Iron and Steel Flashcards
Lecture 11
What are the different types of structural metals (ferrous)?`
Iron (Fe)/steel (Fe+C) are undoubtedly the most important metals in structures
- chiefly for primary load transfer
What are structural metal properties affected by?
the amount of carbon
other alloying elements (Mn, Si)
processing
What is Cast Iron
Generally brittle
high levels of carbon (>1.8%)
leads to low resistance to fracture/low toughness in tension
OK in compression -> designed in compression
Why did the Cast Iron Case Study - Tay Bridge fail?
the Tay Bridge was built in 1871-7
it collapsed on a windy night in December 1879, while a train with 75 people on board was crossing
the failure was traced to the use of cast iron and unexpected loading
made worse as holes for joints were cast in (stress concentration)
Why are we interested in Steel as a structural metal?
An alloy of Fe and carbon
0.008 wt% C to 2.14 wt% C
Annual production (2022) = 1.86 billion tonnes
compared with:
- 20 million tons of aluminium
- 15 million tons of copper
~100 thousand tons of titanium
What is a Blast furnace?
What is Steel processing?
In processing steel, after extraction (mining) of the raw materials there are several stages:
- ironmaking (conventional process - blast furnace - reduction using C, new technologies looking into using H)
- steelmaking - very limited in scale before Bessemer
- secondary steelmaking
- casting
- primary forming
What needs to be reduced to produce steel?
To produce steel the C content needs to be reduced
- low C steel < 0.25 wt% C
- medium C steel 0.25 - 0.6 wt% C
- high C steel 0,6 - 1.4 wt% C
also aims to reduce other impurities (P, Si, Mn, etc)
With steelmaking being done using oxygen injection, name the 2 main steelmaking methods.
basic oxygen steelmaking
electric arc furnace
What is the basic oxygen steelmaking (BOS)
Molten iron from the blast furnace is paired with the converter
oxygen blow - “pig iron” because not an easy iron to work with
oxidation of impurities gives a surplus of heat (can melt some scrap (no further heating is needed)
~ 300 tonnes of steel/cycle
- 30 - 40 minute cycle
- converter in the upright position (lance from the top or blow from the bottom)
the reaction between iron and carbon is exothermic (produced heat, therefore, no need to add more)
What is the electric arc furnace?
main input:
- iron and steel scrap
- steel is the world’s most recycled material
electrodes supply power (500 V, 100,00 A for a 100-tonne furnace) - arc can melt 100 tonnes of cold steel in about 90 minutes
What is the secondary steelmaking/casting?
secondary steelmaking
- further small composition adjustments are made
- create alloys
for further processing want large ingots (blooms) of steel
- continuous casting is most commonly used (about 90% of the world’s steel)
What is steel processing - finishing?
To use the blooms we need to shape them (finishing)
Plate - thickness generally > 6mm (e.g. ship hull plates)
sheets - < 6mm (e.g. car bodies)
rails and sections - complex cross sections (e.g. construction girder)
wire and bars - width and thickness roughly equal
finally, a heat treatment will give the required properties
What is the further shaping of steel processing?
most products have a particular required shape and dimensions
various processes are available to achieve this
each affects the material structure, and so properties
these apply to all metals not just steel and in some cases other materials
What is casting?
requires a melt
involves tipping/injecting the melt into a shaped mould
- sand casting (shaped sand mould)
- die casting (melt injected by a piston into a die)
- injection moulding (melt pushed into mould be a screw plunger)
low cost, but can lead to defects
What is rolling?
properties are better if the material is deformed
this can
- squeeze out porosity
- refine the microstructure
- work harden the material
one method (especially for long products is rolling)
What is forging?
An alternative is to forge the material - probably the oldest metalworking process (e.g. a hammer and anvil)
dies can be flat - open die forging
or shaped - closed die forging, impression die forging
often done with hot metal
produces good microstructures for high-strength
What is extrusion?
extrusion can produce complex shapes in one long direction
steel extrusion needs temperatures > 1000C
extrusion is common for aluminium, polymers, etc
