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definition of metallurgy
a society's ability to develope and use materials is a measure of its technological sopistication and it's technological future
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definition of stress
the force per unit area acting on a part or structure that attempts to change its dimensions
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definition of strain
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
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definition of plastic deformation
a permanent change in the shape of a material due to the application of distorting forces
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different types of forces or stresses
- tensile
- compressive
- shear
- bending or torsion
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definition of tensile
the capacity of a material to be stretched
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definition of compressive
the capacity of a material to be crushed
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definition of shear
the capacity of a material to pull apart in a planear surface
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definition of bending
the capacity of a material to undergo compression, tension and shear forces simultaneously
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definition of torsion
the capacity of being twisted
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8 common mechanical properties
- hardness
- brittleness
- toughness
- strength
- elasticity
- ductility
- malleability
- creep
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definition of hardness
the ability of resistance to surface deformation by penetration, scratching or abrasion
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definition of brittleness
the ability to fracture without visible plastic deformation
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definition of toughness
the ability to withstand sudden impact without failure
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definition of strength
the ability to resist forces
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definition of elasticity
the ability to return to original shape after deformation forces that are less than the plastic limit are removed
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definition of ductility
the ability to under go plastic deformation without fracture as a result of tensile forces
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definition of malleability
the ability to undergo plastic deformation without fracture as a result of compressive forces
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definition of creep
the ability to plastically deform over a long period of time by forces less than the elastic limit when exposed to heat
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methods of testing for hardness
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explain Moh's test
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
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how can you test for toughness
- izod impact test
- charpy impact test
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charpy or izod impact test are a measure for what
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tensometer is a test for what
- strength
- elasticity
- ductility
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basic methods of shaping metals
- hot forming
- cold formimg
- casting
- explosive forming
- powder metallurgy
- machining
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4 hot forming techniques
- forging
- hot pressing
- hot rolling
- extruding
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explain forging
is the plastic working of metal by means of compressive forces
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advantages of hot forging
the creation of residual grain flow makes the piece stronger and tougher
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explain hot pressing
a forging process using steady pressure of a large press instead of repeated hammer blows
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advantages of hot pressing
the component can be made in 1 pressing and as a result is dimensionally more accurate. car bodies made this way
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explain hot rolling
process of passing metal through pairs of rollers to reduce thickness
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advantages of hot rolling
allows a quick reduction in thickness but at the expense of accuracy and surface finish
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explain extruding
involves pushing a solid hot block of metal through a die to produce desired shape
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advantages of extruding
possible to produce very complex shapes with little or no waste
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cold forming techniques
- squeezing
- bending
- shearing
- drawing
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advantages of cold forming
- no heating needed
- better surface finish
- dimensional accuracy is greater
- improved strength
- directional properties can be given to component
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disadvantages of cold forming
- 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
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explain squeezing
includes rolling, cold forging, extrusion, riverting
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explain bending
includes angle bending, roll bending, roll forming, seaming, straightening
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explain shearing
includes blanking piercing, notching, nibbling shaving trimming and cutoff
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explain drawing
includes bar and tube drawing, wire drawing, spinning, embosing,explosive forming, stretch forming
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explain casting
all casting involves the pouring of molten metal into a mould or die
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5 major casting processes
- sand casting
- shell mould casting
- die casting
- centrifugal casting
- investment casting
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explain sand casting
uses sand as the mould material. sand adhering to the casting has to be removed of fettled
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pros and cons of sand casting
- cheap process
- further machining required
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explain shell mould casting
variation on sand casting where the sand is mixed with a resin that is heated to cure it
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pros and cons of shell mould casting
- shells are not reusable
- finished product has a better finish than sand casting
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explain die casting
molten metal is forced into a die under pressurse
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pros and cons of die casting
fine sections and excellent detail can be achieved
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explain investment casting
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
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explain powder metallurgy
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
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basic steps of powder metallurgy
- mixing and preparing the powder
- pressing the powder into the desired shape
- sintering the shape at an elevated temperature
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advantages of powder metallurgy
- 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
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disadvantages of powder metallurgy
- inferior strength properties
- high cost of producing dies
- high cost of materials - offset by lack of waste
- design limitations
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explain explosive forming
sheet metal is placed in a die with an explosive charge which when detonated forces the metal sheet into the shape of the die
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explain machining
is the removal of unwanted pieces of metal in the form of chips
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disadvantages of machining
- inefficient process
- may produce undesirable side effects
- wasteful of material
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name 3 other cutting methods
- laser cutting
- plasma
- chemical etching
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what is a ferrous metal
one where the major constituent is iron example cast iron, wrought iron, plain carbon steels
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what is a non ferrous metal
one where the major constituent is not iron example aluminium, sliver copper bronze
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what is the percentage carbon in plain carbon steels
- 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%
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give a list and examples of steel and uses
- 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
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what is critical temperature
the temperature at which the internal structure of a metal takes on a particular crystalline form
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when steel is above the upper critical temperature it is called
austenite
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steel when quenched from above upper critical temperature is called
martensite
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qualities of martensite are
very hard and brittle is not tough enough for engineering applications
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upper critical temperature is also known as
the transformation temperature
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2 main factors determining the outcome of the heat treatment process are
- the temperature to which the steel is heated
- the rate of cooling
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3 types of heat treatment processes
- annealing
- hardening
- tempering
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describe full annealing
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
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advantages of full annealing
refines the grain structure and improves cold working propertiesas steel is more ductile and more malleable
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describe normalising
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
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advantages of normalising
used to refine grains in preparation for the hardening process
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describe 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
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what is the result of hardening
this very rapid cooling results in a very hard and brittle material called martensite
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describe tempering process
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
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steel hard on the outside but tough on the inside is called
surface hardening
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6 methods to achieve surface hardening
- carburising
- cyanide hardening
- nitriding
- flame hardening
- superficial hardening
- induction hardening
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advantages of surface 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
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briefly explain carburising
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
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briefly explain cyanide hardening
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
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explain nitriding
heated in ammonia rich atmosphere. nitrogen absorbed forming hard wearing nitrides. low temperature so no quenching needed no carbon added
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explain flame hardening
heated to just above UCT with oxy-acetylene flame then quenched, hardening superficial creates martensite on the surface no carbon added
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explain superficial hardening
steel coated with cyanide paste then heated and quenched
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advantages of alloy steels
enhances a materials properties by adding additional elements as well as carbon
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examples of heavy and light non ferrous alloys
- light aluminium magnesium
- heavy titanium copper nickel lead
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advantages of aluminium
- resistant to corrosion
- light
- ductile
- good strenght to weight ratio
- good conductor of electricity and heat
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how does aluminium resist corrosion
due to a presence of a stable oxide film at the surface
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disadvantages of aluminium
- soft
- weak
- low melting point
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