectic and eutectoid reactions are very important in heat treatment of steels.
3- Peritectic L + δ = γ Peritectic reaction , at 1493 deg.C, with low wt% C alloys (almost no engineering importance).
Development of Microstructure in Iron - Carbon alloys
Microstructure depends on composition (carbon content) and heat treatment. In the discussion below we consider slow cooling in which equilibrium is maintained. 1- Microstructure of eutectoid steel
0.76 wt%C, 727 °C γ (0.76 wt% C) ↔ α (0.022 wt% C) + Fe 3 C
When alloy of eutectoid composition (0.76 wt % C) is cooled slowly it forms perlite , a lamellar or layered structure of two phases: α-ferrite and cementite (Fe 3 C).
The layers of alternating phases in pearlite are formed for the same reason as layered structure of eutectic structures: redistribution C atoms between ferrite (0.022 wt%) and cementite (6.7 wt%) by atomic diffusion.
Mechanically , pearlite has properties intermediate to soft, ductile ferrite and hard, brittle cementite.
In the micrograph, the grey areas are Fe 3 C layers, the red phase is α-Ferrite
Hypoeutectoid alloys contain proeutectoid ferrite (formed above the eutectoid temperature) plus the eutectoid perlite that contain eutectoid ferrite and cementite.
3 - M icrostructure of hypereutectoid steel
Compositions to the right of eutectoid (0.76 - 2.14 wt % C) Hypereutectoid ( more than eutectoid ) alloys.
γ → γ + Fe 3 C → α + Fe 3 C
Hypereutectoid alloys contain proeutectoid cementite (formed above the eutectoid temperature) plus perlite that contain eutectoid ferrite and cementite
How to calculate the relative amounts of eutectoidphase ( α or Fe 3 C)
and pearlite?
Fraction of α phase is determined by application of the lever rule across the
entire (α + Fe 3 C) phase field:
Example for hypereutectoid alloy with composition C
Fraction of pearlite (liquid):-
WP = (6.7 – C1) / (6.7 – 0.76)
Fraction of proeutectoid cementite:
WFe3C = (C1 – 0.76) / (6.7 – 0.76)
Effect of Alloying Steel with more Elements:-
Teutectoid changes
Limitations of equilibrium phase diagram
Fe-Fe 3 C equilibrium / metastable phase diagram. Stability of the phases under equilibrium condition only. It does not give any information about other metastable phases. i.e. bainite, martensite. It does not indicate the possibilities of suppression (reduced ) of proeutectoid phase separation. No information about kinetics energy. No information about size. No information on properties.
