Föreläsning 7 - engineering metals Flashcards
What is plastic deformation controlled by?
Dislocation movement. Depending on binding strength and crystal structure. It is easier to deform FCC (many slip systems), more difficult to deform HCP (less slip systems)
What are some results of hardening mechanisms?
- Defects are obstacles for dislocation movement
- Change of microstructure to make dislocation movement more difficult
- Higher yield strength, higher hardness
- Often lower elongation at break
What is cold working?
In metallurgy, cold forming or cold working is any metalworking process in which metal is shaped below its recrystallization temperature, usually at the ambient temperature. Such processes are contrasted with hot working techniques like hot rolling, forging, welding, etc.
We get much higher dislocation density after cold working => higher yield stress
What is hot working?
Re- crystallisation = new dislocation free grains are formed when cold worked metals are heated
Processing at a temperature above the re-crystallization temperature.
Provides deformation without raising the yield stress
Large deformations are possible.
What are the results of casting?
- Casting - structure - Different structures in different parts of the casting.
- Defects - pores, cracks
- Often slightly inferior mechanlical properties than rolled or forged materials
What are the results of welding?
- Welding - the material has melted and solidified- casting structure
- Heat affected zone (HAZ) - altered microstructure, grain growth, altered hardening
- often cracks
What are the results of precipitation hardening?
Heating - alloy elements are dissolved.
Quenching - no atomic movement, metastable
Ageing - particles are precipitated
=> fine dispersion of precipitates
=> higher yield stress
How is steel manufactured?
- Starting: iron oxide
- Reduced sin blast furnace:
iron oxide + carbon + energy => pig iron + carbon monoxide - The carbon content of the pig iron is adjusted to the correct carbon content, the steel is alloyed, scrap can be added
The process requires energy
- process energy
- energy to reduction (is constant)
Recycling saves energy and raw materials, produces less carbon dioxide emissions.
How do we form the steel?
- Continous casting
- Hot rolling
- Cold work
- e.g. Cold rolling
=> sheet metal, rails, beams, rod, thread
How does more carbon affect steel?
Ferrite = Fe, soft
Carbon => Fe_3C, hard, brittle, more carbon gives more Fe_3C
Finer particles of Fe_3C gives higher yield stress
What are some different heat treatments for steel?
Normalization, soft annealing, hardening
What is normalization?
Heating and slow cooling. Provides ‘normal’ microstructure, suitable for constructions where stiffness is important.
Heating a material to an elevated temperature and then allowing it to cool back to room temperature by exposing it to room temperature air after it is heated. This heating and slow cooling alters the microstructure of the metal which in turn reduces its hardness and increases its ductility
What is soft annealing?
Heating to temperature below 723 degrees Celsius => diffusion and grain growth, spheriodized perlite, lower yield strength. Used for steel to be machined and then hardened
What is hardening?
Heating and quenching, tempering. The carbon content determines the proportion of cementite. The temperature and time of determines the size of the cementite particles. Used when hardness and yield strength are important.
Why is steel alloyed?
- Solution harden ferrite
- Increase hardenability. Large alloy atoms delay formation of perlite => more time to harden
- Provide carbides other than Fe_3 with more stable high temperature properties.
- With Cr => chromium oxide layer, corrosion resistance
- Stabalize austenite at room temp.
Describe the properties of structural steels
Low carbon steels
- Low carbon content < 0.25%
- Cheap
- Weldable
- Cannot be hardened
Ex: buildings ships..
HSLA
- High strength low alloy steels
- Manufactured with a thermomechanical process that provides fine microstructure => high yield stress
- Can be welded
- Cannot be hardened
Ex: vehicle plate, bridges
Describe the properties of alloy steels
- Carbon content: 0.25-0.75% C
- Can be hardened
- Alloy increases hardenability
- Harder to weld
Ex: railway rails, hand tools, machine parts
Describe the properties of tool steels
- Carbon content: 0.5 - 1.7% C
- Alloyed to obtain stable carbides at high temp.
Ex: moulds, pressing tools, cutting tools, ball bearings
What is stainless steel?
