Chapter 11: Metal Alloys Heat Treatment Changes Internal Structure Changes Mechanical Properties Therefore, by varying the manner in which metal alloys are heated and cooled, one can produce different combinations of mechanical properties.

Taxonomy of Metals Steels Cast Irons <1.4 wt% C 3-4.5 wt% C Alloys Steels Ferrous Nonferrous Cast Irons Cu Al Mg Ti <1.4wt%C 3-4.5 wt%C Steels <1.4 wt% C Cast Irons 3-4.5 wt% C microstructure: ferrite, graphite cementite Fe 3 C cementite 1600 1400 1200 1000 800 600 400 1 2 4 5 6 6.7 L g austenite +L +Fe3C a ferrite + L+Fe3C d (Fe) Co , wt% C Eutectic: Eutectoid: 0.76 4.30 727°C 1148°C T(°C)

Steels Low Alloy High Alloy low carbon Med carbon high carbon <0.25 wt% C 0.25-0.6 wt% C 0.6-1.4 wt% C heat Name plain HSLA plain plain tool stainless treatable Additions none Cr,V Ni, Mo Cr, Ni Mo Cr, V, Mo, W Cr>11 wt% Example 1010 4310 1040 43 40 1095 4190 Hardenability + ++ +++ TS - + ++ EL + + - - -- Uses auto bridges crank pistons wear drills struc. towers shafts gears applic. saws sheet press. bolts wear dies vessels hammers applic. blades increasing strength, cost, decreasing ductility

Ferrous Alloys Main component is Iron Nomenclature AISI & SAE 10xx Plain Carbon Steels 11xx Plain Carbon Steels (resulfurized for machinability) 15xx Mn (10 ~ 20%) 40xx Mo (0.20 ~ 0.30%) 43xx Ni (1.65 - 2.00%), Cr (0.4 - 0.90%), Mo (0.2 - 0.3%) 44xx Mo (0.5%) where xx is wt% C x 100 example: 1060 steel – plain carbon steel with 0.60 wt% C Stainless Steel -- >11% Cr

Cast Iron Ferrous alloys with > 2.1 wt% C more commonly 3 - 4.5 wt%C low melting (also brittle) so easiest to cast Cementite decomposes to ferrite + graphite Fe3C  3 Fe () + C (graphite) generally a slow process So phase diagram for this system is different (Fig 12.4)

Main Types Of Heat Treatment Spheroidizing Annealing Normalizing Austempering Hardening - Quenching & Tempering Martempering

Heat Treatment Purpose Process Structure Produced Induced Properties spheroidizing Improve machinability Heat long time just below eut. temp spheroidite inc ductility; easily machine annealing Soften slow cooling Coarse pearlite inc ductility; dec strength normalizing refine rain structure air cooling (fast cooling) Fine pearlite ­inc hardness austempering Produce hardness between P & M long time req’d to transform to bainite bainite quenching harden steel rapid cooling martensite very hard; brittle martempering dec susceptibility to cracking stop quenching & cool slowly Prevent cracking tempering decrease brittleness raise temp above martensitic temp Tempered martensite dec hardness; inc ductility of martensite

Austempering

Quenching and Tempering

Martempering

Hardenability--Steels • Ability to form martensite • Jominy end quench test to measure hardenability. 24°C water specimen (heated to g phase) flat ground Rockwell C hardness tests • Hardness versus distance from the quenched end. Hardness, HRC Distance from quenched end

Why Hardness Changes W/Position • The cooling rate varies with position. 60 Martensite Martensite + Pearlite Fine Pearlite Pearlite Hardness, HRC 40 20 distance from quenched end (in) 1 2 3 600 400 200 A ® M P 0.1 1 10 100 1000 T(°C) M(start) Time (s) 0% 100% M(finish)

Hardenability vs Alloy Composition Cooling rate (°C/s) Hardness, HRC 20 40 60 10 30 50 Distance from quenched end (mm) 2 100 3 4140 8640 5140 1040 80 %M 4340 • Jominy end quench results, C = 0.4 wt% C • "Alloy Steels" (4140, 4340, 5140, 8640) --contain Ni, Cr, Mo (0.2 to 2wt%) --these elements shift the "nose". --martensite is easier to form. T(°C) 10 -1 3 5 200 400 600 800 Time (s) M(start) M(90%) shift from A to B due to alloying B A TE

PRECIPITATION HARDENING • Particles impede dislocations. • Ex: Al-Cu system • Procedure: --Pt A: solution heat treat (get a solid solution) --Pt B: quench to room temp. --Pt C: reheat to nucleate small q crystals within a crystals. • Other precipitation systems: • Cu-Be • Cu-Sn • Mg-Al

PRECIPITATE EFFECT ON TS, %EL • 2014 Al Alloy: • TS peaks with precipitation time. • Increasing T accelerates process. • %EL reaches minimum with precipitation time.