ISSUES TO ADDRESS...
• How are metal alloys classified and how are they used?
• What are some of the common fabrication techniques?
• How do properties vary throughout a piece of material that has been quenched, for example?
• How can properties be modified by post heat treatment?
Chapter 11: Metal Alloys - Applications and Processing
Taxonomy of MetalsMetal Alloys
Steels
Ferrous Nonferrous
Cast Irons Cu Al Mg Ti<1.4wt%C 3-4.5wt%CSteels
<1.4 wt% CCast Irons3-4.5 wt% C
Fe3C
cementite
1600
1400
1200
1000
800
600
4000 1 2 3 4 5 6 6.7
L
austenite
+L
+Fe3Cferrite
+Fe3C
+
L+Fe3C
(Fe) Co , wt% C
Eutectic:
Eutectoid:0.76
4.30
727°C
1148°C
T(°C) microstructure: ferrite, graphite cementite
Steels
Low Alloy High Alloy
low carbon <0.25 wt% C
Med carbon0.25-0.6 wt% C
high carbon 0.6-1.4 wt% C
Uses auto struc. sheet
bridges towers press. vessels
crank shafts bolts hammers blades
pistons gears wear applic.
wear applic.
drills saws dies
high T applic. turbines furnaces V. corros. resistant
Example 1010 4310 1040 4340 1095 4190 304
Additions noneCr,V Ni, Mo
noneCr, Ni Mo
noneCr, V, Mo, W
Cr, Ni, Mo
plain HSLA plainheat
treatableplain tool
austenitic stainless
Name
Hardenability 0 + + ++ ++ +++ 0TS - 0 + ++ + ++ 0EL + + 0 - - -- ++
increasing strength, cost, decreasing ductility
Steels
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
System and Composition of Plain Carbon Steel and Alloy Steel
Low carbon
Medium, High carbon
Plain Carbon SteelLow carbon• Good
formability and weldability
• Strengthening by coldwork
• Structure usually pearlite and ferrite
High carbon• Low toughness
and formability• Good hardness
and wear resistance
• Can form martensite by quenching but risk of cracking
Medium carbon• Can be
quenched to form martensite or bainite
• Compromising structure between ductility and strength
Compare to other engineering materials• High strength and stiffness, reasonable toughness, easy to
recycle and low cost• Rust easily, require surface protection
Effect of alloy elements• Bi, Pb - improve machinability• B 0.001-0.003% - powerful hardenability agent• Cr 0.5-2% - increase hardenability, 4-18% - corr. resist.• Cu 0.1-0.4% corrosion resistance• Mn 0.25-0.4% - combine with S to prevent brittleness• Mo 0.2-5% - stable carbides• Ni 2-5% - toughener, 12-20% - corrosion resistance• Si 0.2-0.7% - strength, 2% - spring, higher% - magnetic p.• S 0.08-0.15% - free machining• Ti - fix C in inert particles, reduce mart. hardn. in Cr steels• W - hardness at high temperature• V - stable carbide, inc. str. with remain ductility, fine grain
Alloy Steels
HSLA• Large applications• High yield (nearly twice
of plain C steel), good weldability and acceptable corrosion resistance
• Limited ductility and hardenability
• Resist to form martensite in weld zone
Dual-Phase Steel• Quench from temp.
above A1 but below A3 to form structure of ferrite and martensite
• Strength comparable to HSLA while improve formability with no loss of weldability
• Automotive structure and body application
Alloy SteelsFree-machining steels• S, Pb, Bi, Se, Te or P • Making chip-breaking
discontinuity in structure and a build-in lubrication
• Higher cost may compensated with higher speed and lower wear of cutting tools
• Additives may reduce concerned properties such as strength, ductility
• cold working also improve machinability
Bake-Hardenable steel sheet
• Significant in automotive steel sheet
• Low carbon steel• Good formability and
increase strength after forming with heat exposure in paint-baking process
• Good spot weldability, crash energy, low cost easy recycle
Alloy SteelsMaraging Steels• Super high strength alloy• Typical composition is
0.03% C, 8.5% Ni, 7.5% Co, 0.1% Al, 0.003% B, 0.1% Si, 4.8% Mo, 0.4%Ti, 0.01% Zr, 0.1% Mn, 0.01%S and 0.01%P
• Can be hot worked to get soft, tough, low martensite and easy to machine
• Can be cold worked and aging with a yield of 1725 MPa and %EL 11%
• Weldability
Steel for HighTemp.• Good strength, corrosion
resistance, creep resistance
• Plain C steel – 250 C• Conventional alloy – 350 C• High temp. ferrous alloy
tend to has low carbon (less than 0.1%)
• Can be used at higher than 550 C
Dual Phase Steel
Bake hardenable steel
Tool SteelsWater hardening (W)Cold Work
O – Oil hardeningA – Air hardening
D – High C high CrShock resistance (S)High speed
T – W base, M – Mo baseHot work
H1-H19 – Cr baseH20-H39 – W base
H40-H59 – Mo basePlastic mold (P)Special purpose
L – Low alloyF – carbon-tungsten
• High carbon, high strength ferrous alloy
• Balance of toughness, strength and wear resistance
Tool Steels
Stainless Steels
• Oxide of additive elements is tough, adherent, corrosion resistance and heals itself
Ferritic stainless steel• Normally contain >12% Cr (Cr is ferrite stabilizer)• Corrosion resistance• Limited ductility or formability but weldable (no martensite
can form in weld zone)• The cheapest stainless steel
Series Alloys Structure
200
300
400
500
Cr, Ni, Mn or Ni
Cr and Ni
Cr, (C)
Low Cr (<12%) and (C)
Austenitic
Austenitic
Ferritic or martensitic
Martensitic
Stainless Steels
Martensitic Stainless• The lower content of Cr
lead to more stable of austenite at high temp.
