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Iron Carbon Alloys

Describes Iron Carbon Steel Alloys, the effects of cooling rates on their Strength; Ductility and Crystalography

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Contents

The Constitution Of Iron Carbon Alloys
Critical Point
Crystaline Forms
Cleavage Planes - Crystal Boundaries
Grain Size
Steel Lattice Parameters
Tempering Of Martensite
Page Comments

The Constitution Of Iron Carbon Alloys

1. Iron And Carbon

The following are the three features of adding Carbon to Iron:

Steel is a crystaline substance containing less than 1.5% Carbon.
Carbon gives strength and hardness but at the expense of ductility.
For Steel the Carbon must be present as Iron Carbide. Free Carbon is present in Cast Iron.

2. Chemical Compounds

Carbide of Iron is known as "CEMENTITE" It is a chemical compound of Iron and Carbon $\inline (Fe_3C)$

3. Solution: "austenite"

When steel is heated to a certain temperature the tiny particles of Cementite dissolve or go into solution in the Iron and above this temperature the particles of Iron could not be observed under a microscope. Since the steel was solid this is known as a "Solid Solution" and is called "AUSTENITE". The properties of Austenite are quite different to those of the original mixture of Iron and Cementite.

4. The Iron Carbon Equilibrium Diagram (first Part)

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Before solidification (freezing) can occur the temperature must fall to a point on the lines AB or BC
The Iron Carbon eutectic contains the equivalent of 4.3% Carbon.
The "frozen" eutectic has a composition of Austenite solid solution (Of composition E) and Iron Carbide.
For alloys below 1.8% Carbon each alloy has a range of temperatures over which freezing takes place but the final liquid does not reach eutectic composition and the construction of the solid alloy is entirely Austenite solid solution.
The 1.8% Carbon Criterion is used to distinguish between Steel and Cast Iron.
1) If Carbon in Cast Iron is in the form of Carbide we have "White Cast Iron"

2) If the Carbon is free in the form of Graphite. We have Grey Cast Iron.

Iron Carbon Equilibrium Diagram (second Part)

This concerns the changes which take place when Austenite cools.

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a) Consider the cooling of a 0.5% Carbon Alloy.

At "O" the Alloy consists entirely of Austenite.
On cooling to "X" on the line AE the alloy begins to reject or deposit practically pure Iron. As the temperature continues to fall, more and more pure Iron is deposited until only Austenite solid solut1on of 0.82% Carbon at $\inline 695^0C$ remains.
The remaining Austenite deposits Iron and Carbide of Iron side by side to form the eutectoid of Iron.

b) When a 1.25% Carbon Alloy cools the ( From Y on the EB line) the Austenite begins to deposit Cementite (Carbide of Iron)

Definitions

"FERRITE" Iron precipitated from Austenite as it cools. The name does not apply to the Iron of the eutectoid.
"PEARLITE" The eutectoid mixture of Cementite and Ferrite.
"CEMENTITE" Iron Carbide. Can be deposited from Austenite as it cools.
"EUTECTOID" The eutectic formation arriving from a solid solution as apposed to a liquid one.

Critical Point

The Steel And White Cast Iron

This is the complete diagram. It is the addition of the two previous diagrams with the following exceptions.

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1) Another horizontal line appears at $\inline 1494^0C$ between the limits of 0.1% and 0.5% Carbon. This line represents the "PERITECTIC" reaction of Iron. This simply means that at this constant temperature a reaction takes between the solid $\inline \delta$ solution already deposited containing 0.1% Carbon and liquid containing 0.5% to yield a new solid $\inline \lambda$ solution containing 0.18% Carbon. If the original liquid Iron Carbon Alloy contains less than 0.18% Carbon then all the residual liquid will be used up at the peritectic temperature and the Alloy will consist of the two solid solutions $\inline (\delta \;and\;\lambda$ containing 0.1%and 0.18% Carbon respectively.

If on the other hand the original liquid contains more than 0.18% Carbon then all the solid $\inline \delta$ solution initially deposited is used to give the 0.18W% Carbon $\inline (\gamma$ solid solution and some liquid still remaining of the 0.5% This now cools to deposit more $\inline \gamma$ Iron until solidification is complete

2) The $\inline \delta$ solid solution is a hight temperature form of Ferrite and the $\inline \gamma$ is solid solution of Austenite.

3) It has been assumed up till now that Ferrite is pure Iron. This is infact not true as Iron dissolves about 0.025% Carbon at the eutectoid temperature.

