Metals- Steel Metallurgy Flashcards
3 types of ferrous alloys
Iron- less than 0.008 wt% C in ferrite at room T.
Steels- 0.008-2.14 wt% C (normally under 1wt%), ferrite and Fe3C at room T.
Cast iron- 2.14-6.7 wt% C (normally under 4.5wt%)
Conventional process route for producing large volumes of standard grade steels
Limestone and coke added to iron ore in the blast furnace. This gives liquid iron with about 5% C. Hot metal goes to basic oxygen steelmaking (BOS). Here add scrap, limestone, oxygen, coal. Then goes to secondary refining, continuous casting, shaping
EAF route/ mini mil concept for making steel
Used an electric arc furnace. Oxygen and coal added to scrap or direct reduced iron (DRI). Then goes to secondary refining, continuous casting, shaping. EAF needs a lot of energy to operate. Environmental benefits if use renewable energy and good for recycling. Much more flexible with regard to starting material
Good things about steel
Cheap High strength High stiffness High toughness Easy to join and weld Versatile Recyclable
Bad things about steel
Very dense
Poor corrosion resistance for some types
3 grades of steel
Low carbon: 0.1-0.25% C
Medium carbon: 0.2-0.5% C
High carbon: 0.5-1.6% C
Applications for the 3 grades of steel
Low carbon: is more ductile so beams, car bodies, cans.
Medium carbon: general forgings, shaft, rotors.
High carbon: strong and wear resistant so railway lines, car springs, hammers, saws, drills, cutting tools, dies
How do tensile strength and %elongation vary with carbon content?
Tensile strength increases from under 400MPa to over 900MPa at around 1% C then starts to drop. %elongation decreases from 40% to 5% for 0% C to 0.8% C then doesn’t decrease by as much.
Different phases of pure iron as it is cooled
Immediately after cooling from liquid forms δ phase (ferrite) which is bcc (1534C to 1400C). Then becomes austenite, γ, which is fcc down to 910C. Then becomes α ferrite which is bcc.
Solubility of C in ferrite and austenite
In fcc austenite, C has high solubility as it can fit in the interstitial sites at the centre of the unit cell edges. In bcc ferrite, C has a near zero solubility as the interstitial sites are too small
Formation of pearlite
There is a eutectoid point from the γ phase into α and Fe3C phases at 0.8 wt% C. Slowly cool austenite of this composition and the grains of austenite become grains of alternating plates of ferrite and cementite.
What happens if composition of the steel is less than the eutectoid composition?
Proeutectoid α forms at the grain boundaries of γ. The remaining γ in each grain becomes pearlite
What happens if composition of the steel is more than the eutectoid composition?
Fe3C forms at the grain boundaries of the γ. The reassign γ in each grain becomes pearlite
Formation of martensite
Arises from fast cooling (quenching) of austenite. Normally the interstitial C atoms have time to diffuse out of Fe to form Fe3C. For fast cooling, the C atoms are trapped in the edges of the unit cell which becomes BCT. Results in a more spiky looking microstructure
Why is martensite strong?
It is a non-cubic structure so there are fewer slip systems so dislocation motion is much more difficult.
Interstitial C acts as an obstacle for dislocation movement.
Has a fine grain structure which causes strengthening shown by Hall-Petch equation
Why do we temper martensite?
It is too brittle to be widely used. Needs heat treatment to increase the toughness and maintain as much strength as possible
How does tempering of martensite work?
Use relatively low temperature 600C. At room temperature the diffusion of C in Fe to form Fe3C is very low and C is trapped in a super saturated solid solution. Raising the temperature allows the diffusion of C and the BCT structure transforms to equilibrium BCC
Microstructure of tempered martensite
Strong tough microstructure of Fe3C precipitates within ferrite. Same constituents as pearlite but not in layers (dispersed precipitates).
Compare the hardness and ductility of pearlite, martensite and tempered martensite
Pearlite has lowest hardness and highest ductility. Martensite has highest hardness but very low ductility. Tempered martensite in between for both
TTT diagrams
Time temperature transformation diagram. Temperature vs time graph with different regions for where each phase is formed. Tells you the cooling rate to use or avoid for a particular phase that would form. Highest temperature is eutectoid temperature.
What do the crystal structures of iron affect?
Level of carbon solubility. Whether it is magnetic or non-magnetic
What are stainless steels based on?
Based on the Fe-Cr-Ni system.
Ni added to stabilise FCC phase.
Cr added to stabilise BCC phase (over 12% Cr)
Basic types of stainless steel with compositions
Austenitic: 17-25% Cr, 8-20% Ni
Ferritic: 12-30% Cr, low C
Martensitic: 12-17% Cr, 0.1-1% C
Duplex: 25-30 Cr, 2-7% Ni, Ti, Mo
High strength low alloy steels
Microalloyed steels containing small amounts of V, Ti, Nb. Nb(C,N) particles prevent excessive grain growth during heating and rolling so get fine grain size (grain boundary pinning). A heat treatment that forms NBC particles leads to precipitation hardening
Properties and applications of HSLA steels
Strength levels twice that of normal mild steel. UTS up to 700MPa and strain to failure 10-20%. Tough, formable, weldable.
Automotive body sheet, strip plates, ships, off shore platforms
