deals with the atomic structure of metals

EXAM SUMMARY – METALS

‘Physical metallurgy’ – deals with the atomic structure of metals.

‘Mechanical properties’ – stress-strain, tensile strength, ductility, plasticity etc.

‘Mechanical metallurgy’ – involves the understanding & control of processes such as hot working, cold working & joining.

• These three processes allow metallurgists to control the internal microstructures of metals & through that, the properties of the metals.

PHYSICAL METALLURGY – What is it for different metals of interest?

Grain structure of metals:

• ‘Metallography’ – the technique most widely used to examine the structure of metals.

Three key things to remember:

• Within each grain the atoms are arranged in a regular lattice.
• The orientation of the crystal lattice differs from grain to grain.
• At each grain boundary there’s a mismatch in the atomic arrangement.

Crystal structure of metals:

• Metal ions pack as closely as possible to achieve as high density as possible = only three types of crystal structures possible.

Three possible arrangements:

• Hexagonal close packed (HCP) e.g. magnesium, zinc, titanium.
• Face centered cubic (FCC) e.g. iron above 910C, aluminium, copper, nickel, lead.
• Body centered cubic (BCC) e.g. iron below 910C, chromium, molybdenum.

Changes from one of these structures to another can be brought about by changes of temp, snap freezing etc. – extremely important in metallurgy.

Solutions & compounds:

Metals nearly always alloyed with other elements to obtain better mechanical properties e.g. steel.

Steel can alter its properties when heated – the phase diagram is all about slow heating & cooling of the metal.

Carbon: An interstitial atom that tends to fit into clusters of iron atoms.

• Strengthens steel & gives it ability to harden by heat treatment.
• Major problems in welding over 0.25% – hydrogen cracking.

• 0.05 To 0.29% carbon = mild steel/low carbon steel – structural steels in this region.
• 0.3 – 0.5% = medium carbon steel.
• 0.6+ = high carbon/ultra-high carbon steels.

Austenite: This phase only possible at high temps (above 723C) – face centered cubic atomic structure, which can contain up to 2% carbon.

Ferrite: Body centered cubic atomic structure, which can hold very little carbon – .0001% at room temp – can exist as either alpha or delta ferrite.

Cementite: Very hard intermetallic compound consisting of 6.7% carbon & the remainder iron – very hard but when mixed with soft ferrite it’s reduced a lot.

• Slow cooling gives pearlite – soft, easy to machine but poor toughness.
• Faster cooling gives very fine layers of ferrite & cementite – harder & tougher.

Martensite: If steel is rapidly cooled from austenite the FCC structure changes rapidly to BCC with insufficient time for carbon to form pearlite = Martensite.

• Only parts that cool fast enough form Martensite – in a thick section it will only form to a certain depth.
• Very hard & brittle – unless carbon content is extremely low.

By heat-treating steel you can change its atomic structure therefore changing its properties.

• Ferrite + cementite = up to 2% carbon & 723C.
• Austenite + ferrite = up to 1% carbon – 723-910C.
• Austenite + cementite = 1-2% carbon – 723-1147C.
• Austenite = up to 2% carbon – 723C +.

Tempering of steel: Get rid of unwanted Martensite = heat the steel to 723C – releases the carbon trapped in the Martensite – relieves the stresses.

• This process reduces hardness, increases toughness but reduces tensile strength – the degree of tempering dependent on temp & time.

Annealing of steel: Heat treatment process that produces softening of the structure – heat the steel to austenite & hold to create stable structure – then cooled very slowly to room temp = very soft structure, very large grains – not desirable poor toughness.

Normalising steel: Returns structure back to normal – steel heated until austenite just starts to form then cooled in air = fine grains with uniform pearlite – moderately rapid transformation.

Solid solutions: Some alloying elements can dissolve in the basis metal = solid solutions – two principal classes of solid solution:

• Interstitial solid solution = small atoms fit into spaces between larger atoms.
• Substitutional solid solution = dissolved atoms replace some of the host metal’s atoms because they’re similar.

Some atoms don’t completely fit into host’s lattice = surplus atoms get rejected as an ‘intermediate compound’ e.g. iron carbide in steel – hard & brittle – exert a ‘reinforcing effect’ on the softer metal that they are dispersed in.

