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Microstructures of iron- alloys

Fine 3000X Course pearlite 3000X Spheroidite 1000X

Upper Lower bainite

Microstructures of iron-carbon alloys

Austinite

Tempered X9300 Plate martensitic microstructure X1220

1 Chapter 11 Classifications of thermal processing of alloys

† Basic concepts † process † Heat treatment of † Precipitation

Taxonomy of

Metal Alloys

Ferrous Nonferrous

Steels CastCast Irons Irons Cu Al Mg Ti <1.4wt%C<1.4wt%C 3-4.5wt%C3-4.5wt%C

T(°C) microstructure: 1600 , δ 1400 L γ+L 1200 γ 1148°C L+Fe3C Eutectic: 1000 4.30 α γ+Fe C 800 + 3 α γ 727°C Fe3C ferrite Eutectoid: cementite 600 0.77 α+Fe3C

400 0 1234566.7 (Fe) Co, wt% C

2 Steels

Low High Alloy low carbon med carbon high carbon <0.25wt%C 0.25-0.6wt%C 0.6-1.4wt%C

heat austentitic Name plain HSLA plain plain treatable stainless Cr,V Cr, Ni Cr, V, Additions none none none Cr, Ni, Mo Ni, Mo Mo Mo, W Example 1010 4310 1040 4340 1095 4190 304 Hardenability 0 + + ++ ++ +++ 0 TS - 0 + ++ + ++ 0 EL ++ 0 - - --++ Uses auto bridges crank pistons wear drills high T struc. towers shafts gears applic. applic. sheet press. bolts wear dies turbines vessels hammers applic. V. corros. resistant increasing strength, cost, decreasing

Nonferrous alloys

• Cu Alloys • Al Alloys : Zn is subst. -lower ρ: 2.7g/cm3 (costume jewelry, coins, -Cu, Mg, Si, Mn, Zn additions resistant) - sol. or precip. Bronze: Sn, Al, Si, Ni are strengthened (struct. subst. impurity aircraft parts (bushings, landing & packaging) gear) NonFerrous • Mg Alloys Cu-Be: -very low ρ: 1.7g/cm3 precip. hardened Alloys -ignites easily for strength -aircraft, missles • Ti Alloys -lower ρ: 4.5g/cm3 • metals -high melting T

vs 7.9 for • Noble metals -Nb, Mo, W, Ta -reactive at high T -Ag, Au, Pt -space applic. -oxid./corr. resistant

3 Basic concepts

† Annealing: a heat treatment •heat up • hold for for an extended time • cooling

† Function of annealing: • relieve stress • increase softness, ductility and toughness • produce specific microstructure

Basic concepts

† Lower critical temp: eutectic temp A1

† upper critical temp: A3 and Acm

4 Types of annealing

† Process anneal: negate effect of cold working by recovery/recrystallization † Stress relief: reduce stresses caused by • plastic deformation • nonuniform cooling • transform † Normalize (steels): deform steel with large grains, then normalize to make grains small-- heat up to above critical followed by cool in air to get fine pearlite

Types of annealing(continue)

† Full anneal (steels): make soft steels for good forming by heating to get austenite. Then cool in to get coarse pearlite

† Spheroidizing (steels): make very soft steels for

good machining. Heat just below TE and hold for 15-25h

5 Hardenablility

† Ability of a material of forming martensite

† The Jominy end-quench test

Hardenablility (continue)

† Hardness vs distance from quenched end

6 Hardenability vs alloy content

† Jominy end quench results, c=0.4wt%C

Hardenability vs carbon content

† Jominy end quench results, 8600 series

† Hardenability increases as wt%C increase

7 Quench medium and geometry

† Effect of medium • air, small severity, small hardness • oil, moderate severity, moderate hardness • , large severity, large hardness

† Effect of geometry

Precipitation hardening

† Particles impede dislocations

8 Example: 7150-T651 Al alloy

9