rizam@kukum.edu.my
METAL FABRICATION METHODS-I
CASTING
FORMING
• Forging
(wrenches, crankshafts)
force
die
Ao blank
JOINING
• Rolling
(I-beams, rails)
Ad often at
elev. T
• Drawing
force
• Extrusion
(rods, wire, tubing)
die
Ao
die
Ad
Adapted from
Fig. 11.7,
Callister 6e.
(rods, tubing)
tensile
force
6
FORMING TEMPERATURE
• Hot working
--recrystallization
• Cold working
--recrystallization
--less energy to deform
--oxidation: poor finish
--lower strength
--less energy to deform
--oxidation: poor finish
--lower strength
• Cold worked microstructures
--generally are very anisotropic!
--Forged
(a)
--Swaged
(b)
--Fracture resistant!
(c)
7
METAL FABRICATION METHODS-II
FORMING
CASTING
• Sand Casting
(large parts, e.g.,
auto engine blocks)
• Investment Casting
JOINING
• Die Casting
(high volume, low T alloys)
• Continuous Casting
(simple slab shapes)
(low volume, complex shapes
e.g., jewelry, turbine blades)
plaster
die formed
around wax
prototype
8
METAL FABRICATION METHODS-III
FORMING
• Powder Processing
(materials w/low ductility)
CASTING
JOINING
• Welding
(when one large part is
impractical)
filler metal (melted)
base metal (melted)
fused base metal
unaffected
piece 1
heat affected zone
unaffected
Adapted from Fig.
piece 2
11.8, Callister 6e.
(Fig. 11.8 from
• Heat affected zone:
Iron Castings
Handbook, C.F.
Walton and T.J.
(region in which the
Opar (Ed.), 1981.)
microstructure has been
changed).
9
Thermal processing of metals
Annealing: Heat to Tanneal, then cool slowly.
Outline
• Heat Treatment of Steels
– Hardenability
– Influence of quenching medium, specimen
size, and geometry
• Annealing Processes
– Annealing of ferrous alloys
 Full annealing
 Normalizing
– Process annealing
– Stress relief
• Precipitation Hardening
160
The complete
isothermal
transformation
diagram for an
iron-carbon alloy of
eutectoid
composition.
140
120
100
80
A: austenite
B: bainite
M: martensite
P: pearlite
60
40
20
0
1
10
100
Heat Treatment of Steels
• Conventional heat treatment procedures for
producing martensitic steels involves
– continuous and rapid cooling of an austenitized
specimen in some type of quenching medium,
such as water, oil, or air
• The optimum properties of a steel that has been
quenched and then tempered can be realized only if,
– during the quenching heat treatment, the
specimen has been converted to a high content
of martensite
Non-uniform Cooling Rate during Quenching
• During the quenching treatment,
– it is impossible to cool the specimen at a
uniform rate throughout
– the surface will always cool more rapidly than
interior regions.
• The austenite will transform over a range of
temperatures, yielding a possible variation of
microstructure & properties with position within a
specimen.
Heat Treatments of Steels (Cont’d)
• The successful heat treating of steels to produce a
predominantly martensite microstructure throughout
the cross section depends mainly on three factors:
1. the composition of the alloy
2. the type and character of the quenching medium
3. the size and shape of the specimen
Hardenability of Steels
• Hardenability: A measure of the ability a specific
alloy to be hardened by forming martensite as a
result of given heat treatment
• The Jominy end-quench test:
– to measure hardenability
Jominy End Quench Test
• Ability to form martensite
• Jominy end quench test to
measure hardenability
1”
specimen
(heated to 
phase field)
24°C water
flat ground
4”
Adapted from Fig. 11.10,
Callister 6e.
Hardenability Curves
• Hardness vs. distance from the quenched end
1”
specimen
(heated to 
phase field)
24°C water
flat ground
4”
Adapted from Fig. 11.11,
Callister 6e.
• A steel that is highly hardenable will retain large
hardness values for relatively long distances; a low
hardenable one will not.
Why hardness changes with position?
• The cooling rate varies with position
Adapted from Fig. 11.12, Callister 6e.
Effect of Alloying Elements on Hardenability
(1.85 Ni, 0.80 Cr, &
0.25Mo)
(1.0 Cr & 0.20 Mo)
(0.55 Ni, 0.50 Cr, & 0.20
Mo)
(0.85 Cr)
(plain carbon
steel)
Distance from quench end
Hardenability Curves
for Five Steel Alloys
(Each Containing 0.4
wt% C)
Hardenability
One plain carbon steel (1040) and four alloy steels
•
All five alloys have identical hardness at the
quenched end (57 HRC); this hardness is a function
of carbon content.
•
The shape of the curves relates to hardenability.
The hardenability from low to high:
1040 steel < 5140 steel < 8640 steel < 4140 steel
< 4340 steel
•
A water-quenched specimen of the 1040 plain
carbon steel would harden only to a shallow depth
below the surface, whereas for the other four alloy
steels the high quenched hardness would persist to
a much greater depth.
Effect of Carbon Content on Hardenability
The
hardenability
increases with
the carbon
content.
FIG. 11.14 Hardenability
curves for four 8600
series alloys of indicated
carbon content.
Alloying Elements and Hardenability
• The alloying element content and carbon content
•
change the shapes of the hardenability curve
The principal reason for using alloying elements in the
standard grades of steels is to increase hardenability.
