IE 121 Metal
Asst. Prof. Dr. Oratai Jongprateep
Phases in iron-carbon
alloy
Iron-iron carbide phase diagram
 (BCC) ↔  (FCC) ↔  (BCC) ↔ liquid
Review: Reaction in phase diagram
Phases in Fe–Fe3C phase diagram
 -ferrite - BCC
• Stable form of iron at room T (≤912°C)
• Max. solubility of C is 0.022 wt%
• Soft and relatively easy to deform
 -austenite - FCC
• Stable at 727 ≤ T ≤ 1394°C)
• Max. solubility of C is 2.14 wt% at 1147°C
• Large interstitial lattice positions
• Not stable below the eutectic T unless
cooled rapidly
• Tough and ductile, suitable for hot working
Phases in Fe–Fe3C phase diagram
 –ferrite - BCC
• Stable only at high T, above 1394°C
• Also has low solubility for C
• Tough and ductile
• Not too important technically
Fe3C (iron carbide or cementite)
• Metastable intermetallic compound: at room T- Fe3C
at 650 - 700°C slowly decomposes into -Fe and C (graphite)
• Hard and brittle
Properties versus amounts of phases
Development of
microstructure in ironcarbon alloys
Eutectoid composition
Pearlite occurs at GB of
austenite
Hypoeutectoid composition
Proeutectoid ferrite network
surrounding the pearlite
colonies
Hypereutectoid composition
White proeutectoid cementite
network surrounding the
pearlite colonies
Isothermal
transformation
diagram
T-T-T plot
Isothermal transformation
diagram (T-T-T plot) is generated
from percent transformation
versus logarithm of time
measurements
Pearlite
Ttransf just below TE

Larger T: high diffusion rate

Pearlite is coarser
Ttransf well below TE

Smaller T: diffusion is slower

Pearlite is finer
Mech. properties are different:
fine pearlite is stronger

Adherence /reinforcement effect

Dislocation barrier
Bainite

Consist of ferrite and cementite

A structure intermediate between
pearlite and martensite

Nonlamellar eutectoid structure,
occur in form of needles or plate

Has very fine microstructure: TEM

No proeutectoid phase formed

Formation of banite is competitive
with pearlite, can’t transform from
one to another w/o reheating to
austenite
strips with long rods of Fe3C
Spheroidite

Occurs when pearlite or bainite is
heated at T < TE for a long time (at
700°C for 18 hr)

Fe3C phase appears as sphere-like
particles embedded in a ferrite
phase

Driving force: reduction in Fe3C
phase boundary area

Lower strength/hardness compared
to pearlite due to adherence,
dislocation barrier effect
Mech. prop. of pearlite & spheroidite
Martensite
Occurs from diffusionless
transformation of austenite

Very rapid quenching prevent
diffusion of C

FCC austenite transforms to a
body centered tetragonal (BCT)
martensite

Has platelike or needlelike
appearance

Time -independent
transformation: represented by
a horizontal line in TTT diagram

it is a function of Tquenched:
athermal transformation
60 m

Martentite needles
Austenite
Mech. Prop. of martensite

Hardest, most brittle, abrasive
resistance

Carbon: interstitial solid
solution  impede movement
of dislocation

More percent of carbon
 more hardness
 low toughness and
ductility
Eutectoid iron-carbon alloy and isothermal
heat treatments
Tempered martensite
• Martensite  heat below eutectoid T (250-650C )
• Fe3C particle in a matrix of ferrite
• Softer and more ductile
Quenching Medium & Geometry
• Effect of quenching medium:
Medium
air
oil
water
Hardness
small
moderate
large
Severity of Quench
small
moderate
large
• Effect of geometry:
When surface-to-volume ratio increases:
--cooling rate increases
--hardness increases
Position
center
surface
Cooling rate
small
large
Hardness
small
large
Dept of Mat Eng
22
Cooling Ex.
(a) Rapidly cool to 350°C,
hold for 104s, quench
to Troom
(c
)
(a
)
(b) Rapidly cool to 250°C,
hold for 100s, quench
to Troom
(c) Rapidly cool to 650°C,
(b
)
hold
for 20s, rapidly cool
to 400°C, hold for 103s,
quench to Troom
Dept of Mat Eng
23
Heat treatment of steel
Other Heat Treatment Process
Dept of Mat Eng
25