Arrest Points

The following graph shows the inverse cooling rate for pure Iron. You will note that there is not an even fall in temperature with time.

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1) Since the Iron is pure these arrest points can not be chemical changes and must be due to physical changes in the Iron.

2) For Pearlite to change Austenite the Ferrite must change to gamma Iron.

Notation Of Arrest Points

For convenience the Arrest Points are represented as letters and numbers. Thus the $\inline 1404^0\,C$ change point from delta to gamma is known as $\inline A_{r4}$ and the $\inline 900^0\,C$ arrest as the $\inline A_{r3}$ point etc.

The Heating And Cooling Curves For A 0.2% Carbon Steel.

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Note that the difference in the heating and cooling curves is due to the Thermal Lag or Thermal Hysteresis. This effect is of great importance in the heat treatment of Steel.

The Properties Of The Allotropic Forms Of Iron.

1) Alpha iron is soft and ductile. It is present in steels of the softest and most ductile character as well as in genuine Wrought Iron.

2) Beta Iron is a nonmagnetic form of Alpha Iron but otherewise has the same properties.

3) Gamma Iron and it's Austenitic solid solutions are also soft and plastic - Softer even than Alpha Iron. This is why steel is often taken heated into it's Austenetic region prior to mechanical working.

Crystaline Forms

In it's four Allotropic forms Iron takes up two different Crystal Arrangements.

1) Gamma Iron

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2) Alpha; Beta; and Delta Iron

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3) Notes

a) The above basic cubic formations go to build up the Crystals o Iron. The different formations explains why there is a contraction when Alpha Iron is heated to become Gamma Iron. This is in spite of the normal thermal expansion.

b) In each separate Crystal the axis of the cubes points in the same way but in different crystals the axis will probably be in a different direction. This effect is known as "Orientation".

Cleavage Planes - Crystal Boundaries

1) In Crystals the regular Cubic arrangement of the Atoms results in the formation of planes either parallel with the three axis of the cube or diametrically along those atoms that can most easily slide over one another. These planes are known as "Cleavage Planes" and are a source of weakness.

2) At the Crystal boundaries there are always spare atoms which do not fit into the regular arrangement of the space lattice. This is because there are not always the exact number of atoms available to complete the Cubic arrangement. These spare atoms actually form the Crystal boundaries. i.e. The arrangement of the atoms along the Crystal boundaries are irregular.

FOR PURE METALS AND UNIFORM SOLID SOLUTIONS

3) The above means that there are no cleavage planes along the Crystal boundaries and therefore the boundaries do not split so easily. They are in fact stronger than the Crystals themselves. This explains why fractures take place most easily along cleavage planes. i.e. Through the Crystals and not along the boundaries. Thus the SMALLER the Crystals the stronger the material since there are more boundaries to be broken through.

4) If impurities are present the pure metal will reject them and any other foreign atoms during the cooling process. They then may form a layer or film separating one crystal of the parent material from another.

Conclusion

For a pure metal or alloy the strength is largely the strength of the Crystal boundaries. For a pure metal or uniform solid solution these boundaries are stronger than the Crystals, If however these boundaries contain impurities or other brittle constituents, the fracture may occur along them.

Grain Size

Smaller Grain Size ( Except in Special circumstances) is always associated with increased toughness and strength.
For Steel the final Grain size is affected by the temperature of the steel before cooling started. The higher the temperature above the $\inline A_{c3}$ point the larger will be the Grain size.
However irregular or Course the Steel grains maybe, if they are heated above the $\inline A_{c3}$ point, the large grains will decompose to give small ones of the best possible size.
If Steel is "Soaked" at a temperature of over $\inline 1000^0\,C$ for a considerable time , the grains will be considerably enlarged.
The Cleavage Planes of Iron or Steel Crystals do not all lie in the same direction. If they did the bar could be broken by a light tap.

Steel Lattice Parameters

Austenite 3.6 $\inline A^0$ Non-Magnetic

Ferrite 2.86 $\inline A^0$ Magnetic

Martenite (1.4% C) Tetragonal 2.84 $\inline A^0$ Magnetic 3.04 $\inline A^0$

Tempering Of Martensite

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The Troostite and Sorbite produced by Tempering are not the same as those produced by slow cooling from Austenite. All the products above show small spheres of Carbide instead of the the Plate like structures of Pearlite etc. The impact resistance of tempered spheroidal structures is better than those of of the Plate like products of slow cooling.