• Slow cool may allow carbide of Cr (loss of chromium oxide film)
• Higher cost than ferritic stainless steel due to the heat treatment (austenitization, quench, stress relief and temper
Austenitic Stainless• Ni is austenite stabilizer• The most expensive stainless
due to Ni cost• Mn and N are used as
stabilizer instead of Ni to reduce cost but lower quality
• Non-magnetic, highly corrosion resistance except HCl and other helide acid/salt
• Outstanding formability(FCC)• 304 alloy (18-8) is popular
one, high response to CW
Popular stainless steels
Stainless Steel (1)
Stainless Steel (2)
Precipitation hardenable stainless steel is the special class• Martensitic or austenitic type, modified by addition of
alloying elements like Al to form hard intermetallic compound during temper
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
Fe-C True Equilibrium Diagram
Graphite formation promoted by
• Si > 1 wt%
• slow cooling
•Gray cast iron
•Ductile or Nodular iron
•White iron
•Malleable iron
•Compacted graphite iron
1600
1400
1200
1000
800
600
4000 1 2 3 4 90
L
+L
+ Graphite
Liquid +Graphite
(Fe) Co , wt% C
0.65
740°C
T(°C)
+ Graphite
100
1153°CAustenite 4.2 wt% C
Production of Cast Iron
Types of Cast Iron
Gray iron• graphite flakes• weak & brittle under tension• stronger under compression• excellent vibrational dampening• wear resistant
Ductile iron• add Mg or Ce• graphite in nodules not flakes• matrix often pearlite - better
ductility
Types of Cast Iron
White iron• <1wt% Si so harder but brittle• more cementite
Malleable iron• heat treat at 800-900ºC• graphite in rosettes• more ductile
Types of Cast Iron
Compacted Graphite Iron• Mg/Ce and others are added• Worm-like shape graphite• Microstructure is between
gray cast iron and ductile iron• Sharp edge of graphite
should be avoided• High thermal conductivity• Better resistance to thermal
shock, fracture and fatigue• Lower oxidation at elevated
Temp.
Limitations of Ferrous Alloys
1) Relatively high density
2) Relatively low conductivity
3) Poor corrosion resistance
Nonferrous Alloy
Nonferrous Alloys
NonFerrous Alloys
• Al Alloys-lower : 2.7g/cm3 -Cu, Mg, Si, Mn, Zn additions -solid sol. or precip. strengthened (struct.
aircraft parts & packaging)
• Mg Alloys-very low : 1.7g/cm3 -ignites easily -aircraft, missiles
• Refractory metals-high melting T -Nb, Mo, W, Ta• Noble metals
-Ag, Au, Pt -oxid./corr. resistant
• Ti Alloys-lower : 4.5g/cm3
vs 7.9 for steel -reactive at high T -space applic.
• Cu AlloysBrass: Zn is subst. impurity (costume jewelry, coins, corrosion resistant)Bronze : Sn, Al, Si, Ni are subst. impurity (bushings, landing gear)Cu-Be: precip. hardened for strength
Non-Ferrous Alloys
• Cast Alloy – Forming or shaping by appreciable deformation is not possible, ordinarily by casting. So, brittle.
