Steel and cast iron
Chapter 11 - 1
Chapter 11 - 2
1
Taxonomy of Metals
Metal Alloys
Ferrous
Steels
Steels
<1.4wt%C
<1.4
wt% C
Cast Irons
Cast
Irons
3-4.5 wt%C
3-4.5
wt% C
Cu
Al
d
L
1400
g
austenite
g+L
4.30
g
a+
a800
ferrite
600
400
0
(Fe)
727°C
Eutectoid:
0.76
1
2
Eutectic:
g+Fe3C
Fe3C
cementite
a+Fe3C
3
Ti
L+Fe3C
1148°C
1000
Mg
microstructure:
ferrite, graphite
cementite
T(°C)
1600
1200
Adapted from
Fig. 11.1,
Callister 7e.
Nonferrous
4
5
6
6.7
Co , wt% C
Chapter 11 - 3
Carbon steel
1. Other terms: plain steel, mild steel, low-carbon
steel
2. Available in almost all product forms: e.g. sheet,
strip, bar, plate, tube, pipe etc.
3. Designation: e.g. 1040 steel - 0.40 wt% C
4. Up to 2 wt% C
5. Limitation for other alloying elements:
Si up to 0.6 %,
Cu up to 0.6 %
Mn up to 1.65 %
Chapter 11 - 4
2
Chapter 11 - 5
Hot-rolled steels and typical applications
t11_01b_pg361
Chapter 11 - 6
3
AISI: the American Iron and Steel Institute
SAE: the Sociaty of Automotive and Engineering
ASTM: the American Society for Testing and
Materials
UNS: the Uniform Numbering System
Chapter 11 - 7
High alloys:
Chapter 11 - 8
4
Chapter 11 - 9
Designations, compositions and applications for six tool steels
Chapter 11 - 10
5
Chapter 11 - 11
Chapter 11 - 12
6
Chapter 11 - 13
Chapter 11 - 14
7
Stainless steels
1.
2.
More resistant to rusting and staining than carbon- and low-alloy
steels
Chromium addition, usually above 10 wt% (4-30%)
Chapter 11 - 15
Stainless steels
Chapter 11 - 16
8
Chapter 11 - 17
Chapter 11 - 18
9
Steels
High Alloy
Low Alloy
low carbon Med carbon
<0.25 wt% C 0.25-0.6 wt% C
high carbon
0.6-1.4 wt% C
heat
plain
treatable
Cr,V
Cr, Ni
Additions none
none
none
Ni, Mo
Mo
Example 1010 4310
1040
4340 1095
Hardenability 0
+
+
++
++
TS
0
+
++
+
EL
+
+
0
Name
plain
Uses
auto
struc.
sheet
HSLA
bridges
towers
press.
vessels
plain
crank
shafts
bolts
hammers
blades
pistons
gears
wear
applic.
wear
applic.
tool
Cr, V,
Mo, W
4190
+++
++
--
austenitic
stainless
Cr, Ni, Mo
304
0
0
++
high T
applic.
turbines
furnaces
V. corros.
resistant
drills
saws
dies
increasing strength, cost, decreasing ductility
Based on data provided in Tables 11.1(b), 11.2(b), 11.3, and 11.4, Callister 7e.
Chapter 11 - 19
Ferrous Alloys
Iron containing – Steels - cast irons
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
Chapter 11 - 20
10
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 Æ 3 Fe (a) + C (graphite)
– cementite (Fe3C) a metastable phase
– graphite formation promoted by
• Si > 1 wt%
• slow cooling
Chapter 11 - 21
Fe-C True Equilibrium Diagram
Graphite formation
promoted by
T(°C)
1600
• Si > 1 wt%
L
1400
• slow cooling
Liquid +
Graphite
g+L
1200
g
Austenite
1153°C
4.2 wt% C
1000
g+ Graphite
a+g
800
Cast iron
1. Gray iron
2. Nodular (ductile) iron
3. White iron
4. Malleable iron
5. Compacted graphite iron
(CGI)
0.65
740°C
600
a + Graphite
400
(Fe)
0
1
2
3
4
90
100
Co , wt% C
Chapter 11 - 22
11
Types of Cast Iron
Gray iron
– 1 - 3 % Si, 2.5 – 4% C
– graphite flakes plus ferrite/pearlite
– brittleness due to the flake-like graphite
• weak & brittle under tension
• stronger under compression
• excellent vibrational dampening
• wear resistant
Ductile (nodular) iron
– a small amount (0.05 wt%) of Mg or Ce
– spheroidal graphite precipitates (nodules)
rather than flakes
– matrix often pearlite or ferrite
– Ductility increased by a factor of 20, strength
is doubled
Chapter 11 - 23
Microstructure of pearlite in
the grey iron (Fe-3.3C-2.1Si0.5Cr-0.5Mo-1.0Cu). TEM.
