Materials Engineering Lecture 9: Metals and alloys. Iron alloys

Metals and Alloys There are no really pure metals! Neither there should be. An alloy can be: Mechanical mixture: no chemical bond between components, variable composition, heterogeneous Compound: chemical bond between components, constant composition, homogeneous Solid solution (substitutional or interstitial): some chemical bond between components, variable composition, homogeneous Pearlite (mechanical mixture)

Iron-Carbon Phase diagram a-ferrite: BCC, stable at T<912ºC g-ferrite or austenite: FCC, stable at 912ºC<T<1394ºC d-ferrite: BCC, stable at T>1394ºC Fe3C cementite 6.67 wt% of C = 100% of Fe3C (cementite). Invariant points: 4.30% C, 1147ºC: eutectic: L ↔ g + Fe3C 0.76% C, 727ºC: eutectoid: g(0.76%C) ↔ a (0.022%C) + Fe3C

Eutectoid structure: Pearlite Pearlite is an a-Fe3C solid solution having grain (colony) microstructure Ductile, easily formable, cheap; Not hard, susceptible to corrosion Cables, wires, rails

Hypoeutectoid structures Ferrite + pearlite Cheap plain steels Mild, ductile, easily formable Very easily corrode

Hypereutectoid structures Pearlite + Cementite Hard Brittle The cementite forms a brittle network around the pearlite. This presents difficulty in machining the hypereutectoid steels. Old hypereutectoid structure (1st century CE, Spain)

Austenite Hard and Ductile; non-magnetic Stabilization: Mn, Ni

Austenite transformations Austenite is unstable at T<912ºC. As time goes austenite structure can transform: Cementite + Pearlite Cementite + Bainite (very fine 2 phase needle structure; stronger and harder that pearlite)

Spheroidite Pearlite or bainite steel is heated to ~700ºC (below eutectoid point) and left at that temperature for 18-24 hours Soft and “gummy” This structure is desirable when minimum hardness, maximum ductility and maximum machinability are required.

Martensite Austenite hot steel is rapidly quenched  martensite Non-diffusional transformation => occurs instantaneously Non-equilibrium structure Martensite: BCT (body-centered tetragonal) Hardest and strongest of all steels; the most brittle; can crack on quenching Martensite structure in a 1 century BCE knife Martensite needles in austenite matrix (x1220)

Tempered martensite Martensite is heated to 250-650ºC Martensite (BCT, 1 phase)  tempered martensite (a + Fe3C) Extremely small cementite particles Almost as hard and strong as martensite, but ductile and tough

Phase transformations for iron alloys (review) Austenite Pearlite (a + Fe3C) + proeutectoid Bainite Martensite (BCT) Tempered martensite Slow cooling Rapid cooling Reheat

Ferrous alloys Steels (<2%C) Cast Irons (2 – 6.7%C) Low-Carbon (iron, <0.25%C) Medium-Carbon (0.25-0.6%C) High-Carbon (0.6-1.4%C) Cast Irons (2 – 6.7%C)

Low-Carbon steels Armco Iron: < 0.02%C Mild Steels: 0.1 – 0.2%C All have pearlite-ferrite structure; no martensite Strengthening by cold work Soft, weak, ductile, tough; Good for machining, welding; very cheap Used in automobiles, structural shapes, sheets

Plain low-carbon steels High-Strength Low-Alloy Steels applications Ductility (%EL in 50 mm) Yield Str., MPa Tens. Str., MPa %Other %Mn %C UNS AISI/SAE, ASTM Plain low-carbon steels Automobile panels. Nails, wire 28 180 325 0.45 0.10 G10100 1010 Pipes, sheets, structural 25 205 380 0.20 G10200 1020 Structural (bridges, buildings) 23 220 400 0.20Cu 1.00 0.29 K02600 A36 High-Strength Low-Alloy Steels Bolted structures 21 290 435 <0.30Si, >0.20Cu 1.35 0.28 K12810 A440 Structures for low ambient temperatures 520 0.30Si, 0.08V, 0.02N, 0.03Nb 0.22 K12002 A633 Grade E Truck frames, railway carriages 15 552 655 0.60Si, 0.1V, 0.20Al, 0.015N 1.60 0.18 K11804 A656 Grade 1 AISI: Americal Iron and Steel Institute, SAE: Society of Automotive Engineers; ASTM: American Society for Testing and Materials UNS: Uniform Numbering System

Medium-Carbon and High-Carbon Steels Hardening by austenitizing, quenching then tempering. Structure (for tempered): tempered martensite Strong, poor ductility and toughness High-carbon steels are especially wear-resistant, holding a sharp-cutting edge

Medium and High-Carbon Steels (2) applications Ductility (%EL in 50 mm) Yield Str., MPa Tens. Str., MPa %Other %Mn %C UNS AISI/SAE, ASTM Plain steels Bolts, crankshaft 33-19 430-585 605-780 <1.00 0.40 G10400 1040 Chisels, hammers 24-13 480-980 800-1310 0.80 G10800 1080 Knives, blades 26-10 510-830 760-1280 0.95 G10950 1095 Alloy Steels Springs, hand tools 24-4 710-1770 786-2380 1.65-2.00Ni, 0.40-0.90Cr, 0.20-0.30Mo 0.63 G40630 4063

Stainless Steels >11% Cr, (Ni, Al) Martensitic, ferritic and austenitic Aplication Duct., EL% in 50 mm Y. Str., MPa Tens. Str, MPa Condition %Comp UNS AISI Ferritic Exhaust components, tanks for agricultural sprays 20 205 380 annealed 0.08C, 11.0Cr, 1.0Mn, 0.50Ni, 0.75Ti S40900 409 Valves, combustion chambers 275 515 0.20C, 25Cr, 1.5Mn S44600 446 Austenitic Chemical and food processing, cryogenic vessels 40 0.08C, 19Cr, 9Ni, 2.0Mn S30400 304 Martensitic Rifle barrels, jet engine parts 12 620 485 825 Annealed, Q+T 0.15C, 12.5Cr, 1.0Mn S41000 410

Cast Irons >2.14%C (practically, 3.0-4.5%C) Fe3C  3Fe(a) + C (graphite) Gray iron Ductile (nodular) iron White iron Malleable iron

Gray Iron C: 2.5 – 4.0% Si: 1.0 – 3.0% Graphite flake structure Brittle and weak Especially brittle when chilled Damping vibrations High wear resistance Very cheap

Nodular (ductile) iron Addition of Mg or Ce to gray iron Strong and ductile (almost as a steel) Valves, pump bodies, gears etc.

White and Malleable Iron <1.0% Si, rapid cooling  no graphite, only cementite: white carbon Extremely hard, but very brittle; unmachinable Rollers in roller mills Heating white iron at 800-900ºC for a prolonged time  malleable iron Structure: graphite clusters in pearlite (ferrite) matrix High strength and some ductility Rods, gears, flanges etc.

Musts of this lecture Iron phases (ferrites, austenite, cementite); Iron microstructures (pearlite, bainite, spheroidite, martensite, tempered martensite); Steels and cast irons; Low-carbon steels (properties, application); Medium and High-carbon steels (properties, application); Stainless steels Cast irons