Full annealing
◦ Plain carbon steel with low and moderate percent of
carbon
◦ Heated to 40oC above critical temperature and then
cooled in furnace
◦ Coarse pearlite  soft and ductile and has small
uniform grains

Process annealing
◦ Stress relief
◦ To restore its ductility, so the part can be worked
further into the final desired shape.
◦ Heated below eutectoid temperature (~ 550o C 650o C )

Spheroidizing annealing
◦ Plain carbon steel with high percent of carbon
◦ Heated below eutectoid temperature and cooled
in furnace
◦ Spheroidite cementite in  ferrite matrix  soft

Normalizing
◦ Heated to the austenite region and then cooled
in air
◦ Fine pearlite with small uniform grain
◦ higher strength and toughness, but lower
ductility than full annealing
◦ Reduce compositional segregation in castings of
forging
Hardening of steel and
alloy
Hardenability
• Hardenability : Ability to form martensite
• Jominy end quench test : to measure hardenabilit
1”
specimen
(heated to 
phase field)
24°C water
flat ground
4”
• Hardness
versus distance
from the
quenched end.
Dept of Mat Eng
29
Surface Hardening
• Thermochemical treatments to harden surface of
part (carbon, nitrogen)
• Also called case hardening
• May or may not require quenching
• Interior remains tough and strong
Carburizing
• Low-carbon steel is heated in a carbon-rich
environment
– Pack carburizing - packing parts in charcoal or
coke -makes thick layer (0.025 - 0.150 in)
– Gas carburizing - use of propane or other gas
in a closed furnace - makes thin layer (0.005 0.030 in)
– Liquid carburizing - molten salt bath containing
sodium cyanide, barium chloride - thickness
between other two methods
• Followed by quenching, hardness about HRC 60
Nitriding
• Nitrogen diffused into surface of special alloy
steels (aluminum or chromium)
• Nitride compounds precipitate out
– Gas nitriding - heat in ammonia
– Liquid nitriding - dip in molten cyanide bath
• Case thicknesses between 0.001 and 0.020 in.
with
hardness up to HRC 70
Other case hardening
• Carbonotriding - use both carbon and
nitrogen
• Chroming - pack or dip in chromium-rich
material - adds heat and wear resistance
• Boronizing - improves abrasion resistance,
coefficient of friction
Precipitation Hardening (I)
• 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
Temp.
Pt A (solution heat treatment)
Pt C (precipitate q)
Pt B
Dept of Mat Eng
Time
34
Precipitation Hardening (II)
Dept of Mat Eng
35
Metal Alloy and Cast
Iron
Types of Metal Alloys
Metal alloys
Nonferrous
Ferrous
Steels
Cast iron
Dept of Mat Eng
37
Phase Diagram of Iron-Iron Carbide
Delta iron
Austenite
Ferrite
Dept of Mat Eng
38
Steels
Low Alloy
low carbon
<0.25wt%C
Name
med carbon
.0.25-0.6wt%C
high carbon
0.6-1.4wt%C
plain HSLA plain
heat plain
treatable
none Cr, Ni none
Additions none Cr,V
Ni, Mo
Example 1010
4310
1040
Hardenability 0
+
+
TS
0
+
EL
+
+
0
Uses
High Alloy
auto
struc.
sheet
bridges
towers
press.
vessels
Mo
4340
++
++
-
1095
++
+
-
pistons wear
crank
gears
shafts
applic.
wear
bolts
hammers applic.
blades
tool
austentitic
stainless
Cr, V,
Cr, Ni, Mo
Mo, W
4190
304
0
+++
0
++
-++
drills
saws
dies
Dept of ductility
Mat Eng
increasing strength, cost, decreasing
high T
application
turbines
furnaces
corrosion
resistant
39



+ Cr
: to increase strength,
to reduce corrosion
+ Mo : to improve the strength and
hardness at high temp.
+ Ni : to improve toughness,
good forming
Dept of Mat Eng
Deep drawing 40
Plain Carbon Steel and Low-Alloy Steels
Dept of Mat Eng
41
Plain Carbon Steel and Low-Alloy Steels
Dept of Mat Eng
42
Stainless Steels

Ferritic S.S. :
14-27%Cr and very low C (<0.12%C)
soft, good machinability, magnetic
 no hardenability