• Solutions & compounds = ‘phases’ = a material or part of a material which has the same structure & properties.
• Physical metallurgy all about getting the right phases together with the right dispersion so that you can make the best use of it.
• The microstructure of the metal is what controls its strength & the properties related to strength.
• Energy & equilibrium diagrams extremely important.

MECHANICAL PROPERTIES – What are they for different metals of interest?

– O-A = stays the same.

– A-B = permanent deformation starts to occur.

– If unloaded at B = goes back to C – permanent deformation has occurred.

– A-E = elongation throughout whole material.

– E-F = ‘necking’ prior to fracture (F).

Proof stress = used where some alloys (e.g. aluminium) don’t show appropriate elastic limit.

Tensile strength = the max stress a material can withstand before ‘necking’ occurs.

Necking = large decrease in the original cross-sectional area.

Uniform elongation = the value before deformation is confined to the neck area – prior to this the elongation occurs throughout the whole material.

Ductility = the extent in which the material can be plastically deformed without fracture.

Slip: How plasticity works – works its way through material like moving a heavy carpet.

Strengthening of metals:

Critical property is yield stress – if material requires higher stress to produce yield then the safe working stress is also increased – concerned with how we can make the start of slip more difficult.

How we strengthen metals:

• Grain boundaries
• Strain hardening SEE NOTES
• Annealing
• Dispersion hardening

FORMING – What are the different methods of formation?

Castings:

Most metals can be produced by melting & casting into moulds:

• Shaped & dimensioned according to required component geometry,
• Or a prism of material produced for further casting

When intended for further processing metal solidifies to a coarse grain structure (because little attempt made to control it) = casting defects e.g. porosity, compositional variations, shrinkage – further processing rectifies them.

• ‘Shaped castings’ need more care – normally degassed, grain size carefully controlled, compositional variations minimized by attention to solidification patterns.

Hot working:

***DEFINITION

‘Hot working’ – the working of metals & alloys by rolling, forging, extrusion etc. – depends upon plasticity which is much greater at higher temps i.e. temps above their recrystallization temperature.

• This allows all the common metals to be heavily deformed during hot working, without breaking.

Rolled: For steel & structural members = hot rolling between cylindrical or shaped rolls at temps around 1000C+ – afterwards the members left to cool naturally.

• Result in annealed microstructure & grain sizes – dependent on the weight & temp of deformation & the cooling rate – must be controlled to give products with consistent properties.

The exposure to air at high temps during hot working = heavy film of oxide forms surface.

• Therefore steel sections delivered ‘as rolled’ are covered with iron oxide & need to be shot/sand blasted before receiving protective coating.

Forged: Other articles e.g. engine crankshafts are forged into shape.

Extruded: Many metals can be extruded – advantage of very long lengths with complex sections able to be produced e.g. aluminium glazing bars.

Disadvantages: – The contraction of dimensions on cooling.

• Oxidation.
• In some cases hot-formed parts require further cold forming or machining to meet more demanding tolerances.

Cold forming:

***DEFINITION

‘Cold forming’ – Shaped at temps below the recrystallization temperature of the metal/alloy.

• Creates a lot of dislocations – metal work hardens & its yield point raises – for pure metals & some alloys it’s the only way to increase the yield strength.

Cold drawing: High-strength wire (used in cables) is cold drawn by pulling it through a tapered die.

Deep drawing/stretch forming: Metal sheets shaped into cups, bowls etc. by using these methods.

There is however a limit – beyond which the ductility is exhausted & the metal will fracture – if further cold work required metal must be annealed by heating to a temp where recrystallization occurs, original ductility restored.

• Some metals can’t be cold extruded.

Joining methods:

• Adhesives – Soldering
• Welding – Mechanical fasteners (rivets & bolts)
• Brazing

Welding: Metal components heated locally to melting temp (additional metal may be added), joint then cools – complete continuity between the parts should be present, joint indistinguishable from parent metal & joining material no worse properties than the parent metal.

• Surrounding the weld is a heat-affected zone (HAZ) – in the regions that have exposed to high temps & fast cooling rates, metallurgical changes will occur.

Quality of the joint affected by:

• The structure & properties of the weld metal.
• The structure & properties of the HAZ.