Optical micrograph of a grey
iron (Fe-3.3C-2.1Si-0.5Cr0.5Mo-1.0Cu)
Chapter 11 - 24
12
(a) steel; (b) grey cast iron
Chapter 11 - 25
Types of Cast Iron
White iron
– <1wt% Si, harder but brittle
– eutectic carbide plus pearlite
– large amount of Fe3C formed during
casting
Malleable iron
– the result of annealing white iron
castings, 800º-900º C
– cementite→ graphite precipitates,
clusters or rosettes
– more ductile
Chapter 11 - 26
13
Types of Cast Iron
Compacted graphite iron (CGI)
1.
2.
3.
•
•
•
C: 3.1-4.0%, Si: 1.7-3.0%
Lower content of Mg or Ce
Worm-like (vermicular)
graphite particles
higher thermal conductivity
better resistance to thermal shock
lower oxidation at elevated
temperature
Chapter 11 - 27
Production of Cast Iron
Adapted from Fig.11.5,
Callister 7e.
Chapter 11 - 28
14
Cast iron
Chapter 11 - 29
Which type of cast iron is in the pictures illustrated below ?
Chapter 11 - 30
15
Limitations of Ferrous Alloys
1) Relatively high density
2) Relatively low conductivity
3) Poor corrosion resistance
Chapter 11 - 31
Metal Fabrication
• How do we fabricate metals?
– Blacksmith - hammer (forged)
– Molding - cast
• Forming Operations
– Rough stock formed to final shape
Hot working
• T high enough for
recrystallization
• Larger deformations
vs.
Cold working
• well below Tm
• work hardening
• smaller deformations
Chapter 11 - 32
16
Metal Fabrication Methods - I
FORMING
CASTING
JOINING
• Forging (Hammering; Stamping) • Rolling (Hot or Cold Rolling)
(wrenches, crankshafts)
(I-beams, rails, sheet & plate)
force
roll
die
A o blank
A d often at
elev. T
• Drawing
Ao
roll
force
Ao
Ad
Adapted from
Fig. 11.8,
Callister 7e.
• Extrusion
(rods, wire, tubing)
die
Ad
(rods, tubing)
Ao
tensile
force
force
die
die must be well lubricated & clean
container
ram
billet
die holder
Ad
extrusion
die
container
ductile metals, e.g. Cu,
Al (hot)
Chapter 11 -
33
Metal Fabrication Methods - II
FORMING
CASTING
JOINING
• 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
Chapter 11 - 34
17
Metal Fabrication Methods - II
FORMING
CASTING
JOINING
• Sand Casting
(large parts, e.g.,
auto engine blocks)
Sand
Sand
molten metal
•
trying to hold something that is hot
•
what will withstand >1600ºC?
•
cheap - easy to mold => sand!!!
•
pack sand around form (pattern) of
desired shape
Chapter 11 - 35
Metal Fabrication Methods - II
FORMING
CASTING
JOINING
• Sand Casting
(large parts, e.g.,
auto engine blocks)
Investment Casting
• pattern is made from paraffin.
Sand
Sand
molten metal
• Investment Casting
(low volume, complex shapes
e.g., jewelry, turbine blades)
plaster
die formed
around wax
prototype
• mold made by encasing in
plaster of paris
• melt the wax & the hollow mold
is left
• pour in metal
wax
Chapter 11 - 36
18
Metal Fabrication Methods - II
FORMING
CASTING
• Sand Casting
• Die Casting
(large parts, e.g.,
auto engine blocks)
Sand
JOINING
(high volume, low T alloys)
Sand
molten metal
• Continuous Casting
• Investment Casting
(simple slab shapes)
(low volume, complex shapes
e.g., jewelry, turbine blades)
plaster
die formed
around wax
prototype
molten
solidified
wax
Chapter 11 - 37
Metal Fabrication Methods - III
FORMING
CASTING
• Powder Metallurgy
(materials w/low ductility)
JOINING
• Welding
(when one large part is
impractical)
pressure
filler metal (melted)
base metal (melted)
fused base metal
heat
area
contact
densify
unaffected
piece 1
heat affected zone
unaffected
Adapted from Fig.
piece 2
11.9, Callister 7e.
