A phase diagram is a graphical depiction of the several phases of a material that occur at a specific temperature, pressure, or weight percentage.
Understanding the iron (Fe)-iron carbide (Fe3C) phase diagram is crucial in metallurgy and materials science for predicting the microstructure and characteristics of steel and other iron-based alloys.
When working with iron and iron alloys, particularly steel, as many Xometry customers do, it’s important to understand the phases, or structures, that they go through as they’re heated, cooled, and shaped, as well as how these affect their strength, hardness, brittleness, and overall useability.
What is Iron Carbide Phase Diagram?
The iron-carbon phase diagram is commonly used to describe the various stages of steel and cast iron. Both steel and cast iron are composed of iron and carbon. Additionally, both alloys include a tiny quantity of trace elements.
The graph is fairly intricate, but since we are restricting our research to Fe3C, we will only look at up to 6.67 weight percent of carbon.
This iron-carbon phase diagram shows carbon concentrations by weight on the X-axis and a temperature scale on the Y-axis.
Figure exhibits the Fe-C equilibrium diagram, which depicts diverse structures (obtained during heating and cooling), phases, and microscopic components of various types of steel and cast iron.
The fundamental structures, importance of numerous lines, and crucial spots are addressed below.
Structures Of Iron Carbide Phase diagram
The primary microscopic elements of iron and steel are listed below:
Austenite:
Austenite is a solid mixture of free carbon (ferrite) and iron in gamma iron. After heating the steel to the top critical temperature, the structure forms into austenite, which is hard, ductile, and non-magnetic.
It is capable of dissolving enormous amounts of carbon. It falls between the critical and transfer regions when steel is heated and cooled. It is generated when steel contains up to 1.8% carbon at 1130°C. Cooling below 723°C causes it to convert into pearlite and ferrite.
Austenitic steels cannot be toughened using standard heat treatment procedures and are nonmagnetic.
Ferrite:
Ferrite has little to no carbon in iron. It refers to pure iron crystals that are soft and ductile. The gradual cooling of low carbon steel below the critical temperature results in a ferrite structure. Ferrite does not harden when cooled quickly. It is really soft and magnetic.
Cemite:
Cementite is a chemical combination of carbon and iron known as iron carbide (Fe3C). Cast iron contains 6.67% carbon and has the entire structure of cementite.
Free cementite is present in all steels containing more than 0.83% carbon. It rises with an increase in carbon%, as seen in the Fe-C Equilibrium diagram. It’s incredibly hard.
The hardness and brittleness of cast iron are thought to be caused by the presence of cementite. It lowers tensile strength.
This occurs when carbon forms certain combinations with iron in the form of iron carbides, which are exceedingly hard in nature. The presence of cementite in cast iron largely determines its brittleness and hardness. It is magnetic below 200 °C.
Pearlite:
Pearlite is a eutectoid alloy composed of ferrite and cementite. It appears in medium and low carbon steels as a mechanical combination of ferrite and cementite in the ratio of 87:13. Its hardness rises with the amount of pearlite in the ferrous material.
Pearlite is comparatively strong, hard, and ductile, whereas ferrite is weak, soft, and ductile. It is composed of alternating bright and dark plates. These layers are alternating between ferrite and cementite.
When seen under a microscope, the surface resembles a pearl, which is why it is named pearlite. Hard steels are composed of pearlite and cementite, whereas soft steels are composed of ferrite and pearlite.
As the carbon level exceeds 0.2%, the temperature at which the ferrite is initially rejected from austenite decreases until, at or above 0.8% carbon, no free ferrite is rejected from austenite. This steel is known as eutectoid steel and has a pearlite composition.
As iron with varying percentages of carbon (up to 6%) is heated and cooled, the following phases depicting the lines reveal the structure of iron and how it charges.
Significance of Transformations Lines
Line ABCD:
The line ABCD indicates that melting has been completed above this line when heating the iron. The molten metal is entirely in liquidus state.
The metal below this line and above line AHJECF is both solid and liquid. The solid metal is referred to as austenite. Thus, the line ABCD shows the temperatures at which melting is deemed complete.
Beyond this line, the metal is completely molten. It is not a horizontal line since the melting temperature varies with carbon concentration.
Line: AHJECF:
This line indicates that metal melts at this temperature. This line is not horizontal; therefore, the melting temperatures will vary with carbon concentration. Below this line and above line GSEC, the metal is solid and has an austenite structure.
Line PSK:
This line, which occurs at 723°C and is horizontal, is known as a lower critical temperature line because it marks the start of steel transformation. Carbon% has no effect on it, which implies that steel with varied carbon percentages will convert at the same temperatures.
The transformation range extends from above the line to GSE. This line indicates that steel with a carbon content of up to 0.8% will begin to convert from ferrite and pearlite to austenite when heated.
Line ECF:
It is a line at 1130°C that indicates that cast iron has a percentage of C ranging from 2% to 4.3%. Cast iron will have austenite + ledeburite and cementite + ledeburite below and above line SK, respectively.
FAQs
What is the phase diagram for iron and carbon?
The iron-carbon phase diagram is simply a graphic that depicts the various configurations (known as phases) of iron or steel when they undergo extremely hot or cold treatments.
What are the five phases of iron?
The beta designation follows the Greek letter succession of phases in iron and steel: α-Fe, β-Fe, austenite (γ-Fe), high-temperature δ-Fe, and high-pressure hexaferrum (ε-Fe).
Which phase is the hardest in the iron-carbon diagram?
Explanation: Cementite, also known as iron carbide (Fe₃C), contains 6.67% carbon by weight; hence, when the X-axis extends up to 6.70 wt% carbon, the intermediate complex cementite is created. It is the toughest structure to create in the iron-iron carbide diagram. It possesses an orthorhombic crystal structure.
How can I understand a TTT diagram?
The left curve symbolizes the beginning of a transformation, while the right curve shows the end of the change. The region between the two curves represents the transition of austenite into various forms of crystal formations. (Transformations: austenite to pearlite, austenite to martensite, and austenite to bainite.)
What exactly is meant by eutectic point?
The eutectic point is the lowest temperature at which the liquid phase remains stable at a given pressure. A eutectic system is a homogeneous, solid combination of two or more substances that forms a superlattice and melts or solidifies at a lower temperature than any of the individual constituents’ melting points.