Metals Flashcards
metals - density
metals are generally most dense
stiffness
the resistance to elastic deformation measured in pascals
metals - stiffness
have a high modulus
strength
the resistance to permanent deformation measured in pa
metals -strength
strong but not the strongest
toughness
resistance to cracking, fracture
metals - toughness
are the toughest
metals -thermal conductivity/electrical conductivity
can have much higher conductivity, but not always
metals - cost
metals have a v large range of prices
the strengthening mechanisms
1 atoms in solution, different sizes deflect and create distortions (alloy)
2 dispersion strengthening, adding enough of another element to make a second phase
3 work hardening
4 reduce grain size,more boundaries present , more deflections
casting
pouring a melt into a mould
low cost but can lead to defects
rolling
properties are better if material is deformed - stronger squeezes out pores refines microstructure work hardening good for producing long bits, eg rails
forging
hot softened metal pushed/ flattened into shape
produces good microstructure high strength
extrusions
can produce complex materials
steel needs high temp so not practical method
common for aluminium or polymers
austenite (gamma)
high temp phase
fcc structure
(can be present in stainless steels and non magnetic metals)
Ferrite (alpha)
high purity- a lot of carbon
v small region in phase diagram
bcc structure
present in most steels not as a single phase
cementite (fe3C)
a ceramic like material, intermetallic
line compund, fixed composition
effects all mechanical properties
pearlite
not a single phase
two phase region of ferrite and cementite
has practical implication on how fraction travel in materials
martensite
not on phase diagram
isn an equi phase
produced by fast cooling
hard and brittle
Extraction - Blast furnace
iron ore+coke+limestone
hot air blown in bellow
coke acts as a heat source, reducing agent accepting electrons from other reactions.
liquid metal and waste (slag) tapped of periodically
5% C - poor strength, not useful
Steel making - Basic oxygen
molten iron poured into converter oxygen blow oxidation of impurities, gives more heat (can melt some scrap) ~300 tonne, 30-40 min cycle large scale
Steel making - electric arc furnace
iron and scrap steel
electrodes supply power
will melt 100 tonnes in 90 mins
secondary steel making
small composition adjustments
creates certain alloys
casting
requires a melt
tipping/injecting melt into a mould
low cost BUT can lead to defects
rolling
properties better if deformed - work hardening -squeezes out pores -refines microstructure -work hardening good for long products, eg rails
forging
dies can be flat, open die forging
shaped and closed die forming
often done with hot softened metal
produces good microstructure
extrusion
produces complex cross sections in one long direction
not good for steel, need high temp
commonly used for polymers and aluminium
Pearlite - phases
ferritie and Fe3C alternating plates, lamellae plates are boundaries, stronger slow cooling final product can be controlled
martensite - phases
quick cooling
no time for ferritie and Fe3C to form carbon doesnt fit in bbc ferrite
structure is distorted- v hard brittle material
wrought steels
steels shaped by deformation carbon is low= weld-ability high strength low alloy (hsla) -grain size reduction -second phase strongest and toughest of regular steels
cast steels
not used structurally
casting is easier and cheaper
martensite at the surface, v brittle difficult to machine
stainless steel
has protective surface oxide
expensive -used for aesthetics/long-life structures
strong
good in certain application, not structural tho
non ferrous structural metals
all metals not based on fe
mostly more expensive
have attractive properties
aluminium
lightweight usually alloyed for strengthening food strength to weight ratio susceptible to degradation from heat used in, transport -panelling -internal fitting -cladding -bridges -roofs
titanium
more expensive than aluminium
high strength to weight ratio
used for -cladding
-aircraft
copper
not primary load support use din -roofing -flashing and cladding easy to form difficult to weld oxidation gives a green surface colour
