Material Science Exam 3

Thermodynamic condition that must be met for a state of equilibrium to exist

Equilibirum only exists if a minimum free energy exists for a specified temperature, pressure, and composition.

What is the distinction between hypoeutectoid and hypereutectoid steels?

Hypoeutectoid steels = carbon concentration < eutectoid

Hypereutectoid steels = carbon concentration > eutectoid

What is the eutectoid?

The point (~0.77% carbon concentration) at which austenite transforms simultaneously into ferrite and cementite, creating a perfect pearlite microstructure.

In a hypoeutectoid steel, both eutectoid and proeutectoid ferrite exist. Explain the difference between them.

Proeutectoid ferrite - the microconstituent, formed above the eutectoid temperature

eutectoid ferrite - a constituent of pearlite formed below the eutectoid temperature

What will the carbon concentration of proeutectoid and eutectoid ferrite be in a hypoeutectoid steel?

0.022 wt% C. All ferrite will be at the same solubility concentration. Since eutectoid ferrite exists, it must be at the eutectoid temperature (the horizontal line).

What are the two stages involved in the formation of particles of a new phase?

Nucleation - formation of very small particles of the new phase(s), they are stable and capable of new growth.

Growth - the increase in size of new phase particles.

In terms of heat treatment and the development of microstructure, what are two majorlimitations of the iron-iron carbide phase diagram?

1. Non equilibrium martensite is not shown

2. Shows no information about time-temperature relationships for the formation of pearlite, bainite, and spheroidite, all of which are composed of the equilibrium ferrite and cementite phases.

Rank the hardness & strength fof spheroidite, bainite, and perlite.

Bainite > Perlite > Spheroidite

What do perlite, bainite, and spheroidite all have in common?

All consiste of alpha-fettite and cementite phases

Explain the structure of perlite

ferrite and cementite exist in alternating layers

Explain the structure of bainite

fine, parallel needles of cementite in a ferrite matrix

Explain the structure of spheroidite

small sphere particles of cementite in a ferrite metrix

Explain why fine pearlite is harder and stronger than coarse pearlite, which in turn is harder and stronger than spheroidite.

More boundary area between the phases = Harder and Stronger alloy.

Fine pearlite is the hardest because thin layers = more phase boundary area. Pearlite in general has more boundary area than spheroidite.

Give two reasons why martensite is so hard and brittle.

1. Few operable slip systems in the body centered tetragonal crystal structure.

2. Most of the carbon is in solid solution -> solid solution hardening effect

Explain the solid solution hardening effect

increasing the strength of a material by dissolving foreign atoms into a lattice structure

Steps of hardening steel with total heat treatment

1. Heat austenite above the upper critical temperature

2. Quench to relatively low temperature

3. Temper at a temperature below eutectoid

4. Cool to room temperature

Steps to precipitation hardening

1. Solution heat treat by heating into solid solution phase region

2. Quench to a relatively low temperature

3. Precipitation harden by heating to a temperature within the solid two-phase region

4. Cool to room temperature

For the four steps of the heat hardening of steel process, list the microstructures

1. Austenite

2. Martensite

3. Tempered Martensite

4. Tempered Martensite

For the four steps of the precipitation hardening of steel process, list the microstructures

1. Single phase

2. Single phase - supersaturated

3. Small plate like particles of a new phase within a matrix of the original phase

4. Same as step 3

Describe the mechanical characteristics at the steps of heat treatment hardening of steel

1. Not relevant

2. Steel becomes hard and brittle upon quenching

3. Alloy softens slightly to become more ductile during tempering

4. No significant changes upon cooling to room temperature

Describe the mechanical characteristics at the steps of precipitation hardening of steel

1. Not relevant

2. Alloy is relatively soft

3. Alloy initially hardens with increasing time, becomes more brittle. May soften with overaging

4. Alloy may continue to harden or overage at room temperature.

Eutectic reaction

L → alpha + beta

Eutectoid reaction

gamma → alpha + Fe3C

Peritectic reaction

L + alpha → beta

Rank the strength/hardness of the microstructures of steel

Martensite

Bainite

Fine Pearlite

Coarse Pearlite

Ferrite

Spheriodite

Rank the ductility of the microstructures of steel

Spheroidite
Ferrite

Course Pearlite

Fine Pearlite

Bainite

Martensite