Both of these affected by the rate of cooling after welding – slower the rate of cooling the closer the structure is to equilibrium (the greater the thermal mass the faster the cooling rate).

• Has to be designed to incorporate welding & its affects.

Brazing/soldering/adhesives: All put a thin film of material on the parent material that then bonds together.

• Good brazed/soldered joints should have as much strength as the parent metal.
• High forces are needed to break this film of liquid – provided film is thin enough.

Mechanical fastening (pinning): Some metals don’t weld/solder/braze well e.g. cast iron – bolting/riveting used – both rely on friction.

Bolts:

• Tightened bolt forces the two members together & the friction between nut & bolt at the threads holds it in place.
• High strength friction grip (HSFG) bolts used in structural steelwork combines bolting & riveting – nut is tightened to place the bolt into tension & this tensile pre-stress acts in the same way as the tensile stress in the rivet.

Rivets:

• The hot rivet is hammered into prepared holes – as it cools it contracts & develops a tensile stress, which locks the members together.

OXIDATION & CORROSION – What causes these matters?

Corrosion occurs in two forms:

• Dry oxidation
• Wet corrosion

Dry oxidation:

Earth’s atmosphere is an oxidizing one – nearly all metals deteriorate because of the presence of oxygen – general oxidizing reaction – M+O=MO.

Takes place in two steps:

• The metal forms an ion, releasing electrons & the electrons are accepted by oxygen to form an ion.
• The ions then attract one another to form a compound.

The oxygen ions attach themselves to the surface to form a thin layer of oxide, then:

• The oxygen ions must diffuse inwards,
• OR, the metal ions & electrons must diffuse outwards through the oxide to meet oxygen at the surface.

The rate of oxidation determined by how fast the oxygen gets in (or the metal gets out if it proceeds faster), – this is controlled by the thickness $ structure of the oxide skin.

• On some metals the oxide occupies less volume than the parent metal & on others it occupies more – both resulting in the brittle oxide breaking away & exposing fresh metal.
• However some metals the oxide matches the metal volume = thin films form that act as a barrier to further oxidation e.g. aluminium, stainless steel.

Wet corrosion:

In the presence of moisture the situation changes drastically & the loss of material by corrosion = very appreciable.

• Like dry oxidation – wet corrosion involves ionization but, if the ions are soluble in the corroding medium (usually water) the metal steadily corrodes.
• No water &/or no oxygen = NO WET CORROSION!

For iron/steel rusting in oxygenated water/moist air, the water on or near the metal surface acts as the electrolyte of the corrosion cell & the anode & cathode are close together. The oxide is formed & deposited near but not directly on the metal surface – allowing the corrosion to be continuous.

Ionization produces a change in electric potential – this can be measured relative to a reference value – metals which are more positive are anodic &will corrode, metals which are negative are cathodic & won’t/won’t as much.

• Anodic e.g. magnesium, zinc, zinc, iron.
• Cathodic e.g. gold, platinum, silver, copper.

When pairs of metals are immersed a galvanic cell arises – one (the lower on the table) becomes the cathode & unreactive & the other (the higher on the table) becomes the anode & corrodes.

• The further away metals are on the table the more they will react with each other.
• We can use this composition cell to our advantage e.g. the use of zinc to galvanize steel – zinc is more anodic than iron & on exposure to atmosphere the zinc corrodes first – protecting the steel.

A variation in corrosion – ‘concentration cell’ of preferential corrosion can occur due to differences in the electrolyte or deposition on the surface e.g. ‘waterline corrosion’ – the surface layers of the water richer in oxygen & become the cathode – the lower oxygen-deficient layers are anodic & corrosion occurs here.

Control of corrosion:

Corrosion problems start & must be dealt with at the design stage.

Three requirements:

• Understand the environment in which the metal must work.
• Consider the ‘design life’. SEE NOTES
• Select the most appropriate method of control.

Protection against corrosion:

Three ways: SEE NOTES

• Design – take steps prior to the structure being erected.
• Isolation from the environment – applying one or more protective coatings to a suitably prepared surface.
• Cathodic protection – use of external power source to make the metal cathodic to its surroundings.

THEIR DIFFERENCES & USES – Why would you use them over others?