• Heat affected zone:
point contact
at low T
densification
by diffusion at
higher T
(region in which the
microstructure has been
changed).
(Fig. 11.9 from Iron
Castings
Handbook, C.F.
Walton and T.J.
Opar (Ed.), 1981.)
Chapter 11 - 38
19
Thermal Processing of Metals
Annealing: Heat to Tanneal, then cool slowly.
• Stress Relief: Reduce
• Spheroidize (steels):
stress caused by:
-plastic deformation
-nonuniform cooling
-phase transform.
Make very soft steels for
good machining. Heat just
below TE & hold for
15-25 h.
Types of
Annealing
• Process Anneal:
Negate effect of
cold working by
(recovery/
recrystallization)
• Full Anneal (steels):
Make soft steels for
good forming by heating
to get g, then cool in
furnace to get coarse P.
• Normalize (steels):
Deform steel with large
grains, then normalize
to make grains small.
Based on discussion in Section 11.7, Callister 7e.
Chapter 11 - 39
Heat Treatments
800
a) Annealing
Austenite (stable)
T(°C)
TE
A
b) Quenching
P
600
c) Tempered
Martensite
B
A
400
Adapted from Fig. 10.22, Callister 7e.
0%
10
0%
50
%
0%
M+A
200
50%
M+A
90%
a)
b)
10
-1
10
10
time (s)
3
10
5
c)
Chapter 11 - 40
20
f11_10_pg389
Chapter 11 - 41
Chapter 11 - 42
21
Hardenability--Steels
• Ability to form martensite
• Jominy end quench test to measure hardenability.
flat ground
specimen
(heated to g
phase field)
24°C water
Adapted from Fig. 11.11,
Callister 7e. (Fig. 11.11
adapted from A.G. Guy,
Essentials of Materials
Science, McGraw-Hill Book
Company, New York,
1978.)
Rockwell C
hardness tests
Hardness, HRC
• Hardness versus distance from the quenched end.
Adapted from Fig. 11.12,
Callister 7e.
Distance from quenched end
Chapter 11 - 43
Why Hardness Changes With Position
Hardness, HRC
• The cooling rate varies with position.
60
40
20
0
1
2
3
distance from quenched end (in)
T(°C)
600
A
fi
0%
100%
P
Adapted from Fig. 11.13, Callister 7e.
(Fig. 11.13 adapted from H. Boyer (Ed.)
Atlas of Isothermal Transformation and
Cooling Transformation Diagrams,
American Society for Metals, 1977, p.
376.)
400
200
M(start)
A fi M
1
te a
rli Pe
lite ea +
ar P ite
Pe Fine ens
t
ar ite
M ens
t
ar
0.1
M
0 M(finish)
10
100
1000
te
rli
Time (s)
Chapter 11 - 44
22
Hardenability vs Alloy Composition
(table 11.2a p363)
• "Alloy Steels"
10
3
2 Cooling rate (°C/s)
60
100
80 %M
4340
50
40
4140
8640
40
10
Adapted from Fig. 11.14, Callister 7e.
(Fig. 11.14 adapted from figure furnished
courtesy Republic Steel Corporation.)
100
Hardness, HRC
• Jominy end quench
results, C = 0.4 wt% C
20
5140
0 10 20 30 40 50
Distance from quenched end (mm)
800
(4140, 4340, 5140, 8640)
--contain Ni, Cr, Mo
(0.2 to 2wt%)
--these elements shift
the "nose".
--martensite is easier
to form.
T(°C)
TE
600
A
shift from
A to B due
to alloying
B
400
M(start)
M(90%)
200
0 -1
10 10
3
10
5
10 Time (s)
Chapter 11 - 45
Quenching Medium & Geometry
• Effect of quenching medium:
Medium
air
oil
water
Severity of Quench
low
moderate
high
Hardness
low
moderate
high
• Effect of geometry:
When surface-to-volume ratio increases:
--cooling rate increases
--hardness increases
Position
center
surface
Cooling rate
low
high
Hardness
low
high
Chapter 11 - 46
23
Summary
• Steels: increase TS, Hardness (and cost) by adding
--C (low alloy steels)
--Cr, V, Ni, Mo, W (high alloy steels)
--ductility usually decreases with 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.
Chapter 11 - 47
24