Lecture Notes AT308,Inc.LN#:021

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IRON CARBON STRUCTURES

DEFINITIONS OF IRON-CARBON STRUCTURES

Austenite is a non-magnetic solid solution of carbon in FCCGamma iron. It can dissolve up to 1.7% carbon at2065oF, but is unstable at room temperature.

Delta iron is the BCC form of iron. It exists only at elevatedtemperatures (above 2600oF)

Ferrite also known as Alpha Iron is in BCC structure. It existsbelow 1666oF and above 1414oF is non-magnetic. It candissolve 0.025% carbon at 1333oF but only 0.008% carbonat room temperature.

Cementite intermetallic compound of iron and carbon Fe3C(containing 6.67% carbon). It is very hard andbrittle.

Figure 1

Figure 2

Figure 3

AT308,Inc. Lecture NotesLN#:021

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Ledeburite is the eutectic mixture of austenite and cementite. Itcontains 4.3% carbon. Ledeburite begins to form at2065oF upon rapid cooling.

Pearlite consists of iron carbide in a ferrite matrix. It is fully pearliteeuctectic contains 0.83 % carbon and requires slowcooling to form.

GENERAL PROPERTIES

Rc Tensile Strength ElongationScale in psi in 2in.

Austenite 40 150,000 10%Ferrite 0 40,000 40%Pearlite 20 120,000 20%

Figure 4

Figure 5

Lecture Notes AT308,Inc.LN#:021

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GRAIN SIZE CHANGES for Steels (not cast iron)

From the iron-carbon diagram it can be seen that Steels have two basic forms

A. Pearlite and FerriteB. Pearlite and Cementite

The discussions below willstart at room temperature,heat to an elevated temperature, andreturn to room temperature.

For PEARLITE and FERRITE Steels

Starting from room temperature,

consists of pearlite and ferrite

As temperature rises no change in grain size until the lower critical temperature (indicated byA1 in Figure).

At lower critical temperature,1. pearlite recrystallizes to (very fine grained) austenite2. the alpha iron associated with the pearlite only begins changing to gamma iron.3. the ferrite (approximately 70% of the structure) remains unchanged.

At upper critical temperature (A3 in Figure)

1. the ferrite transform from an alpha iron to gamma iron and is absorbed by thepreviously recrystallizes austenite. (grain growth is prevented by thetransformation of the ferrite)

Above upper critical temperature

1. all alpha iron is changed to gamma iron and is in solid solution with the austenite.2. grain growth begins and is continued to the solidus line.

AT308,Inc. Lecture NotesLN#:021

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For PEARLITE and FERRITE Steels (continued)

Upon slow cooling from above upper critical temperature to just slightly above the UCT.

1. no change in grain size

At Upper critical temperature

1. the austenite begins precipitating ferrite grains at the boundaries.

As the temperature continues to drop to lower critical temperature

1. austenite precipitation of ferrite continues, increasing the size and number of theferrite grains and decreasing the size and number of the austenite grains.

At the lower critical temperature

1. all remaining austenite is transformed to pearlite.

For PEARLITE and CEMENTITE Steels (Steels with carbon greater than 0.83% carbon)

From room temperature to Lower critical temperature

no change in grain size

At lower critical temperature

pearlite changes to austenite (alpha iron changes to gamma iron and Fe3C crystalsdissolve)

Basically no change to cementite

At Upper critical temperature

cementite is dissolved with an increase in austenite gain size.