Metals divided into:

• Ferrous (based on iron).
• Non-ferrous (based on metals other than iron).

Main ferrous alloys used are cast iron & steel:

• Cast irons contain more than 1.7% carbon.
• Steels contain up to about 1.7% carbon.
• Structural steels contain about 0.25% carbon.

Main non-ferrous alloys in civil engineering are based on aluminium or copper.

Extraction of iron:

Iron is extracted from naturally occurring ores (like all metals).

• High temps (1600C) needed to allow the collection of carbon monoxide and result in pig iron (industrially useless – 4% carbon). Then remelted & with controlled oxidation using air blast the carbon content is reduced to 2-4% – cast into sand moulds = cast iron.

Cast Irons:

Usually brittle & best used in compression rather than tension.

• Used mainly in pipes & fittings for services & for civil engineering – tunnel linings & mine shaft tubing (specifically SG cast iron).

Four main types:

• White cast iron.
• Grey cast iron. SEE NOTES
• Speroidal graphite (SG) cast iron.
• Malleable irons.

Joining of cast irons: Extremely difficult to weld – bolting is the safest method.

Steel:

• Consists of iron, carbon, manganese & silicon.

Steel making involves very complex thermochemistry – the basic reaction is simply that of reducing the carbon content further (for structural steels down to approx. 0.25%), by a controlled oxidation process.

• To keep the reaction going a considerable amount of oxygen must be used – some dissolves in the liquid steel.
• If the oxygen isn’t removed this would form a hard, brittle iron oxide – thus when the required carbon content is reached the oxygen is ‘fixed’ as an oxide which is eventually removed as slag.

Manganese & silicon used to fix the oxygen in this way – steels that have been treated in this way are called ‘killed steels’.

• Manganese also gets rid of Sulphur that forms iron sulphide – causes ‘hot shortness’ where steel cracks disastrously.
• If elements other than the normal manganese & silicon are added – the steel becomes ‘alloy steel’ & if certain elements (chromium & nickel) are added in quantity it becomes ‘stainless steel’.

Structural steels:

Processed into the required section shapes & lengths by hot rolling $ their microstructures are the same as normalized steel.

• Consist of two phases: ferrite & cementite – combined together into laminar regions of alternating layers.
• Steel containing these regions, has been properly prepared & has a colored pearly appearance = pearlite – 0.8% carbon – steels containing less carbon are mixtures of ferrite & regions of pearlite.

The properties of steels are strongly affected by how much pearlite they contain (or their carbon content) – as pearlite levels rise = tensile strength rises, hardness rises, elongation falls.

Structural steels can go through a ductile to brittle transition as the temperature of use changes – shown by impact tests at different temps.

• Aim is to formulate steels that’s ductile to brittle transition temp (DBTT) is low & which can be joined successfully by welding.
• To achieve this the carbon content must be low, must be a high ratio of manganese to carbon & a small grain size of the ferrite – yield stress increases by reducing grain size.

To produce & maintain fine grain sizes control of the following is important:

• Temps of hot rolling.
• Amounts of deformation imposed.
• Cooling rates.

Cold rolled steels:

Cold rolled steel of very low carbon content is used to produce many lightweight sections e.g. lintels, angle sections.

• Strength derived from work hardening of the ferrite.
• Good control over section shapes & sizes possible.
• Welding will locally anneal the material = changes to the properties in the HAZ.
• ‘Cor ten’ – a special type of steel – contains small amount of copper – when exposed to rainwater it rusts & produces a hard, adherent & protective oxide layer.

Heat-treated steels:

Carbon content > 0.3% = properties of the steel can be varied by heat treatment i.e. by fast cooling (quenching in water/oil) from a high temp, followed by reheating to temps not exceeding 650C (tempering).

• The fast cooling = martensite – hard, brittle, all of carbon trapped in it.
• Reheating more than 650C = carbon releases, becomes softer, more ductile.
• By varying the tempering temperature & controlling the amount of carbon left in the martensite = great control over the properties of the steel can be achieved.

These steels don’t find great application in structural engineering – except HSFG bolts which are supplied in the hardened & tempered condition – therefore these bolts shouldn’t be reheated or the effects of the heat treatment may be jeopardised – could easily become brittle.

