M ATERIAL S CIENCE. I NTRODUCTION (C OOLING ) Observation of a pure metal cooling from the liquid state to its solid state show that it does it in a particular.

MATERIAL SCIENCE

INTRODUCTION (COOLING)

Observation of a pure metal cooling from the liquid state to its solid state show that it does it in a particular well defined way.

As soon as the freezing point is reached, nucleii begin to form at random throughout the cooling liquid and crystal begin to form in a very special way.

As soon as the nucleii are initiated the formation of crystals begins with the nucleii spreading in three directions, this process is called....

Crystal formation

CRYSTAL FORMATION

HARDENING. (COOLING CURVE)

0ºC

Liquid

Freezing Point

Time

Solid

Temp

Gas

Evaporation/fusion

HARDENING. (COOLING CURVE)

450ºC

0ºC

910ºC

Liquid

720ºC

Porridge/mixture

Freezing Point

Time

Crystallisation

420ºC

Temp

Gas

Phase Diagrams

CRYSTAL STRUCTURE

EQUILIBRIUM (PHASE) DIAGRAM

Copper and Nickel Phase diagram

Question: A liquid contain equal amounts of the alloy will begin to solidify at what temperature?

1312 ºC

Question: What temperature does it become completely solid?

1248 ºC

EQUILIBRIUM (PHASE) DIAGRAM

Plain carbon steel

equilibrium phase

diagram

EQUILIBRIUM (PHASE) DIAGRAM

Ferrite/Iron: This is a solid solution containing no more than 0.04% carbon dissolved in a Body Centred Cubic formation lattice.Ferrite can be regarded as almost pure iron and is very soft, ductile and easily worked.

= Crystal Structure

720ºC

Pearlite +

Cementite

0.83%Pearlite

Pearlite +

Ferrite

= Pearlite Crystal structure

Pearlite: This structure exist at the eutectoid of 0.83% carbon and consist of alternate layers of ferrite and Cementite. The formation of Pearlite takes place by the breakdown of Austenite below a temperature of 720ºC

= Cementite Crystal Structure

720ºC

Pearlite +

Cementite

0.83%Pearlite

Pearlite +

Ferrite

Cementite: This structure exist above 0.83% carbon and is very hard and brittle and is usually found on the crystal boundary

Eutectoid point

Pearlite +

Cementite

Pearlite +

Ferrite

Austenite

Austenite: This is a solid solution of carbon, in face centre cubic and iron. The maximum carbon content is 1.7% at 1130ºC. Austenite only exist in plain carbon steels above the UCP and is a soft non-magnetic compound

1130ºC

1.7%

0.83% carbon 100% Pearlite

720ºC Lower Critical Point (LCP)

Pearlite +

Cementite

Pearlite +

Ferrite

Austenite

Austenite + Cementite

Austenite + Ferrite

910ºC

Eutectoid point

Upper Critical Point (UCP)

Pearlite changes to Austenite above 720ºC

TWO THINGS THAT ARE NOT ON THE PHASE DIAGRAM

Martensite, most commonly refers to a very hard  form of steel crystalline structure, but it can also refer to any crystal structure that is formed by displacive transformation. It includes a class of hard minerals occurring as lath- or plate-shaped crystal grains. When viewed in cross section, the lenticular (lens-shaped) crystal grains are sometimes incorrectly described as Acicular, needle shape.

Martensite is formed by rapid cooling (quenching) of austenite which traps carbon atoms that do not have time to diffuse out of the crystal structure.

Bainite: A fine non-lamellar structure, bainite commonly consists of cemenite and dislocation-rich ferrite. The high concentration of dislocations in the ferrite that are present in bainite makes this ferrite harder than it normally would be.

The temperature range for transformation to bainite (250–550 °C) is between those for pearlite and martensite. When formed during continuous cooling, the cooling rate to form bainite is more rapid than that required to form pearlite, but less rapid than is required to form martensite in steels of the same composition

TWO THINGS THAT ARE NOT ON THE PHASE DIAGRAM

Cooling diagram for Hardening plain carbon steel

Cooling

This cooling process forms crystal which in turn form grains

Metals have a crystalline structure - this is not usually visible but can be seen on galvanized lamp posts for example.

When a metal solidifies from the molten state, millions of tiny crystals start to grow through the Dendritic Growth Process.

The longer the metal takes to cool the larger the crystals grow in the process.

These crystals form the grains in the solid metal.

Each grain is a distinct crystal with its own orientation. A crystal on a crystal.

