Steel Flashcards
What does the lever rule show?
The relative amount of each phase in the 2 phase region
What do tie lines show
The composition of phases in 2 phase regions
What are the key steel manufacturing processes?
- Blast Furnace
- Basic Oxygen converter
- Electric arc furnace
What are the main slow cooled microstructures?
- Austenite
- Ferrite
- Cementite
- Pearlite
What are the main rapid cooled microstructures
Martensite
Tempered martensite
What kind of reaction is the cooling of steel?
Eutectoid
One solid to two different solids
What temperature does iron transform from bcc -> fcc
912 C
What is the solubility of iron in ferrite?
(alpha-iron, bcc) 0.02 wt%
austenite (gamma-iron, fcc): 2.1 wt%
What are the 3 eutectoid compositions?
<0.8% - hypoeutectoid
0.8% - eutectoid
>0.8% - hypereutectoid
What happens in the hypoeutectoid composition?
(Ferrite and pearlite)
Ferrite grains formed from austenite
Remaining austenite goes to ferrite and cementtiete
Pearlite is formed
What is pearlite?
Fine lamenar structure formed from ferrite and cementite - it is like a metal-ceramic nano-composite
What determines the fineness of the ferrite?
the cooling conditions
What temperature is pearlite formed at?
723 C
What is the eutectoid composition of steel?
@723 C Austenite –> ferrite + Fe3C
Pearlite
What is the hypereutectoid composition of steel?
austenite +Fe3C
Below 723 C:
Ferrite + Fe3C
cementite and pearlite
What are the properties of hypereutectoid steel?
Brttle, as conitnuous brittle phase
What kind of steels do we generally use in engineering?
Binary alloys of iron and carbon @<0.8 wt % C
How much manganese, phosphorous and sulphur do carbon steels usually contain?
Manganese (0.45 - 0.9 %)
Phosphorous (0.025 - 0.06 %)
Sulphur (0.03 - 0.05 %)
Do phase diagrams show rapid cooling?
No
What is the effect of rapid cooling on the composition steel?
The crystal lattic tries to switch from fcc (austenite) to bcc (ferrite).
Excess carbon –> distorted body centered lattice –> martensite
What is the benefit of iron carbon martnesites?
- Many interfaces
- Interiors of ‘laths’ heavily - - twinned or dislocated
- high and strongly varying stresses
very high resistance to dislocation motion
What are the poperties of martensite?
Hard and brittle
too brittle to be useful
What can be done to make martensite less brittle?
can be tempered
What are the applications of tempered matrensite?
Crankshafts, spanners, high tension bolts
What is tempering of martensite?
Heat treatment at 200-600 C
allows C atoms to diffuse out of martnsite
alpha’ -> alpha + Fe3C
How is Fe3C present in tempered martensite?
Uniform distribution of fine round precipates –> high strength and toughness
What happens if martensite is tempered <350 C
Precipitates as epsilon-carbide
~Fe2C5
Internal stresses dissapear
What happens if martensite is tempered between 350 - 500 C
epsilon carbide precipitates to form cementite
What happens if martensite at >500 C?
Cementite precipitates coarser, larger and more widely spaced
“spheriodised”
What makes stainless steel?
Adding >11% wt % Cr
What protects against corosion in stanless steel?
Adherent Cr2O3 film
prootection against corrosion and oxidation
What are most stainless steels?
Austenitic
What is a useful test for Austenitic stainless steel?
It is not magnetic
How is molten steel produced
- Limestone + Iron ore
- into sinter plant (fueled by coke ovens)
- Blast furncae produces molten iron
- this is fed into a basic oxygen vessel
- this produces molten steel
How does an electric arc furnace work?
Recycled/ scrap steel fed in
electric arc furnace feeds to secondary steelmaking
This then produces molten steel
How does a blast furnace work?
- Iron ore coke and limestone added at the top
- Oxygen in air reacts with coke to produce carbon monoxide
- This passes up the furnace transforming iron oxide to iron
- liquid iron collects at the bottom of the furnace, covered by a layer of molten slag
- the iron is tapped at the bottom, andnd the slag is also tapped off from a seperate hole
- This produces pig iron
What is the function of a basic oxygen converter?
Further reduces C content
8m in diammeter and 11m high
it is lined with ~1m of magnesite bridge
To what C content does a basic oxygen converter convert C to
0 and 1.5%
What is the process behing and electric arc furnace?
Charged with “cold” material - normally scrap or iron from blast furnace
- steel is tipped into the EAF
- lid is swung into a position that contains electroes
- electric current is passed throuh the electrodes to form an arc
- O2 is blown into the steel Lime and fluorspar are added to form slag
- the furnace is tilted to remove slag
- molten steel in porued into a ladle for secondary steel making
How much steel does and EAF make in 90 minutes?
150 tonnes
How much CO2 is produced per tonne of steel?
1.7 tonnes
How many tonnes of CO2 per year come from steel?
2.73 billion
