Heat Treatment

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Heat Treatment

• ISAT 430

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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.

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Heat Treatment

• Principal heat treatments
• Annealing
• Martensite formation in steel
• Precipitation hardening
• Surface hardening

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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

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The Iron Carbon System

• Steels, ferrous alloys, cast irons, cast steels
• Versatile and ductile
• Cheap
• Irons (lt 0.008 C)
• Steels (lt 2.11 C)
• Cast irons (lt6.67 mostly lt4.5C)
• The material properties are more than composition
  they are dependent on how the material has been
  treated.

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The Phase Diagram
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Fe - C

• Iron melts at 1538C
• 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 727C
• Soft and ductile
• Magnetic up to the Curie temperature of 768C

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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.

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Fe - C

• Austenite (gamma ? iron)
• Between 1394 and 912C 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.

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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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Martensite formation in Steel

• The diagram at left assumes slow equilibrium
  cooling.
• Each phase is allowed to form
• Time is not a variable.

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Martensite formation in Steel

• 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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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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The Time Temperature Transformation Curve
(TTT)
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The Time Temperature Transformation Curve
(TTT)

• Composition Specific
• Here 0.8 carbon
• At different compositions, shape is different

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0.8C
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The Time Temperature Transformation Curve
(TTT)

• 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.

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The Time Temperature Transformation Curve
(TTT)

• If cooling is rapid enough austenite is
  transformed into Martensite.
• FCC gt 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

• The extreme hardness comes from the lattice
  strain created by carbon atoms trapped in the BCT

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Tempered Martensite

• Step 1 -- Quench in the martensitic phase
• Step 2 soak
• Fine carbide particles precipitate from the iron
  carbon solution
• Gradually the structure goes BCT gt BCC

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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
• These cause rate of cooling differences

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Quenching Media2

• Cooling capacities of typical quench media are
• Agitated brine 5.
• Still water 1.
• Still oil 0.3
• Cold gas 0.1
• Still air 0.02

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Other quenching concerns

• 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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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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Surface Hardening2

• The common procedures are
• Carburizing
• Nitriding
• Carbonnitriding
• Chromizing and boronizing

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Carburizing

• Heating a low carbon steel in the presence of
  carbon rich environment at temperature 900C
• 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.

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Nitriding

• Nitrogen is diffused in the surface of special
  alloy steels at temperatures around 510C.
• Steel must contain elements that will form
  nitride compounds.
• Aluminum
• Chromium
• Forms a thin hard case without quenching
• Thicknesses 0.001 in 0.020 in.

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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.

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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.