INTRODUCTION TO GRAIN STRUCTURES

GRAIN STRUCTURES

Grain structures are altered by the working of the material in its solid metallic state. In particular: -

1. Hot working and cold working

2. Heat treatment

3. Over stressing due to continued working

All of these processes have an effect on grain size, grain growth and orientation of the crystal structure

In metalworking, rolling is a metal forming process in which metal stock is passed through a pair of rolls.

Rolling and other forms of metal forming is classified according to the temperature of the metal rolled.

If the temperature of the metal is above its recrystallization temperature, then the process is termed as hot rolling.

If the temperature of the metal is below its recrystallization temperature, the process is termed as cold rolling.

In terms of usage, hot rolling processes more tonnage than any other manufacturing process, and cold rolling processes the most tonnage out of all cold working processes.

HOT WORKING

RECRYSTALLIZATION TEMPERATURE

The lower limit of the hot working temperature is determined by its recrystallization temperature. As a guideline, the lower limit of the hot working temperature of a material is 0.6 times its melting temperature (on an absolute temperature scale). 

In our case we are going to use plain carbon steel with 0.83% carbon.

What is the re-crystallisation temperature?

720ºC x 0.6 = 432ºC

EFFECTS ON STRUCTURE

COLD WORKING

Deformed crystals

EFFECTS OF COLD WORKING Breaks down the crystal structure

Destroys the lattice structure

Deformation occur along the crystal edges

Much more pressure is required to work the material

The elasticity limit is exceed

Work hardening occurs when not required.

Internal stress occurs known as residual stress.

To combat these effects the material has to be annealed

HEAT TREATMENT (ANNEALING)

Annealing, in metallurgy and materials science, is a heat treatment that alters a material to increase its ductility and to make it more workable.

It involves heating material to 30 to 50ºC above its upper critical temperature, maintaining a suitable temperature, depending on the mass of the material, then it is cooled very slowly, usually leaving it in the furnace when switch off.

Annealing can induce ductility, soften the material, relieve internal stresses, refine the structure by making it homogeneous, and improves cold working properties.

There are two "softening" processes commonly used when metalworking: normalizing and annealing. The objective of both processes is to soften the metal and to make it less brittle. This makes further work on the piece easier and safer.

Deformed crystals

Apply heat here

ANNEALING

Normalizing is the heating of steel to 30-50ºC above its upper critical point (UCP) followed by an air cool.

The cooling is faster than annealing and this is the main difference between the two processes.

This limits the grain growth to a more refined grain structure and a better quality of material.

The hardness and strength of normalised steel are better than that of annealed steel but it looses out where ductility is concerned.

SOFTENING PROCESSES (NORMALISING)

THE IRON CARBON PHASE DIAGRAM

1300

1200

1000

1100

900

800

700

600

500

400

200

300

100

0.2 0.4 0.6 0.8 1.41.0 1.2 1.6 1.8

% of carbon in the steel

Temperature in ºC

Normalizing

Annealing and Hardening 940ºC to 960ºC

Lower Critical Point

Upper Critical Point

Re-crystallisation range

720ºC

910ºC

400 to 450ºC

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HEAT TREATMENT (HARDENING PRESENTATION)

The Hall–Petch method

Work hardening

Solid solution strengthening

Precipitation hardening

Martensitic transformation hardening

Quenching Hardening

You must talk about the following in your presentation:-

1. Describe the process using photos and text.2. The lattice formation (BCC) (FCC) etc.3. Temperatures used in the process and why.4 The percentage carbon in the steel5. How the steel was cooled (speed)6 What effect the process has on the steel

(crystals)

HEAT TREATMENT (HARDENING)

1300

1200

1000

1100

900

800

700

600

500

400

200

300

100

0.2 0.4 0.6 0.8 1.41.0 1.2 1.6 1.8

% of carbon in the steel

Temperature in ºC

Lower Critical Point

Upper Critical Point 910ºC 720ºC

Hardening temperature range 940ºC to 960ºC

Do not

Will harden but temperatures varies according to carbon content

These steels will harden but have a constant temperature

0.83% carbon0.3% carbon

PLASTICS

PLASTICS

PLASTICS

PLASTICS

Thermo Plastics comprise long-chain molecules held together by weak bonds (Figure a). When heat is applied, the molecules "slide past" one another and the polymer softens. On cooling, the molecules cannot slide past each other easily and the polymer hardens

PLASTICS

Thermo Setting long chain molecules, however, are linked together by small molecules via strong chemical bonds, a process sometimes referred to as vulcanization (Figure b). This three- dimensional network is so rigid that the molecules cannot move very much even when the polymer is heated. Thus, TSs do not soften when heated.