• Wrought Alloy – amenable to mechanical deformation
Sometimes the heat treatability of an alloy is frequently mentioned as “heat treatable”
Cu and its alloys• 3 important properties are
high electrical and thermal conductivity, useful strength with high ductility and corrosion resistance
• Heavily than iron• Pure Cu – wire, cable• Cu-Zn – brass – popular
alpha brass - ductile, form.
beta brass – Zn rich, brittle• Cu-Ni – high thermal
conductivity, high strength at high temperature
• Cu-Sn - bronze
Al and its alloys• The most important of non-
ferrous metal• Light weight, corrosion
resist., good elec./thermal cond., workability, recycle
• Serious weakness is low modulus of elesticity
• Pure Al – soft, ductile• Alloy for mechanical appl.
strength as HSLA level • Alloy for corrosion resist.
difficult to weld• Al-Li – high strength, great
stiffness, lighter weight
Mg and its alloys• Lightest of commerc. Metal• Pure Mg – weak • Alloy – poor ductility, wear,
creep and fatigue• Modulus less than Al• In positive side, high
strength/weight ratio, high energy absorption, good damping of noise/vibration
• Higher purity alloy – good corrosion resistance
• Formability – at high temp.• Good machinability/weldab.• Fire hazards
Ti and its alloys• Strong, light weight,
corrosion resistance• Good mechanical
properties up to 535 C• High cost, fabrication
difficulty, high energy content and high reactivity at elevated temperature
• Fabrication can be by casting, forging, rolling, extrusion or welding
Refractory metal• Extremely high melting T• Nb (2468 C), Mo, W (3410
C), Ta• Ta-Mo to improve corrosion
resistance
Super alloys• Use in aircraft turbine
component• Difficult to form and
machine• Special methods are used,
EDM, electrochemical, ultrasonic m/c
Noble metals• Au, Ag, Pt, Pd, Rh, Ru, Ir
and Os• Expensive
Miscellaneous nonferrous• Ni (coating)• Pb• Sn• Alkaline
Metal Fabrication
• How do we fabricate metals?– Blacksmith - hammer (forged)– Molding - cast
• Forming Operations – Rough stock formed to final shape
Hot working vs. Cold working• T high enough for • well below Tm
recrystallization • work hardening
• Larger deformations • smaller deformations
FORMING
roll
AoAd
roll
• Rolling (Hot or Cold Rolling) (I-beams, rails, sheet & plate)
Ao Ad
force
dieblank
force
• Forging (Hammering; Stamping) (wrenches, crankshafts)
often atelev. T
Metal Fabrication Methods - I
ram billet
container
containerforce
die holder
die
Ao
Adextrusion
• Extrusion (rods, tubing)
ductile metals, e.g. Cu, Al (hot)
tensile force
AoAddie
die
• Drawing (rods, wire, tubing)
die must be well lubricated & clean
CASTING JOINING
FORMING CASTING JOINING
Metal Fabrication Methods - II
• Casting- mold is filled with metal– metal melted in furnace, perhaps alloying
elements added. Then cast in a mold – most common, cheapest method– gives good production of shapes– weaker products, internal defects– good option for brittle materials
• Sand Casting (large parts, e.g., auto engine blocks)
Metal Fabrication Methods - II
• trying to hold something that is hot
• what will withstand >1600ºC?
• cheap - easy to mold => sand!!!
• pack sand around form (pattern) of desired shape
Sand Sand
molten metal
FORMING CASTING JOINING
plasterdie formedaround waxprototype
• Sand Casting (large parts, e.g., auto engine blocks)
• Investment Casting (low volume, complex shapes e.g., jewelry, turbine blades)
Metal Fabrication Methods - II
Investment Casting
• pattern is made from paraffin.
• mold made by encasing in plaster of paris
• melt the wax & the hollow mold is left
• pour in metal
wax
FORMING CASTING JOINING
Sand Sand
molten metal
plasterdie formedaround waxprototype
• Sand Casting (large parts, e.g., auto engine blocks)
• Investment Casting (low volume, complex shapes e.g., jewelry, turbine blades)
Metal Fabrication Methods - II
wax
• Die Casting (high volume, low T alloys)
• Continuous Casting (simple slab shapes)
molten
solidified
FORMING CASTING JOINING
Sand Sand
molten metal
Continuous casting
CASTING JOINING
Metal Fabrication Methods - III
• Powder Metallurgy (materials w/low ductility)
pressure
heat
point contact at low T
densification by diffusion at higher T
area contact
densify
• Welding (when one large part is impractical)
• Heat affected zone: (region in which the microstructure has been changed).
piece 1 piece 2
fused base metal
filler metal (melted)base metal (melted)
unaffectedunaffectedheat affected zone
FORMING
Annealing: Heat to Tanneal, Soaking, then cool slowly.
