Again for the Iron-iron Carbide Phase Diagram

1. INTRODUCTION

In this article we're going to explain how to build the fe - carbon phase diagram.

The Fe - C diagram (also chosen the iron - carbon phase or equilibrium diagram) is a graphic representation of the respective microstructure states of the blend iron - carbon (Fe-C) depending on temperature and carbon content.

To explain this diagram, an introduction about metal structures and pure iron must be washed.

2. Unit CELL CUBIC STRUCTURES

Before to explain the Iron-C diagram nosotros've to introduce the two structures that are involved in the diagram and precisely:

1. BODY-CENTERED CUBIC (BCC) structure, in which there'due south an atom at each corner of the cube and i in the center;
2. FACE-CENTERED CUBIC (FCC) construction, in which there's an atom at the corner of each unit jail cell and one in the centre of each face, merely at that place'southward no cantlet at the middle of the cube;

Hither beneath we tin can see the primary differences between the two structures and relevant backdrop:

three. IRON

Iron (Fe) is an allotropic metal that modify beliefs based on temperature. It's a relatively soft and ductile metal. Information technology exists in more than i type of lattice structure (BCC/FCC). In fact there're iv allotropic transformations and five phases that iron tin can assume. All of these can be hands represented using a COOLING Curve where the following phases can be identified:

1. LIQUID Stage (Fifty), above 1539 °C iron is in the liquid phase;
2. DELTA FERRITE (δ-Fe), if we slowly cooling downwards the pure iron below his melting signal (1539 °C) information technology will crystallize into a phase, which has a body-centered cubic (BCC) structure;
3. GAMMA FERRITE (γ-Atomic number 26), cooling downwardly further until 1400 °C, iron assume a new phase called gamma (austenite), which has a face up-centered cubic (FCC) structure;
4. ALFA FERRITE non magnetic (α-Iron), at 910 C an below the iron assume a new structure called alfa ferrite, which is once more a BCC structure;
5. ALFA FERRITE magnetic (α-Fe), under 770 C, iron assume a magnetic properties maintaining the BCC structure of alfa ferrite;

4. Atomic number 26-C PHASE DIAGRAM

Every bit mentioned before the Atomic number 26-C stage diagram is the representation of all the phases and structures of the blend iron - carbon based on the different temperature and carbon content.

This diagram is particular important in order to:

• Classify the alloy into dissimilar groups like steel and bandage atomic number 26;
• Identify the unlike phases of iron;
• Identify the microstructure of the different phases;
• Provide the data regarding solidification and heat treatments;

We're going to see how build the diagram step past pace starting to the essential temperatures indicated regarding the pure iron.

Temperature (°C) is indicated in the y-centrality and weight carbon content (wt % C) is indicated in the ten-axis. In the diagram is represented a maximum value of 6.67% C every bit there'due south no particular application in engineering for carbon content above this value.

Now nosotros've to introduce the iii invariant and isothermal reactions as indicated here below:

1. PERITECTIC REACTION, the liquid (at 0.5% C) and δ-Iron (at 0.1% C) phases transform into a austenite (with 0.ii% C) at temperature of 1493 °C;
2. EUTECTIC REACTION, the liquid solidifies as a stage mixture of austenite (with 2.1% C) and cementite (with 6.67% C) at temperature of 1147 °C;
3. EUTECTOID REACTION, austenite (at 0.viii% C) transforms into a stage mixture of ferrite (with 0.02% C) and cementite (with 6.67% C) at temperature of 727 °C;

The 3 invariant reactions tin be easily represented in the diagram every bit three horizontal lines with stock-still temperature as indicated here below:

At present we can place all the phases of the diagrams which are:

1. LIQUID PHASE (L)
2. DELTA FERRITE PHASE (δ-Fe), is an interstitial solid solution of C in δ-Fe (BCC) in the high temperature region of the diagram. It'due south stable at temperature above 1400 °C and melts at temperature in a higher place 1539 °C;
3. GAMMA FERRITE PHASE (γ-Atomic number 26), is an interstitial solid solution of C in γ-Fe (FCC). It's as well called austenite. Is not stable below 910 °C. The max solubility of C is about two.i% at 1147 °C. It's has high formability;
4. ALFA FERRITE PHASE (α-Fe), is an interstitial solid solution of C in α-Iron (BCC). It's a stable form of iron at room temperature. The max solubility of C is about 0.02% at 727 °C. It's a fairly and ductile phase, actually the softest one structure of the diagram;
5. CEMENTITE (Fe3C), also called as iron carbide. It's an inter metallic compound of fe and carbon with orthorhombic crystal organization. It'south very hard and brittle stage, actually the hardest construction of the entire diagram;

5. PROPERTIES OF THE DIFFERENT PHASES

Based on the microstructures each phase has unlike properties as indicated hither below:

6. General Classification

Based on the general form of the diagram nosotros can start to classify the alloy Fe-C into these groups:

• IRON, with a carbon content less than 0.008% in α-Fe structure at room temperature;
• STEEL, with a carbon content between 0.008 and 2.1% (generally less than ane%) in α-Fe + Fe3C structure at room temperature;
• CAST Atomic number 26, with a carbon content between 2.one and 6.67% (generally less than iv.5%);

In the future articles we are going to talk about the different types of steels and cast irons that can be identified through the diagram.

Author: Eng. Matteo Sporchia

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