Unformatted text preview:

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

View Full Document

UJ CMA 03A3 - Summary

Download Summary
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...

Join to view Summary and access 3M+ class-specific study document.

We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view Summary 2 2 and access 3M+ class-specific study document.


By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?