Stainless steels:

A wide range of ferrous alloys – all contain at least 12% chromium – produces a stable, passive oxide film.

• Other alloying elements such as nickel & molybdenum may also be present.

Three basic types:

• Martensitic: low carbon steel, 13% chromium – heat treatable, can be made very hard – unweldable.
• Ferritic: 13% chromium & very low carbon – not heat treatable – reasonably ductile, middle strength – unweldable.
• Austenitic: low carbon, 18% chromium & 8% nickel – not heat treatable – reasonably ductile, good strength – can weld.

All three have good corrosion resistance – except in situations of low oxygen.

Aluminium:

The term ‘aluminium’ is normally used to include aluminium alloys.

Pure aluminium & some alloys can be readily rolled & extruded into long lengths with complex cross sectional shapes e.g. cladding, window frames.

– Alloys may be cast or wrought while structural sections produced only by extrusion.

Expensive:

• Only competes with steel where its properties can be exploited i.e. lightness, strength, durability, appearance.
• Most economic use of lightweight materials is in high ratio of self-weight to live-load structures i.e. roofs, footbridges & long span structures, & where the lightness of the material offers advantages in transport, handling & erection.
• Good durability of aluminium – good in polluted & coastal areas – high initial cost may be offset by reduced maintenance.

• Young’s modulus lower than steel (70GPa to 210GPa) – meaning greater deflection under a given load & lower thermal stress developed by a given rise in temp.
• Density significantly lower than steel.
• Doesn’t show definitive yield stress like mild steel – the working stress usually defined as that stress at which a small amount of plastic deformation has occurred – ‘proof stress’ – the stress corresponding to a plastic deformation of 0.1 or 0.2%.

Three types of aluminium alloys:

• Casting alloys – based on eutectic alloy system – solidifies over narrow temp range making very suitable for casting into moulds that cause rapid solidification.
• Wrought non-heat treatable – properties controlled by how much they have been strain (work) hardened.
• Age hardenable – properties can be changed by heat treatment – all about dislocation movement.

Ageing = heated to 550C copper dissolves into solution & remains in there when rapidly cooled = fine dispersion of a hard, intermediate compound forming with small, evenly dispersed particles = max resistance to dislocation movement & yield stress now a lot higher than pure aluminium.

Over ageing: Ageing process speeded up by reheating to 150C – but if reheated to high = particles clump together = dislocations pass easily & yield strength lowered.

• Durability of aluminium alloys greater than steel.
• Corrosion resistance depends on the alloys composition & heat treatment – fully heat treated alloys most susceptible – need protection.

Joining:

• Welding of casting & non-heat-treatable alloys possible but usual care taken – when welding heat-treated alloys the thermal cycle will inevitably produce an over-aged structure.
• Overall preferable to use bolting/riveting especially for joints made on site – steel bolts may be used but should be protected by zinc coating.
• Arrangements must be made to keep the bolts/rivets (if not aluminium) electrically isolated from the aluminium – bimetallic corrosion can lead to rapid attack of the aluminium.

Structural forms:

• Most readily available & special sections can be produced by extrusion more readily than steel – expensive, only justified when large quantities are needed – although the use of special sections allows more freedom & scope e.g. extruded widow frame section.

Copper & alloys:

Main use where its compatibility with water, high thermal conductivity & high electrical conductivity are important e.g. domestic water services, heating, sanitation etc.

• High resistance to corrosion, pleasing color of its oxide but expensive.

Two alloy families widely used:

• Brass
• Bronze

Brass:

• Alloy of copper & zinc with other additions to produce enhanced strength.
• Two main classes: alpha brasses (70% copper, 30% zinc) & alpha-beta brasses (60% copper, 40% zinc).
• Both stronger than pure copper, 70:30 brass is extremely ductile.
• Not heat-treatable – difficult to weld – better soldered/brazed.
• Not as corrosion resistant as pure copper but cheaper.

Bronze:

• Alloys of copper & tin – with a range of possible additions to achieve specific properties.
• Usually formed by casting – some of the less highly alloyed bronzes ductile enough to enable sheet metal working.
• Stronger & harder than copper & brass, high corrosion resistance & weldable by inert gas processes – all expensive.