Quenching Mediums (1): Water
• The most commonly used quenching medium
• Inexpensive and convenient to use
• Provide very rapid cooling
• Especially used for low-carbon steel, which requires
a very rapid change in temperature in order to obtain
good hardness and strength
• Can cause internal stresses, distortion, or cracking
Quenching Mediums (2): Oil
• More gentle than water
• Used for more critical parts, such as parts that have
thin sections or sharp edges
• Razor blades, springs, and knife blades
• Does not produce steel that is as hard or strong as
steel quenched by water
• Less chance of producing internal stresses,
distortion, or cracking
• More effective when oil is heated slightly above
room temperature to 100°F or 150°F (40°C or 65°C):
reduced viscosity
Quenching Mediums (3): Air
• More gentle than oil
• Does not produce steel that is as
hard or strong as steel quenched
by water or oil
• Less chance of producing
internal stresses, distortion, or
cracking
• Generally used only on steels
that have a very high alloy
content
Special alloys (such as Cr and
Mo) are selected because they
are known to cause materials to
harden even though a slower
quenching method is used
The heated sample is
placed on a screen. Cool air
is blown at high speed from
below it.
Effect of Quenching Medium
Medium
Severity of Quench
Hardness
air
small
small
oil
moderate
moderate
water
large
large
The severity of quench: water > oil > air
Quenching Operations in Heat Treatment
Removing Samples from
a Heat-Treating Furnace
Parts Quenched in a Group
(Figs. 11-3 and 11-4 in Metallurgy Fundamentals, by D. A. Brandt and J. C. Warner)
Effect of Part Size
FIG. 11.17 Radial hardness profiles for (a) 50 mm (2 in.) diameter cylindrical
1040 and 4140 steel specimens quenched in mildly agitated water, and (b) 50
and 100 mm (2 and 4 in.) diameter cylindrical specimens of 4140 steel quenched
Effect of Part Geometry
• When surface-to-volume ratio increases
cooling rate increases
hardness increases
Position Cooling rate
center
small
surface
large
Hardness
small
large
Annealing Processes
• Annealing: a heat treatment in which a material is
exposed to an elevated temperature for an extended
time period and then slowly cooled.
• Three stages of annealing
1. Heating to the desired temperature
2. Holding or “soaking” at that temperature
3. Cooling, usually to room temperature
Purposes for Annealing
1. Relieve Internal Stresses
• Internal stresses can build up in metal as a
result of processing.
– Stresses may be caused by previous
processing operations such as welding, cold
working, casting, forging, or machining.
• If internal stresses are allowed to remain in a
metal, the part may eventually distort or crack.
• Annealing helps relieve internal stresses and
reduce the chances for distortion and cracking.
Purposes for Annealing (Cont’d)
2. Increasing Softness, Machinability, and Formability
• A softer and more ductile material is easier to
machine in the machine shop.
• An annealed part will respond better to forming
operations.
3. Refinement of Grain Structures
• After some types of metalworking (particularly
cold working), the crystal structures are
elongated.
• Annealing can change the shape of the grains
back to the desired form.
The Iron–Iron Carbide Phase Diagram
P
2.14
E
4.30
L + Fe3C
F
G
M
O
N
H
0.76
0.022
Cementite Fe3C
C
6.70
Temperature Regime of Steel Heat Treatment
FIG. 11.9 The iron-iron
carbide phase diagram
in the vicinity of the
eutectoid, indicating
heat treating
temperature ranges for
the plain carbon steels.
• Most heat treating operations begin with heating the alloy
into the austenitic phase field to dissolve the carbide in
the iron
• Steel heat treating practice rarely involves the use of
temperatures above 1040°C (1900°F)
Precipitation hardening of metals
(Heat treatment)
• Strength and hardness of some metal alloys
may be enhanced by the formation of extremely
small uniformly dispersed particles of a second
phase within the original phase matrix.
• This strengthening is accomplished by phase
transformations induced by heat treatment.
• Age hardness is also used to designate the
process since strength develops with time,
or as the alloy ages.
Precipitation hardening (Cont.)
• Requisite features of phase diagrams of
alloy systems for precipitation hardening:
– Appreciable maximum solubility of one
component in the other (on the order of
several percent).
– Solubility limit that rapidly decreases in
concentration of the major component with
temperature reduction.
– Composition of precipitation-hardenable alloy
must be less than the maximum solubility.
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
Precipitation hardening (Cont.)
• Types of precipitation hardening
process:
– Solution heat treatment
– Precipitation heat treatment.
Temperature vs. time plot showing both solution and
precipitation heat treatment for precipitation hardening.
Precipitation hardening (Cont.)
Solution heat treatment:
• Quenching (rapid cooling) prevent diffusion and
the formation of a new phase. Resulting phase
will be supersaturated in B atoms (alloy relatively
soft and weak). a Phase is retained at room
temperature for relatively long periods (long
diffusion rate at room temperature).
Precipitation heat treatment:
• The characteristics of the  phase and the
strength and hardness of the alloy depends on
the precipitation temperature T2 and the aging
time at this temperature.
Precipitation effect on TS, %EL
• 2014 Al Alloy:
• TS peaks with
precipitation time.
• Increasing T accelerates
process.
• %EL reaches minimum
with precipitation time.
Effect of aging on dislocation
motion
• Peak-aged
--avg. particle size = 64b
--closer spaced particles
efficiently stop dislocations.
• Over-aged
--avg. particle size = 361b
--more widely spaced
particles not as effective.
SUMMARY
• 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.
• Hardenability
--increases with alloy content.
• Precipitation hardening
--effective means to increase strength in
Al, Cu, and Mg alloys.