1. 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.

2. 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)

3. Steels Low Alloy High Alloy low carbon Med carbon high carbon
<0.25 wt% C
wt% C
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

4. 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 ( %), Cr ( %), Mo ( %)
44xx Mo (0.5%)
where xx is wt% C x 100
example: steel – plain carbon steel with 0.60 wt% C
Stainless Steel -- >11% Cr

5. Cast Iron Ferrous alloys with > 2.1 wt% C
more commonly 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)

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

7. 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

8. Austempering

9. Quenching and Tempering

10. Martempering

11. 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

12. 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)

13. 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

14. 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

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