Stainless steel is an alloy of iron that is resistant to rusting and corrosion. It contains at least 11% chromium and may contain elements such as carbon, other nonmetals and metals to obtain other desired properties.
Stainless steel’s resistance to corrosion results from the chromium, which forms a passive film that can protect the material and self-heal in the presence of oxygen.
Stabalize austenite at room temp.
What are the three types of stainless steel?
Ferritic
- Inexpensive
Austenitic
- Best corrosion resistance
- Low temp properties
Martensitic
- can be hardened
Describe the properties of cast iron
-2-4% C
- Common types: grey and ductile iron
- The microstructure consists of perlite and graphite
- In grey iron there is graphite as flakes => Lower E-module (1/2 steel), relatively brittle, vibration damping
- In ductile iron graphite is spherical => higher toughness
Ex machine parts, machine stand
Describe the properties of aluminuim
- Weight less than steel, density is 2.7 kg/dm^3
- FCC structure => good plastic malleability
- Good machineabilty
- Good electrical and heating conduction
- Corrosion protection
- Al reacts with O_2 and forms a protective oxide layer on the surface
What types of aluminium alloys are there?
Forging alloys:
- Both heat treatable and non-heat-treatable
Casting alloys:
- Non- heat- treatable
What are the properties of non-heat-treatable forging alloys?
Use:
Solution and deformation hardening.
Significantly lower yield stress, lower price => Less durable applications, building sheet, packaging, electrical conductors.
What are the properties of heat-treatable forging alloys?
Use:
Heat treated = separation hardening
- high yield limit, higher price
What are the properties of cast alloys?
Use:
Alloyed with Si => authetics that provide strength and wear resistance
cast components in aerospace, machine parts, household products
What are the properties of magnesium?
- Low density 1.8 kg/dm^3
- HCP structure => limited plastic malleabibilty
- Good machineability
- Good casting, most of it is used as castings
- Forms porous oxide layer, poorer corrosion protection
- Combustible - but only as powder or thin sheet metal
- Energy intensive production
Magnesium alloys use: 75% castings, 25% forging, ex: aerospace, sports, automotive, cameras
What are the properties of titanium?
- Average density 4.1 kg/dm^3
- Excellent strength
- Excellent corrosion protection
- Expensive due to manufacturing process
- Used in pure form or alloy
- Only biocompatible metal (not negative for body)
ex: chemical industry, implants, sport
What are the properties of copper and some copper alloys?
- High density 8.9 kg/m^3
Copper: Excellent malleability, high electrical and thermal conductivity
Brass: alloyed with 5-40% Zn, good shape and machineability
Bronze: alloyed with 5-25% Sn, good strength but little ductility, often cast
Good tribological properties
What are the properties of nickel and super-alloys?
- Exceptional good high-temp. properties with good oxidation and corrosion properties
Alloyed with: Cr, Co, Al, Ti, Mo, Zr, Fe, Hf
Compare stiffness and density for engineering metals
Steel and nickel: highest E-modulus
Steel, copper and nickel based alloys have high density
Cast iron has half E-modulus of steels
Titanium, Aluminium and magnesium lower E-modulus in proportion to density
Compare yield strength for engineering metals
Steel has highest yield stress
Titanium has high yield stress, considering density
Aluminium and magnesium have lower yield stress in proportion to density
Compare price strength for engineering metals
Cast irons have lowest price
Carbon steels an low alloy steels have low price
Aluminium, magnesium and stainless steels are more expensive
Titanium is significantly more expensive
Compare max service temp. for engineering metals
Super alloys: 1000 degrees C
Stainless steels: 750 degrees C
Low alloy steels: 500 degrees C
Aluminium and magnesium: 200 degrees C
Compare CO_2 footprint for engineering metals
Magnesium and titanium at the top
Steels and cast-iron have lowest footprint
(primary production)
Why are steel and aluminium used so often?
Steel: good mechanical properties, cheap, low environmental impact per kg, many variants for different applications
Aluminium: lower density, good price, easy to machine, good corrosion protection
Other metals are selected when their special characteristics are needed.
