Baixe Metalurgia bsica e outras Notas de estudo em PDF para Engenharia Metalúrgica, somente na Docsity! Engineering “C” – High Perfomance Alloys: HT 2000 S.G. Roberts 2: 1 The region of most interest is around the eutectoid reaction γ ⇒ α + Fe3C. Steels – the Basics Revisited Ferrite α Fe BCC δ Fe BCC 0 200 400 600 800 1000 1200 1400 1600 1800 0 1 2 3 4 5 6 7 Austenite γ Fe FCC Liquid Cementite Fe3C α Fe + Cementite γ Fe + Cementite γ Fe + Liquid ( Liq uid + G ra ph ite ) wt% CarbonFe T (ºC) 5 10 15 20 25 at% Carbon Most steels are based on the “metastable” phase diagram Fe – Fe3C. Engineering “C” – High Perfomance Alloys: HT 2000 S.G. Roberts 2: 2 Ferrite α Fe BCC 400 500 600 700 800 900 1000 1100 1200 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 α + Fe3C γ + Fe3C α + γ Austenite γ Fe FCC wt% Carbon T (ºC) Eutectoid (0.8wt%C) Carbon Steels 50 – 500 µm Pearlite 1-10µm Engineering “C” – High Perfomance Alloys: HT 2000 S.G. Roberts 2: 5 Quenched steels - Martensite 0 100 200 300 400 500 600 700 800 0.1 1 10 100 1000 10000 γ γ + α γ + α + C α + C Ms M50 M90 50%1% 99% T (ºC) time (s) Time – Temperature – Transformation diagram for “2340” steel: 0.37% C, 0.7% Mn, 3.4% Ni. Slow cooling: 1: α nucleates at γ grain boundaries and grows into γ grains. 1 2: Cementite starts to form: α and C grow together into γ grains as “pearlite”. 2 3: 50% of the γ has been transformed 3 4: Decomposition of the γ into α and pearlite is complete. 4 Engineering “C” – High Perfomance Alloys: HT 2000 S.G. Roberts 2: 6 Quenched steels - Martensite 0 100 200 300 400 500 600 700 800 0.1 1 10 100 1000 10000 γ γ + α γ + α + C α + C Ms M50 M90 50%1% 99% T (ºC) time (s) Time – Temperature – Transformation diagram for “2340” steel: 0.37% C, 0.7% Mn, 3.4% Ni. Rapid cooling (“quenching”): 1: γ is supercooled past the diffusion- cotrolled transformation “nose” . Too fast for α to nucleate. 1 2 2: γ now well below normal transformation T, but diffusion is very slow; carbon is “stuck” in supersaturated solution. 3 3: At Ms temperature, γ⇒α free energy change big enough to force rapid diffusionless transformation to near-α structure but with trapped carbon atoms – “martensite” 4 4: γ ⇒ martensite transformation is complete. Engineering “C” – High Perfomance Alloys: HT 2000 S.G. Roberts 2: 7 Formation of Martensite The face-centred cubic structure can distort to give a body centred structure. Each BCC unit cell is directly related to the “parent” fcc unit cells. Engineering “C” – High Perfomance Alloys: HT 2000 S.G. Roberts 2: 10 Tempering Martensite 400 0 800 1000 1200 100 200 300 400 500 600 700 600 20 40 60 80UTS σY εfract Impact energy σ σσσ Y , U TS (M Pa ) ε εεε f ra ct (% ), Im pa ct e ne rg y (J ) Tempering temperature (ºC) Effects of tempering on a 0.5% C, 0.7% Mn steel, quenched into water from 830ºC Low temperatures (<~350ºC): carbon precipitates as “ε-carbide” (~Fe2C5) internal stresses disappear Moderate temperatures (~350ºC - ~500ºC) ε-carbide precipitates transform to cementite High temperatures (>~500ºC) cementite precipitates coarsen (larger and more widely spaced) “Spheroidised” Tempering trades off strength for toughness. Compromise selected depends on application. Typical tempering range for “engineering” steels is 250 - 450ºC. Lower T (and higher C) for blades, etc; Higher T (and lower C) for shafts, gears, etc. Engineering “C” – High Perfomance Alloys: HT 2000 S.G. Roberts 2: 11 Alloying elements in Steels ++ + (in γ) + + Grain refine? +++ + +++ +++ Solid soln Strength? N, C N, C N, C C C C N Carbides / Nitrides? >12% in “high speed” tool steels Mostly used in maraging steels Promotes bainite >20% in maraging steels >10% for corrosion resistance Deoxidiser, esp. for high C steels Deoxidiser, esp. for low C steels Deoxidiser, Desulphuriser Other +Nb +V +Ti +W +Co ++αMo +γNi ++++αCr +Si +Al +++ γMn Harden- ability StabilisesElement Engineering “C” – High Perfomance Alloys: HT 2000 S.G. Roberts 2: 12 “Engineering” Nickel-Chromium Steels • Nickel alone grain refines, but tends to promote graphitisation of carbides. • Chromium alone forms stable carbides, but tends to promote grain growth. • Ni stabilises FCC γ phase, Cr stabilises BCC α phase. • Both elements – are good solid solution hardeners, – provide corrosion resistance (esp Cr) – retard the γ ⇒ α + Fe3C transformation • A balanced mixture (2-3x as much Ni as Cr) gives steels which can be – easily quenched to give martensite, – tempered to give a good trade-off between strength and ductility – can be case-hardened (carbon diffused in to give ~0.9% C in surface) to give extra wear resistance. • High alloy content (>4% Ni, 1.5% Cr) can give air- hardening steels. Typical compositions and applications: 0.3% C, 0.6% Mn, 3.0% Ni, 0.8% Cr, [0.65%Mo] (Mo to avoid “temper brittleness” in larger section components) Quench into oil from ~830ºC, temper 550-650ºC. σY 600-800 MPa, UTS 800-1000 MPa εF 16-20%, Impact ~50 J Highly stressed general engineering parts: shafts, conrods, rockers etc. 0.12% C, 0.45% Mn, 3.3% Ni, 1% Cr Carburise at ~850ºC, quench into oil from ~770ºC, temper 150ºC. σY 850 MPa, UTS 900 MPa εF 13%, Impact ~40 J Case hardening variant – high core strength, hard wearing surface. Engineering “C” – High Perfomance Alloys: HT 2000 S.G. Roberts 2: 15 High Strength Low Alloy (HSLA) Steels 0 20 40 60 80 100 120 140 0 0.02 0.04 0.06 0.08 0.1 0.12 V Ti Nb microalloy content (%) fe rri te g ra in s iz e (µ m 2 ) HSLA steels are “microalloyed”: Typically 0.1 – 0.25% C, 1.0 – 1.7% Mn, with very small additions of V, Ti, Nb. Use carefully controlled hot-rolling in the γ- phase to achieve a fine grain size. In hot rolling, there is competition between: - work hardening: reduces grain size; - diffusion controlled grain growth. Nb(C,N) has very low solubility in γ-Fe, comes out of solution as as fine grain boundary precipitates as rolling temperature drops; these pin movement of grain boundaries. Result: small γ-Fe grains during rolling. Grain size of resultant α-Fe is even finer. Engineering “C” – High Perfomance Alloys: HT 2000 S.G. Roberts 2: 16 High Strength Low Alloy (HSLA) Steels As well as strengthening via grain refinement, microalloyed steels are often heat-treated to give a fine dispersion of hard precipitates. On cooling from FCC γ-Fe to BCC α-Fe, solubility of C decreases markedlly. Some C is precipitated as very fine (and very hard) particles of NbC, VC, etc. at the γ – α interface as the α grows “stepwise” into the γ. Typical properties: σY: 450 – 650 MPa (depends on C, N content) UTS: 550 – 700 MPa εF: 10 – 20% Strength level about 2x that of “normal” mild steel, tough, formable and weldable. Also not much more expensive than the equivalent mild steel. Applications: Automotive sheet (body panels), strip, plate. Off-shore platforms, ships. γ α Check data 0.3 µm 0.1 µm Fe: 0.75%V, 0.15%C; after 5 mins at 750ºC Engineering “C” – High Perfomance Alloys: HT 2000 S.G. Roberts 2: 17 “High Speed” Tool Steels • Typical composition: Fe: 18% W, 4% Cr, 1% V, 0.75%C (+ maybe 6-10% Co) • Substutional alloying elements modify Fe-C phase diagram • 1: “Soak” just below eutectic