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.
