metallurgy

a society's ability to develope and use materials is a measure of its technological sopistication and it's technological future

the force per unit area acting on a part or structure that attempts to change its dimensions

the measurable or visible result of stress/load applied to a structure. strain is measured by any dimensional change to a part or structure as a result of a load. expressed as a ratio new dimension/original dimension

a permanent change in the shape of a material due to the application of distorting forces

• tensile
• compressive
• shear
• bending or torsion

the capacity of a material to be stretched

the capacity of a material to be crushed

the capacity of a material to pull apart in a planear surface

the capacity of a material to undergo compression, tension and shear forces simultaneously

the capacity of being twisted

• hardness
• brittleness
• toughness
• strength
• elasticity
• ductility
• malleability
• creep

the ability of resistance to surface deformation by penetration, scratching or abrasion

the ability to fracture without visible plastic deformation

the ability to withstand sudden impact without failure

the ability to resist forces

the ability to return to original shape after deformation forces that are less than the plastic limit are removed

the ability to under go plastic deformation without fracture as a result of tensile forces

the ability to undergo plastic deformation without fracture as a result of compressive forces

the ability to plastically deform over a long period of time by forces less than the elastic limit when exposed to heat

• rockwell
• brinell
• vickers

is used to compare the hardness of a material where the ability of one material to scratch another material is a relative measure of a material's hardness

• izod impact test
• charpy impact test

• brittleness
• toughness

• strength
• elasticity
• ductility

• hot forming
• cold formimg
• casting
• explosive forming
• powder metallurgy
• machining

• forging
• hot pressing
• hot rolling
• extruding

is the plastic working of metal by means of compressive forces

the creation of residual grain flow makes the piece stronger and tougher

a forging process using steady pressure of a large press instead of repeated hammer blows

the component can be made in 1 pressing and as a result is dimensionally more accurate. car bodies made this way

process of passing metal through pairs of rollers to reduce thickness

allows a quick reduction in thickness but at the expense of accuracy and surface finish

involves pushing a solid hot block of metal through a die to produce desired shape

possible to produce very complex shapes with little or no waste

• squeezing
• bending
• shearing
• drawing

• no heating needed
• better surface finish
• dimensional accuracy is greater
• improved strength
• directional properties can be given to component

• higher forces are needed
• heavier and morep powerful machinery is required
• ductility of metal is less because it is cold
• metal surface must be clean and scale free
• strain or work hardening occurs and annealing (heating to soften) may be required before further shaping
• directional properties given may be detrimental
• undesirable residual stresses may be produced

includes rolling, cold forging, extrusion, riverting

includes angle bending, roll bending, roll forming, seaming, straightening

includes blanking piercing, notching, nibbling shaving trimming and cutoff

includes bar and tube drawing, wire drawing, spinning, embosing,explosive forming, stretch forming

all casting involves the pouring of molten metal into a mould or die

• sand casting
• shell mould casting
• die casting
• centrifugal casting
• investment casting

uses sand as the mould material. sand adhering to the casting has to be removed of fettled

• cheap process
• further machining required

variation on sand casting where the sand is mixed with a resin that is heated to cure it

• shells are not reusable
• finished product has a better finish than sand casting

molten metal is forced into a die under pressurse

fine sections and excellent detail can be achieved

a wax pattern is made for each casting. the wax is coated in refactory material that sets hard, usually by baking. the heat from this melts the wax thus creating the mould for the casting. the metal is poured in and allowed to cool and solidify and then the mould is broken to extract the cast item

fine powder is pressed into shape in a die at high pressure. the compacted powder is heated at a temperature below the melting point of the major constituent, known as sintering

• mixing and preparing the powder
• pressing the powder into the desired shape
• sintering the shape at an elevated temperature

• no waste
• no machining needed
• only semi skilled labour needed
• unique properties can be obtained
• unique alloys can be created
• high production rates
• high degree of unformity
• complex shapes can be produced

• inferior strength properties
• high cost of producing dies
• high cost of materials - offset by lack of waste
• design limitations

sheet metal is placed in a die with an explosive charge which when detonated forces the metal sheet into the shape of the die

is the removal of unwanted pieces of metal in the form of chips

• inefficient process
• may produce undesirable side effects
• wasteful of material

• laser cutting
• plasma
• chemical etching

one where the major constituent is iron example cast iron, wrought iron, plain carbon steels

one where the major constituent is not iron example aluminium, sliver copper bronze

• low carbon up to 0.25%
• medium carbon 0.25% to 0.55%
• high carbon 0.55% to 1.5%
• cast iron 1.5% to 5%

• mild steel - lightly stressed parts
• structural steel - used for building and bridges
• casting steel - medium strenght good machinability. casting
• construction steel - axles crankshafts gears
• tool steel- drills files knives
• cast iron - piston rings

the temperature at which the internal structure of a metal takes on a particular crystalline form

austenite

martensite

very hard and brittle is not tough enough for engineering applications

the transformation temperature

• the temperature to which the steel is heated
• the rate of cooling

• annealing
• hardening
• tempering

steel is heated to just above transfomation temperature and held there for a time. the cooling rate is controlled either in a furnace or for non critical parts by putting the steel in an inert material such as lime or ashes

refines the grain structure and improves cold working propertiesas steel is more ductile and more malleable

steel is heated to just above the transformation temperature and held there for a time the cooling rate is not controlled in that it takes place in still air

used to refine grains in preparation for the hardening process

the hardness of the steel is controlled by the rate of cooling through the critical range, and this is determined by the quenching liquid used

this very rapid cooling results in a very hard and brittle material called martensite

the steel is heated to well below the lower critical temperature and then cooled. the cooling is not critical but is normally done in cool still air

surface hardening

• carburising
• cyanide hardening
• nitriding
• flame hardening
• superficial hardening
• induction hardening

would be used where we need the steel to be resistant to wear at the surface but tough enough to withstand sudden shock loads in normal operation

steel packed in a sealed box with carburising compound rich in carbon, heated then cooled carbon is added to the surface needs to be harden and tempered

steel heated in sodium cyanide bath, may be quenched further heat treatment may be applied if needed. iron nitrides formed at the surface and some carbon absorded

heated in ammonia rich atmosphere. nitrogen absorbed forming hard wearing nitrides. low temperature so no quenching needed no carbon added

heated to just above UCT with oxy-acetylene flame then quenched, hardening superficial creates martensite on the surface no carbon added

steel coated with cyanide paste then heated and quenched

enhances a materials properties by adding additional elements as well as carbon

• light aluminium magnesium
• heavy titanium copper nickel lead

• resistant to corrosion
• light
• ductile
• good strenght to weight ratio
• good conductor of electricity and heat

due to a presence of a stable oxide film at the surface

• soft
• weak
• low melting point