line, forming g and dissolving as much carbon as possible (some W, V carbides remain) • 2: Quench into oil bath • 3: Temper to 500-600ºC. 400 800 1000 1200 1400 0 0.2 0.4 0.6 0.8 α + complex carbides γ + complex carbides α + γ Austenite γ Fe FCC wt% Carbon T (ºC) Fe: 18% W, 4% Cr, 1% V. 1 3 2 600 Engineering “C” – High Perfomance Alloys: HT 2000 S.G. Roberts 2: 20 Bainitic high strength Steels What is Bainite ? - As in “diffusionless” formation of Martensite, units cells of FCC γ – Fe transform by minor atomic re-arrangements to BCC α-Fe - but carbon has to come out of solution for this to happen. 3µm - As in “diffusional” formation of Ferrite /Pearlite, excess carbon comes out of solution to form carbides - but carbides formed are finely-divided and closely-spaced within or on the bainite “laths”. Engineering “C” – High Perfomance Alloys: HT 2000 S.G. Roberts 2: 21 Bainitic steels: Applications Typical Bainitic steel: “A533B” – Fe: 0.24%C, 1.4%Mn, 0.5%Mo, 0.5%Ni. σY: 400 – 450 MPa UTS: 700 – 800 MPa εF: 12 – 20% KIc: >100 MPa on “upper shelf” ~40 MPa on “lower shelf” Beware brittle-ductile transition! Uses: pressure vessels (e.g. Nuclear reactor containment). Engineering “C” – High Perfomance Alloys: HT 2000 S.G. Roberts 2: 22 Maraging Steels Do not contain carbon (0.03% max) Hardening is via intermetallic precipitates “Design philosophy” • Start with Fe – 18-20% Ni – will give 100% martensite on air cooling – not very hard (σy ~ 700MPa) and not brittle – as no C. • Add 2-3% Mo and 1% Ti – ageing at 475-525ºC (several hours) gives Ni3Ti, Fe7Mo6 – Ti also ties up any residual carbon – depress MS, MF temperatures. • Add ~8% Co – depresses solubility of Mo in Fe; hence more precipitates – elevates MS, MF temperatures. • Other possible additions: – V, Nb, W, Cu… more precipitates – Cr – corrosion resistance 0 200 400 600 800 0 10 20 30 % Ni T (ºC) 90% 10% 10% 90% α γ γ α Martensitic transformations in Fe - Ni Engineering “C” – High Perfomance Alloys: HT 2000 S.G. Roberts 2: 25 Maraging Steels - Applications • Aerospace – Rocket motor cases – Jet engine and helicopter drive shafts – Landing gear – Hinges for swing-wings – Shock absorbers on Lunar Rover! • Automotive – Drive shafts – connecting rods – Engine valves • Tooling – Extrusion rams – Machine tool gears – Al and Zn casting dies – Index plates – Rolls – Splined shafts – …etc. BM006 - Maraging steel face•Maraging steel face offers a super hard striking surface. Engineering “C” – High Perfomance Alloys: HT 2000 S.G. Roberts 2: 26 Creep-resistant Steels • Commonly used in power-generating turbines, etc. • Large, highly-stressed components, requiring long life (20+ years) • Steam temperatures now >500ºC Conventional plain C steels begin to lose strength rapidly above ~ 300ºC Not much use above ~ 400ºC Family of Cr – Mo steels for use in 300 – 600ºC range: – 0.15% C, 1.2% Cr, 0.5% Mo – 0.15% C, 2.5% Cr, 1% Mo – 0.15% C, 9% Cr, 1% Mo Used in quenched and heavily tempered state. (may have up to 0.75% V) For the most highly stressed components (turbine blades), use: – martensitic stainless (12%Cr) – austenitic stainless (better creep resistance) – precipitation hardened stainless: eg. “FV520B”, Fe: 0.05% C, 5.5% Ni, 14% Cr, 1.6% Mo, 1.5% Cu, 0.3% Nb. Upper temperature limit “steels” – “Inconels” – ~ 40% Ni, Cr, Co. 0 400 800 1200 1600 200 400 600 800 1000 Precipitation hardening steels Inconels Austenitic Martensitic / Ferritic Low-C steels 10 00 hr R up tu re S tre ss (M Pa ) Temperature (ºC) Engineering “C” – High Perfomance Alloys: HT 2000 S.G. Roberts 2: 27