Austenitic S.S. :
Cr, Ni added
good for forming, not magnetic

Martensitic S.S. :
12-18%Cr and high C (up to 1.2%C)
good hardenability, magnetic
Dept of Mat Eng
43
Designations,Compositions And
Applications for Stainless Steels
Dept of Mat Eng
44
Cast Irons
Nodular Iron
Gray Iron
White Iron
Malleable Iron
45
Cast Irons
Gray cast iron :
◦ C in the form of graphite
◦ used in the engine block
Nodular cast iron :
◦ 3.5%C, 2.5%Si and Mg, Na, Ce, Ca, Li etc.
◦ give the nodular (spherical) graphite
◦ good toughness for crankshaft, rocker arm and piston
White cast iron
◦ If the cast iron has been quenched, the white cast iron
occurs which has high compressive strength and
corrosion resistance
Malleable cast iron
◦ because white cast iron is too hard  will be annealed
 Malleable cast iron
Dept of Mat Eng
46
Nonferrous Alloys
• Cu Alloys
• Al Alloys
Brass: Zn is subst. impurity -lower r: 2.7g/cm3
(costume jewelry, coins,
-Cu, Mg, Si, Mn, Zn additions
corrosion resistant)
-solid solution or precipitation
Bronze: Sn, Al, Si, Ni are
strengthened (structure
substitutional impurity
aircraft parts
(bushings, landing
& packaging)
gear)
• Mg Alloys
Nonferrous
Cu-Be:
-very low r :1.7g/cm3
precipitation hardened Alloys
-ignites easily
for strength
-aircraft, missiles
• Ti Alloys
• Refractory metals
-lower r: 4.5g/cm3
vs 7.9 for steel • Noble metals -high melting T
-Nb, Mo, W, Ta
-reactive at high T -Ag, Au, Pt
-space application- oxidation/corrosion resistant
Dept of Mat Eng
47
Ni-based Superalloys
High temperature heat-resistance
alloys (760-980oC), which can retain
high strengths at elevated
temperatures, good corrosion
resistance and good oxidation
resistance.
 There are three types of Ni-base
superalloys; nickel base, nickel-iron
base and cobalt base.
 Applications:
◦ Aircrafts, space vehicles, rocket
engines
◦ Industrial gas turbines,
◦ Nuclear reactors, submarines
◦ Steam power plants, petrochemical
equipment

48
Processing of Steel
Production of steel from iron ore
Iron ore: Hermatite Fe2O3, Magnetite Fe3O4
Coke, limestone
Blast furnace
Pig Iron
Basic Oxygen furnace
Steel
up to 1.2% of carbon
Fe2O3 + 3CO 2Fe + 3CO2
4% of carbon along with other
impurities
Cupola Furnace
Cast Iron
2-4% of carbon
Refining steel from iron ore

Coke = reducing agent to
produce raw pig iron (contain
4% of C +other impurities)

Basic Oxygen furnace: pig
iron + 30% of steel scrap
Oxidizing the carbon and
other impurities

Cupola furnace: metal,
coke and flux

Steel: up to 1.2% of C

Cast iron: contain 2-4% of C
Processing-Fabrication
Forming operations
Forging
Rolling

Introduce plastic deformation
by using mech. force

Types of forming

Hot: repeatable

Cold: good surface
finishing, mech. prop.,
expensive
Drawing
Extrusion
Rolling

The ingots are heated and
hot-rolled into slabs, billets, or
blooms

The slabs are subsequently
hot- and cold-rolled into steel
sheet and plate

The billets are hot- and coldrolled into bars, rods, and wire

The blooms are hot- and cold-
rolled into shapes such as I
beams and rails
Thick metal sheet
Steel or metal casting

Molten steel is either cast in stationary mold or
continuously cast into long slabs from which long
sections are periodically cut off (called ingot).
Steel casting
Most popular
High production rate
Ingot  slab
Casting processes
Sand casting
Continuous casting
Die casting
Casting processes
Investment
casting
Casting processes: advantages and limitations
Process
Advantages
Limitations
Sand
Almost any metal is
cast; no limit to size,
shape or weight; low
tooling cost.
Some finishing required;
somewhat coarse finish,
wide tolerances.
Investment
Intricate shapes; close
tolerance parts; good
surface finish.
Part size limited;
expensive patterns,
molds, and labor.
Die
Excellent dimensional
accuracy and surface
finish; high production
rate.
Die cost is high; part
size limited; usually
limited to nonferrous
metals; long lead time.
Powder met. & joining
Mostly used in metals with
high Tmp and low ductility