Transcript Heat Treatment - James Madison University

Heat Treatment
ISAT 430
Heat Treatment

Three reasons for heat treatment

To soften before shaping

To relieve the effects of strain hardening

To acquire the desired strength and toughness
in the finished product.
Spring 2001
Dr. Ken Lewis
ISAT 430
Module 6
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Heat Treatment

Principal heat treatments
 Annealing
 Martensite formation in steel
 Precipitation hardening
 Surface hardening
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ISAT 430
Module 6
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Annealing


Process

Heat the metal to a temperature

Hold at that temperature

Slowly cool
Purpose

Reduce hardness and brittleness

Alter the microstructure for a special property

Soften the metal for better machinability

Recrystallize cold worked (strain hardened) metals

Relieve induced residual stresses
Spring 2001
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ISAT 430
Module 6
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The Iron Carbon System





Steels, ferrous alloys, cast irons, cast steels
 Versatile and ductile
 Cheap
Irons (< 0.008% C)
Steels (< 2.11% C)
Cast irons (<6.67% [mostly <4.5%]C)
The material properties are more than composition –
they are dependent on how the material has been
treated.
Spring 2001
Dr. Ken Lewis
ISAT 430
Module 6
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The
Phase
Diagram
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Dr. Ken Lewis
ISAT 430
Module 6
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Fe - C

Iron melts at 1538°C

As it cools, it forms in sequence
Delta ferrite
 Austenite
 Alpha ferrite


Alpha ferrite

Solid solution of BCC iron

Maximum C solubility of 0.022% at 727°C

Soft and ductile

Magnetic up to the Curie temperature of 768°C
Spring 2001
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ISAT 430
Module 6
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Fe - C



Delta ferrite
 exists only at high temperatures and is of little
engineering consequence.
Note that little carbon can be actually interstitially
dissolved in BCC iron
Significant amounts of Chromium (Cr), Manganese
(Mn), Nickel (Ni), Molybdenum (Mb), Tungsten (W),
and Silicon (Si) can be contained in iron in solid
solution.
Spring 2001
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ISAT 430
Module 6
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Fe - C

Austenite (gamma  iron)
 Between 1394 and 912°C iron transforms from
the BCC to the FCC crystal structure.
 It can accept carbon in its interstices up to
2.11%
 Denser than ferrite, and the FCC phase is much
more formable at high temperatures.
 Large amounts of Ni and Mn can be dissolved
into this phase
 The phase is non-magnetic.
Spring 2001
Dr. Ken Lewis
ISAT 430
Module 6
9
Fe - C



Cementite

100% iron carbide Fe3C

Very hard

Very brittle
Pearlite

Mixture of ferrite and cementite

Formed in thin parallel plates
Bainite

Alternate mixture of the same phases

Needle like cementite regions

Formed by quick cooling
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ISAT 430
Module 6
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Martensite formation in Steel
Spring 2001
Dr. Ken Lewis

The diagram at left
assumes slow equilibrium
cooling.

Each phase is allowed to
form

Time is not a variable.
ISAT 430
Module 6
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Martensite formation in Steel

Spring 2001
Dr. Ken Lewis
However

If cooling is rapid
enough that the
equilibrium reactions
do not occur

Austenite transforms
into a non-equilibrium
phase

Called Martensite.
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Module 6
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Spring 2001
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ISAT 430
Module 6
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Fe - C

Martensite
 Hard brittle phase
 Iron carbon solution whose composition is the
same as austenite from which it was derived
 But the FCC structure has been transformed
into a body center tetragonal (BCT)
 The extreme hardness comes from the lattice
strain created by carbon atoms trapped in the
BCT
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ISAT 430
Module 6
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The Time – Temperature – Transformation
Curve (TTT)
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ISAT 430
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The Time – Temperature – Transformation
Curve (TTT)
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
Composition Specific

Here 0.8% carbon

At different compositions,
shape is different
ISAT 430
Module 6
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0.8C
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Module 6
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The Time – Temperature – Transformation
Curve (TTT)



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Dr. Ken Lewis
At slow cooling rates the
trajectory can pass through the
Pearlite and Bainite regions
Pearlite is formed by slow
cooling

Trajectory passes
through Ps above the
nose of the TTT curve
Bainite

Produced by rapid
cooling to a temperature
above Ms

Nose of cooling curve
avoided.
ISAT 430
Module 6
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The Time – Temperature – Transformation
Curve (TTT)


Spring 2001
Dr. Ken Lewis
If cooling is rapid enough
austenite is transformed into
Martensite.

FCC > BCT

Time dependent diffusion
separation of ferrite and
iron carbide is not
necessary
Transformation begins at Ms
and ends at Mf.

If cooling stopped it will
transition into bainite and
Martensite.
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Martensite hardness

Spring 2001
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ISAT 430
The extreme hardness
comes from the lattice
strain created by
carbon atoms trapped
in the BCT
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Tempered Martensite
Spring 2001
Dr. Ken Lewis

Step 1 -- Quench in the
martensitic phase

Step 2 – soak

Fine carbide particles
precipitate from the iron –
carbon solution

Gradually the structure
goes BCT > BCC
ISAT 430
Module 6
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Quenching Media

The fluid used for quenching the heated alloy effects
the hardenability.
 Each fluid has its own thermal properties
Thermal conductivity
 Specific heat
 Heat of vaporization


Spring 2001
These cause rate of cooling differences
Dr. Ken Lewis
ISAT 430
Module 6
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Quenching Media2

Cooling capacities of typical quench media are




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Agitated brine
Still water
Still oil
Cold gas
Still air
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5.
1.
0.3
0.1
0.02
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Other quenching concerns



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Fluid agitation
 Renews the fluid presented to the part
Surface area to volume ratio
Vapor blankets
 insulation
Environmental concerns
 Fumes
 Part corrosion
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ISAT 430
Module 6
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Surface Hardening



Refers to a “thermo chemical” treatment whereby
the surface is altered by the addition of carbon,
nitrogen, or other elements.
Sometimes called CASE HARDENING.
Commonly applied to low carbon steels
 Get a hard wear resistant shell
 Tough inner core
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ISAT 430
Module 6
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Surface Hardening2

The common procedures are:

Carburizing

Nitriding

Carbonnitriding

Chromizing and boronizing
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ISAT 430
Module 6
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Carburizing





Heating a low carbon steel in the presence of carbon rich
environment at temperature ~ 900°C

Carbon diffuses into the surface

End up with a high carbon steel surface.
Pack parts in a compartment with coke or charcoal
Gas carburizing

Uses propane (C3H8) in a sealed furnace
Liquid carburizing

Used NaCN, BaCl2
Thickness 0.005 in. to 0.030 in.
Spring 2001
Dr. Ken Lewis
ISAT 430
Module 6
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Nitriding

Nitrogen is diffused in the surface of special alloy
steels at temperatures around ~510°C.
 Steel must contain elements that will form nitride
compounds.
Aluminum
 Chromium


Forms a thin hard case without quenching

Spring 2001
Thicknesses 0.001 in – 0.020 in.
Dr. Ken Lewis
ISAT 430
Module 6
28
Chromizing


Diffuse chromium into the surface 0.001 – 0.002 in.
Pack the parts in Cr rich powders or dip in a molten
salt bath containing Cr salts.
Spring 2001
Dr. Ken Lewis
ISAT 430
Module 6
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Boronizing


Performed on tool steels, nickel and cobalt based
alloy steels.
When used on low carbon steels, corrosion
resistance is improved.
Spring 2001
Dr. Ken Lewis
ISAT 430
Module 6
30