Steels revision

Blast furnace

A counterflow chemical reactor where hot air is fed in and reacts with coke and iron oxide = pig iron

Steps for in blast furnace

1. Coke reacts with O2

   2C +O2 → 2CO

2. T increases + iron oxide is reduced

   3Fe2O3 + CO → 2Fe3O4 + CO2

3. Reduced further

   Fe3O4 + CO → 3FeO + CO2

4. Reduced further

   FeO + CO → Fe + CO2

5. Direct reduction at high T

   FeO + C → Fe + CO

6. Indirect reduction

   FeO + CO → Fe + CO2

7. Sulfur impurities are removed by the lime

   FeS + CaO + C → Fe + CaS + CO

8. SiO2 impurities are removed by the lime, producing slag

9. Pig iron Fe content » Steel

10. Also has XS that needs to be removed

What is indirect reduction driven by?

The burning of coke in the air, producing CO

BOF Process

1. Molten pig iron is inserted in converter

2. O2 is blown into furnace through water-cooled lance

3. O2 oxidises C in pig iron to make CO + CO2 → helpful as reaction = very exothermic so allows for some scrap steel to be added

4. Powdered lime = added to form foaming slag to oxidise leftover phosphorus

5. Mg = added to desulfurize pig iron to produce MgS which is raked off

Secondary steel making/ EAF

1. Molten steel transferred to ladle furnace to be stirred with argon fed in from bottom, heated using arcs from C electrodes → helpful as reaction = very exothermic so allows for some scrap steel to be added

2. T increased to 1800 deg + a new reducing CaC slag is added

3. May be in vac conditions for low O2 environment

High industrial electricity costs

Can be turned off when electricity costs are high

Alternative Ironmaking

• Top Gas Recycling : Separating CO + CO2 so CO can be recycled.

• Oxygen blast furnace : Replace air with pure O2 to avoid parasitic N2 in cycle

• MIDREX : DRI using CH4, making CO2 + H20- used when natural gas prices = low, producing raw material for EAFs as clean sub for scrap iron (solid briquetted iron)

• Hydrogen plasmas: uses reducing properties of H2 plasma to allow for hybrid reduction

Nucleation

• 1st random formation of thermodynamic new phase

• peak = max energy needed to put into system

• Energy barrier = surface energy

• Balanced out by XS Gibbs free energy per unit vol when vol of nucleus increases

• Eqn for energy: W = -4/3 pi r³ delta G + 4 pi r² * sigma

Equilibrium cooling

• Fast diffusion in solid + liquid

• All compositions given by phase diagram

Scheil cooling

• Slow diffusion in solid so solid evolves during solidification

• infinitely fast diffusion in liq

• different components of alloys don’t have enough time to equilibrate completely

Constitutional supercooling

• Slow diffusion in liquid comp to velocity of growth front

• liquid remains in supercooled state instead of normal solidification

• arises due to impurities

• disrupts formation of solid crystal structure

• can solidify once nucleation occurs

Casting processes

• Dendrites form in diff orientations based on their nucleation sites

• several adj dendrites can form single grain when solidification = complete

• Growing single crystal + avoiding dendrites = very difficult as must solidify slow enough to grow with planar interface

• Mosaicity -The spread of dendrite orientations in crystal

• Nucleants/ innoculants = added to provide more places for nucleation to occur

• Dendrite tips - can break off + be nucleation sites

Continuous casting

• Patented by Henry Bessemer

• Most common method of casting long products

• due to solute rejection ( from Scheil cooling), the composition must be homogenised so its the same everywhere

• Thinner slabs allow energy savings of up to 40% as solidification = faster, smaller primary dendrite arm spacing so lower homogenisation time

• process compression results in significant cost reductions

Equation for diffusion

D = Do * exp(-Q/ RT), X = (DT)^1/2

Examples of different casting:

• Sand casting ( manual process- doesn’t scale well)

• High pressure die casting

Why does casting contain porosity ?

Because metals have > solubility for gases in liq than solid- so during solidification, gas that gets rejected from solid can be trapped.

Examples of inclusions ( undesired foreign bodies)

• Compounds that remain in liquid metal from refining process

• Compounds from erosion of casting/ pouring moulds.

Additive manufacturing

• uses alloy powder (expensive)

• laser additive manufacturing machines ( cheap but slow)

• most suited for low rate production of small, high value intricate parts in non-critical applications

Hot working

1. After continuous casting + homogenizing, steel must be hot rolled to 3mm

2. reducing grain size by recrystallisation

3. recovery = removal of strain energy by rearranging dislocations into lower energy configs / allowing opposite-sense dislocations to annihilate

4. Recrystallisation = formation of new strain free grains, nucleating at grain boundaries + other high energy defects

5. grain growth (final process) - overall energy state = further reduced by reducing total area of grain boundaries.

6. M = EI/R

Galvanizing

• Coating material with Zn

• Zn oxidises instead of Fe in steel

• lifetime can be extended by increasing coating thickness

Paints

• Prone to spallation

• Paint blisters + rust spots start to appear

Stainless steels

• Can regenerate its own protective coating

• interstitial O sites are filled, making them good barriers to O2 diffusion

• Passivation : If scale = scratched, quickly regen by base metal

• addition of Cr forms Cr2O3 scale

Bearing steels

• Essential for mechanical devices in motion; need high strength and fatigue resistance.

• Classic bearing steel is "1C-1.5Cr" (SAE52100), with around 0.25% Mn and 0.3% Si.

• Yield stress around 1400 MPa and UTS over 2 GPa, but limited ductility.

• Control of O content and alumina inclusions is crucial for bearing steel performance.

• Recent developments focus on large offshore wind turbines and geared turbofans in jet engines, with a greater emphasis on structural integrity

Hardenability of steel

• ability to partially/ completely transform from austenite to martensite (when cooled in a given quenching medium)

• measures depth + distribution of hardness via formation of martensite in steel upon quenching from high T

What increases hardenability of steel?

Greater % of C in steel

Increase in austenite grain size

• decreases no. grain boundaries

• so available no. nucleation sites decreases

• so rate of formation of pearlite decreases

• so more opportunity for martensite to form

Ceramic toughness and failure stress

• Ceramics have a toughness around 1 MPa√m.

• For a flaw size of 2 µm, the failure stress is 400 MPa.

• Maximum achievable strength for sintered ceramic powders is limited without toughening mechanisms.

• SiC-SiC fibre-reinforced ceramics use fibers for crack blunting rather than strength increase.

Steel Toughness and Flaw Tolerance

• Most steels have a toughness around 100 MPa√m.

• At 2 GPa stress, a flaw size of 800 µm is required for failure in the strongest steels.

• Detectable flaws via dye penetrant testing or ultrasonic evaluation are around 100 µm.

• Overstressed steel will plastically deform and fail progressively, not suddenly.

• Steels in pressure vessels (e.g., gas cylinders, airplane fuselages, nuclear reactors) have a "leak-before-break" failure mode.

Ceramics and Glasses Failure Risk

• Ceramics and glasses need to avoid overstress and crack growth due to environmental degradation.

• Engineering controls are essential to survive sudden failures.

High integrity melting processes

Techniques to eliminate inclusions and reduce trace elements in steel:

• Vacuum Induction Melting (VIM): Melting under vacuum to reduce impurities.

• Vacuum Arc Remelting (VAR): Minimizes macrosegregation and porosity, uses a consumable electrode.

• Electroslag Remelting (ESR): Similar to VAR but with a slag layer to further reduce oxidation and filter inclusions.