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 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

C RYSTAL FORMATION

H ARDENING. (C OOLING C URVE ) 0 ºC Liquid Freezing Point Time Solid Temp Gas Evaporation/fusio n

H ARDENING. (C OOLING C URVE ) 450 ºC 0 ºC 910 ºC Liquid 720 ºC Porridge/mixture Freezing Point Time Crystallisation 420 ºC Temp Gas

Phase Diagrams

C RYSTAL S TRUCTURE

E QUILIBRIUM (P HASE ) D IAGRAM 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

E QUILIBRIUM (P HASE ) D IAGRAM Plain carbon steel equilibrium phase diagram

E QUILIBRIUM (P HASE ) D IAGRAM 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

720 ºC Pearlite + Cementite 0.83% 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 720 ºC

= Cementite Crystal Structure 720 ºC Pearlite + Cementite 0.83% 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 1130 ºC 1.7%

0.83% carbon 100% Pearlite 720 ºC Lower Critical Point (LCP) Pearlite + Cementite Pearlite + Ferrite Austenite Austenite + Cementite Austenite + Ferrite 910 ºC Eutectoid point Upper Critical Point (UCP) Pearlite changes to Austenite above 720 ºC

T WO 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 T WO 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. I NTRODUCTION TO G RAIN S TRUCTURES

G RAIN S TRUCTURES 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. H OT W ORKING

R ECRYSTALLIZATION T EMPERATURE 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? 720 ºC x 0.6 = 432ºC

E FFECTS ON S TRUCTURE

C OLD WORKING Deformed crystals

E FFECTS OF C OLD 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

H EAT T REATMENT (A NNEALING ) 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 50 ºC 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 A NNEALING

Normalizing is the heating of steel to ºC 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. S OFTENING P ROCESSES (N ORMALISING )

T HE I RON C ARBON P HASE D IAGRAM % of carbon in the steel Temperature in ºC Normalizing Annealing and Hardening 940 ºC to 960ºC Lower Critical Point Upper Critical Point Re-crystallisation range 720 ºC 910 ºC 400 to 450 ºC Copy this one now

H EAT T REATMENT (H ARDENING P RESENTATION ) 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 steel 5. How the steel was cooled (speed) 6 What effect the process has on the steel (crystals)

H EAT T REATMENT (H ARDENING ) % of carbon in the steel Temperature in ºC Lower Critical Point Upper Critical Point 910 ºC 720 ºC 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% carbon 0.3% carbon

P LASTICS

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

P LASTICS 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.