The document discusses various methods for increasing the strength of metals, including hardening, quenching, annealing, tempering, normalizing, and austempering. It provides details on the processes involved and the microstructural and property changes that result from each method. Solid solution strengthening, strain hardening, grain size refinement, precipitation hardening, dispersion hardening, and phase transformations are also summarized as six major mechanisms for increasing metal strength.

1. L 08
 Hardening
 Quenching
 Annealing
 Tempering
 Normalizing
 Austempering
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2. Six major mechanisms are available to increase
the strength of metals
 Solid-solution strengthening
 Strain Hardening
 Grain size refinement
 Precipitation Hardening
 Dispersion Hardening
 Phase Transformation
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3. Solid solution Strengthening
 In solid solution strengthening, a base metal
dissolves other atoms, either as substitutional
solutions, where the new atoms occupy sites
in the host crystal lattice, or as interstitial
solutions, where the new atoms squeeze into
“holes” between the atoms of the base
lattice.
 The amount of strengthening depends on the
amount of atoms involved.
 Since distortion of the host structure makes
dislocation movement more difficult, the
greater the size difference, the more effective
the addition.
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4.  Produces an increase in strength by means of plastic
deformation under cold-working conditions.
Grain Size Refinement
 Because grain boundaries act as barriers to
dislocation motion, a metal with small grains tend to
be stronger than the same metal with larger grain.
 Thus grain size refinement can be used to increase
strength, except at elevated temperatures, where
grain growth can occur & grain boundary diffusion
contributes to creep and failure.
 It is important to note that grain size refinement is
one of the few processes that can improve strength
without a companion loss of ductility & toughness
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5. 10/22/2011
PRECIPITATION HARDENING
 In precipitation hardening or age
hardening, strength is obtained from a non
equilibrium structure that is produced by a
three-step heat treatment (For detail pl see
5.4).
DISPERSION HARDENING
 Strength obtained by dispersing second
phase particles throughout a base material is
known as dispersion hardening. To be
effective, the dispersed particles should be
stronger than the matrix, adding strength
through both their reinforcing action & the
additional interfacial surfaces that present
barriers to dislocation movement.
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6.  Involves those alloys that can be heated
to form a single phase at elevated temp
& subsequently transform to one or more
low-temp phases upon cooling.
 When this feature is used to increase
strength, the cooling is usually rapid &
the phases that are produced are usually
of a non equilibrium nature.
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7.  Austenitizing – again taking a steel with 0.6%
carbon or greater and heating to the austenite
region.
 Rapid quench to trap the carbon in the crystal
structure – called martensite. Quench
requirements determined from isothermal
transformation diagram (IT diagram).
 Depending on how fast steel must be
quenched (from IT diagram), the heat treater
will determine type of quenching required:
 Water (most severe)
 Oil
 Molten Salt
 Gas/ Air (least severe)
 Adding 10% sodium hydroxide or salt will
have twice the cooling rate!
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8.  primary purpose is to soften the steel and
prepare it for additional processing such as
cold forming or machining. If already cold
worked - allows recrystallization.
 What does it do?
1. Reduce hardness
2. Remove residual stress (stress relief)
3. Improve toughness
4. Restore ductility
5. Refine grain size
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9.  Process Steps:
1. Heat material into the austenite region (i.e.
above 1600F) – rule of thumb: hold steel for
one hour for each one inch of thickness.
2. Slowly furnace cool the steel – DO NOT
QUENCH
3. Slow cooling allows the C to precipitate out
so resulting structure is coarse pearlite with
excess ferrite.
4. After annealing steel is quite soft and
ductile.
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10.  Normalizing – use when max softness
not require and cost savings desired
(faster than anneal). Air cooled vs.
furnace cooled.
 Process Anneal – not heated as high as
full anneal.
 Stress Relief Anneal – lower temp
(1,000F), slow cooled. Large castings,
weldments.
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11. 10/25/2011
TEMPERING
 Steel is usually harder than necessary and
too brittle for practical use after being
hardened. Severe internal stresses are set
up during the rapid cooling of the metal.
 Steel is tempered after being hardened to
relieve the internal stresses and reduce its
brittleness.
 Tempering consists of heating the metal to
a specified temperature and then
permitting the metal to cool.
 The rate of cooling usually has no effect on
the metal structure during tempering.
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12.  Therefore, the metal is usually
permitted to cool in still air.
 Temperatures used for tempering
are normally much lower than the
hardening temperatures.
 The higher the tempering
temperature used, the softer the
metal becomes.
 High-speed steel is one of the few
metals that becomes harder
instead of softer after it is tempered.
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13.  Ferrous metals are normalized to relieve the
internal stresses produced by machining,
forging, or welding.
 Normalized steels are harder and stronger than
annealed steels. Steel is much tougher in the
normalized condition than in any other
condition.
 Parts that will be subjected to impact and parts
that require maximum toughness and
resistance to external stresses are usually
normalized.
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14.  Normalizing prior to hardening is beneficial in
obtaining the desired hardness, provided the
hardening operation is performed correctly.
 Low carbon steels do not usually require
normalizing, but no harmful effects result if
these steels are normalized.
 Normalizing is achieved by heating the metal
to a specified temperature (which is higher
than either the hardening or annealing
temperatures), soaking the metal until it is
uniformly heated, and cooling it in still air.
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15.  Austempering is an isothermal treatment that
produces a bainite structure in some plain-
carbon steels.
 The process provides an alternate procedure to
quenching and tempering for increasing the
toughness and ductility of some steels.
 In the Austempering process the steel is first
austinitized, then quenched in a molten salt
bath at a temperature just above the Ms
temperature of the steel, held isothermally to
allow the austenite-to-bainite transformation to
take place, and then cooled to room
temperature in air.

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16.  The final structure of an austempered eutectoid
plain-carbon steel is bainite. The advantages of
austempering are :
 Improved ductility and impact resistance of
certain steels over those values obtained by
conventional quenching and tempering and
 Decreased distortion of the quenched
material.
 The disadvantages of austempering over
quenching and tempering are the need for a
special molten salt bath and the fact that the
process can be used for only a limited number
of steels.
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