Thermal Processing of Metals
Types of Annealing
• Process Anneal: Negate effect of cold working by (recovery/ recrystallization)
• Stress Relief: Reduce stress caused by:
-plastic deformation -nonuniform cooling -phase transform.
• Normalize (steels): Deform steel with large grains, then normalize to make grains small. (air cool)
• Full Anneal (steels): Make soft steels for good forming by heating to get , then cool in furnace to get coarse P.
• Spheroidize (steels): Make very soft steels for good machining. Heat just below TE & hold for
15-25 h.
Fe-Fe3C diagram
a) Annealing
b) Quenching
Heat Treatments
c)
c) Tempered Martensite
time (s)10 10 3 10 510 -1
400
600
800
T(°C)
Austenite (stable)
200
P
B
TE
0%
100%50%
A
A
M + A
M + A
0%
50%
90%
a)b)
Hardenability--Steels• Ability to form martensite• Jominy end quench test to measure hardenability.
• Hardness versus distance from the quenched end.
24°C water
specimen (heated to phase field)
flat ground
Rockwell Chardness tests
Har
dnes
s, H
RC
Distance from quenched end
• The cooling rate varies with position.
Why Hardness Changes with Position
distance from quenched end (in)Ha
rdn
ess
, H
RC
20
40
60
0 1 2 3
600
400
200A M
A
P
0.1 1 10 100 1000
T(°C)
M(start)
Time (s)
0
0%100%
M(finish) Martensite
Martensite + Pearlite
Fine Pearlite
Pearlite
Hardenability vs Alloy Composition• 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.
Cooling rate (°C/s)
Har
dne
ss, H
RC
20
40
60
100 20 30 40 50Distance from quenched end (mm)
210100 3
4140
8640
5140
1040
50
80
100
%M4340
T(°C)
10-1 10 103 1050
200
400
600
800
Time (s)
M(start)M(90%)
shift from A to B due to alloying
BA
TE
Equivalent distance and Bar diameter
(Quenched in water) (Quenched in oil)
Radial hardness profile
(Quenched in water) (Quenched in oil)
• Effect of quenching medium:
Mediumairoil
water
Severity of Quenchlow
moderatehigh
Hardnesslow
moderatehigh
• Effect of geometry: When surface-to-volume ratio increases: --cooling rate increases --hardness increases
Positioncentersurface
Cooling ratelowhigh
Hardnesslowhigh
Quenching Medium & Geometry
0 10 20 30 40 50wt% Cu
L+L
+L
300
400
500
600
700
(Al)
T(°C)
composition range needed for precipitation hardening
CuAl2
A
Precipitation Hardening• Particles impede dislocations.• Ex: Al-Cu system• Procedure:
--Pt B: quench to room temp.--Pt C: reheat to nucleate small crystals within crystals.
• Other precipitation systems: • Cu-Be • Cu-Sn • Mg-Al
Temp.
Time
--Pt A: solution heat treat (get solid solution)
Pt A (sol’n heat treat)
B
Pt B
C
Pt C (precipitate
• 2014 Al Alloy:
• TS peaks with precipitation time.• Increasing T accelerates process.
Precipitate Effect on TS, %EL
precipitation heat treat time
tens
ile s
tren
gth
(MP
a)
200
300
400
1001min 1h 1day 1mo 1yr
204°C
non-
equi
l. so
lid s
olut
ion
man
y sm
all
prec
ipita
tes
“age
d”
few
er la
rge
prec
ipita
tes
“ove
rage
d”149°C
• %EL reaches minimum with precipitation time.
%E
L (2
in s
ampl
e)10
20
30
0 1min 1h 1day 1mo 1yr
204°C 149°C
precipitation heat treat time
Metal Alloy Crystal Structure
Alloys• substitutional alloys
– can be ordered or disordered– disordered solid solution– ordered - periodic substitution
example: CuAu FCC
Cu
Au
• Interstitial alloys (compounds) – one metal much larger than the other – smaller metal goes in ordered way into
interstitial “holes” in the structure of larger metal
– Ex: Cementite – Fe3C
Metal Alloy Crystal Structure
• Steels: increase TS, Hardness (and cost) by adding --C (low alloy steels) --Cr, V, Ni, Mo, W (high alloy steels) --ductility usually decreases w/additions.• Non-ferrous: --Cu, Al, Ti, Mg, Refractory, and noble metals.• Fabrication techniques: --forming, casting, joining.• Hardenability --increases with alloy content.• Precipitation hardening --effective means to increase strength in Al, Cu, and Mg alloys.
Summary
Top Related