StuDocu is not sponsored or endorsed by any college or universitySummary - lecture All - metals, alloys and steelConstruction Materials (University of New South Wales)StuDocu is not sponsored or endorsed by any college or universitySummary - lecture All - metals, alloys and steelConstruction Materials (University of New South Wales)Downloaded by tebogo mkhulisi ([email protected])lOMoARcPSD|13801716METALSProperties of metals- Ductile – can be drawn into wire- Malleable – can be hammered into sheets- Lustre – shiny- Good conductors – thermal + electricity- High density – high mass per unit volume- High melting point- High reaction – corrosion in presence of H2O and O2 (oxidation)Copper- Ductile- Very high thermal and electrical conductivity- Pure copper is soft and malleable Tin- Not easily oxidised in air- Often used to coat other metals to prevent corrosion (tin plating of steel)- Low toxicity (used for tin cans)Alloy- Combination of elements- Alloys disrupt the crystals slipping planes, thus effectively locking one part of the crystal against another- Not many atoms are needed to do this, so allows with a few (%) of another metal can produce considerable metallic changes in properties (i.e. greater strength) compared to the original pure metal (softer)- The locking atoms are those of the alloying metal (I.e. small amounts of tin cause this locking process in copper thus producing the harder material called bronze)Zinc- Zinc baths used for hot-dip galvanizing- Zinc plating for corrosion-resistant steelBrass alloy- Brass = 64% copper + 36% zinc- Often used for corrosion resistant decorative purposes such as hardware- It is much harder and stronger than its original components- Machines easily Titanium- High strength to weight ratio- Low density- Lustrous- Corrosion resistantDownloaded by tebogo mkhulisi ([email protected])lOMoARcPSD|13801716Iron- Body central cubic crystalline structure- Soft (add carbon to overcome)- MalleableCast iron- Not structurally sound- Brittle- Heavy- Molten iron poured into cast mould (hence name) to solidify- Usually made from pig iron:o Intermediate product of melting iron oreo Very high carbon content (3.5% - 4.5%)o Very brittleo Intended for re meltingSteel- Key to making steel from iron is to limit % Carbon- Steel is an alloy of Iron and Carbon- No carbon > iron is soft- Too much carbon > iron is brittle- Correct amount of carbon > produces steelStructural steel- Beams & columns, channels, angles, hollow sections - Long tail on stress strain diagram is good- Yield – the capacity up to yielding of the steel (increases with more working/rolling – the more you roll steel, the stronger it becomes)- Rolling steel squashes the grains thereby making the steel strongero Larger grains = weakero Smaller grains = strongerResidual stresses- Significant stresses locked inside the section during rolling because cooling rates vary along the section- Examples of the extent of stressing are categorised as:o HR (hot rolled)o CF (cold formed)o SR (stress relieved)o HW (heavily welded)Steel metallurgy- Austeniteo 912OC to 1394OCo Face centred cubic (FCC) – atoms located at each of the corners and the centres of all the cubic facesDownloaded by tebogo mkhulisi ([email protected])lOMoARcPSD|13801716o In this state the metal is non magnetic, has high formability and in particular high solubility of carbon (over 2% can be dissolved into the iron structure)- Pearliteo As the temperature drops, Austenite – converts to body central cubic (BCC)o The resulting structure is a fine mixture of ferrite and cementite (pearlite)o 0.87% carbon- Ferrite:o Soft and ductile solid solution of iron (magnetic)o Contains up to 0.008% carbon at room temperatureo Atomic shape of this material is Body central cubic (BCC)o BCC > easily distorted structure, thus soft/ductile- Cementiteo When more carbon is present in the ferrite matrix, the excess carbon joins with the iron to form iron carbideo This is a hard brittle compound with about 6.7% carbonCooling rate vs. grain size- Cooling swiftly will leave iron carbide finely dispersed and produce a fine grained pearlite (until the martensite critical temperature is reached)o Fine grained steels are usually stronger steels- Cooling slowly will give a coarser particleo Coarse grains generally means weaker steel- Cooling a hypo-eutectoid steel (less than 0.77% C) results in a lamellar-pearlite structure of iron carbide layers with a ferrite (pure iron) in-between- If it is hyper-eutectoid steel (more than 0.77% C) then the structure if full pearlite with small grains (larger than the pearlite lamella) of cementite scattered throughoutEutectoid- As temperature lowers more pearlite is formed until the microstructure is 100% fully pearlite - Eutectoid point – the three-phases are in equilibriumSpherodizing- Spherodite forms when carbon steel is heated to approx. 700OC for over 30 hoursMartensite- When carbon steel is cooled rapidly, it forms a hard, higher strength layer with needle like crystals in a darker matrixBainite- Intermediate of pearlite and martensite in terms of harndessDownloaded by tebogo mkhulisi ([email protected])lOMoARcPSD|13801716Summary- Iron: weak > ass carbon (strength)- Structural steel (0.4-0.5% C)- Machinery steel (>0.8% C)- Major phases o Ferrite: almost pure iron, very soft and ductileo Cementite: iron carbide, very hard and brittleo Pearlite: alternating layers of ferrite and cementiteo Bainite: a more brittle pearlite type phaseo Martensite: a hard brittle phase produced by very fast coolingo Austenite: a ductile high temperature phaseAnnealing heat treatment- Reduces hardness, relieved stresses in metal- Heating steel to a certain temperature- Leaving the steel element at this temperature for prescribed period- Cooling at a predetermined rate- The three steps above helps to remove coarseness of grain and relieve internal i.e. residual stresses induced by rolling or uneven coolingNormalizing- Carbon steel is heated to approx. 55OC for 1 hour- Ensures steel completely transforms to Austenite- The steel is then air-cooled, which is a cooling rate of approximately 38OC per minute- This results in a fine pearlitic structure, and a more-uniform structure- Normalized steel has a higher strength than annealed steel and higher ductilityQuenching- Increases hardness of steel- Involves immersion of hot steel into water or oil bath- Steel does not revert from Austenite phase
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