lOMoARcPSD 13801716 Summary lecture All metals alloys and steel Summary lecture All metals alloys and steel Construction Materials University of New South Wales Construction Materials University of New South Wales StuDocu is not sponsored or endorsed by any college or university StuDocu is not sponsored or endorsed by any college or university Downloaded by tebogo mkhulisi tebogomkhulisi1 gmail com lOMoARcPSD 13801716 METALS Properties 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 Alloy Not easily oxidised in air Often used to coat other metals to prevent corrosion tin plating of steel Low toxicity used for tin cans 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 baths used for hot dip galvanizing Zinc plating for corrosion resistant steel Zinc Brass alloy Brass 64 copper 36 zinc Often used for corrosion resistant decorative purposes such as hardware Machines easily It is much harder and stronger than its original components Titanium High strength to weight ratio Low density Lustrous Corrosion resistant Downloaded by tebogo mkhulisi tebogomkhulisi1 gmail com lOMoARcPSD 13801716 Body central cubic crystalline structure Malleable Soft add carbon to overcome Iron Cast 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 ore o Very high carbon content 3 5 4 5 o Very brittle o Intended for re melting Steel Steel is an alloy of Iron and Carbon Key to making steel from iron is to limit Carbon No carbon iron is soft Too much carbon iron is brittle Correct amount of carbon produces steel Structural 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 stronger o Larger grains weaker o Smaller grains stronger Residual 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 Austenite o 912OC to 1394OC o Face centred cubic FCC atoms located at each of the corners and the centres of all the cubic faces Downloaded by tebogo mkhulisi tebogomkhulisi1 gmail com lOMoARcPSD 13801716 o 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 o As the temperature drops Austenite converts to body central cubic o The resulting structure is a fine mixture of ferrite and cementite Pearlite BCC pearlite o 0 87 carbon Ferrite o Soft and ductile solid solution of iron magnetic o Contains up to 0 008 carbon at room temperature o Atomic shape of this material is Body central cubic BCC o BCC easily distorted structure thus soft ductile Cementite o When more carbon is present in the ferrite matrix the excess carbon joins with the iron to form iron carbide o This is a hard brittle compound with about 6 7 carbon Cooling 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 particle o 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 throughout As temperature lowers more pearlite is formed until the microstructure is 100 fully pearlite Eutectoid point the three phases are in equilibrium Spherodite forms when carbon steel is heated to approx 700OC for over 30 hours When carbon steel is cooled rapidly it forms a hard higher strength layer with needle like crystals in a darker matrix Intermediate of pearlite and martensite in terms of harndess Eutectoid Spherodizing Martensite Bainite Downloaded by tebogo mkhulisi tebogomkhulisi1 gmail com lOMoARcPSD 13801716 Summary 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 ductile o Cementite iron carbide very hard and brittle o Pearlite alternating layers of ferrite and cementite o Bainite a more brittle pearlite type phase o Martensite a hard brittle phase produced by very fast cooling o Austenite a ductile high temperature phase Annealing 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 cooling Normalizing 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 ductility Quenching Increases hardness of steel Involves immersion of hot steel into water or oil bath Steel does not revert from Austenite phase into ferrite cementite phase as happens with slow cooling At high temperatures alloying metals are completely dissolved in the base metal so quenching traps the alloying metals within the crystal