Annealing (PowerPoint)

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					                   Annealing
• Annealing is the process of heating a metal
  which is in a metastable of distorted structural
  state, to a temperature which will remove the
  instability or distortion and then cooling is so that
  the room temperature structure is stable and/or
  strain free.
• Classification:
  -Full annealing
  -Bright annealing
  -Box annealing
  -Isothermal (cycle) annealing
  -Spheroidise annealing
  -Subcritical annealing
          Purpose of annealing
• Inducing a completely stable structure
• Refining and homogenizing the structure
• Reducing hardness
• Improving machinability
• Producing desired microstructure
• Removing residual stresses
• Removing gases
• Improving mechanical,physical,electrical and
  magnetic properties
• Improving cold working, characteristics for
  facilitating further cold work
Full annealing
• The term full annealing is used for the treatment that involves
  heating a steel to its austenitic state before subjecting it to slow
  cooling.
• PROCESS:The process consists of heating the steel to above A3
  temperature for hypoeutectoid steels and above A1 temperature
  for hypereutectoid steels by 30-500C,holding at this temperature
  for a definite period and slow cooling to below A1 or to room
  temperature.To insure equalization of temperature throughout
  the cross section of the component and complete austenization,
  a holding period of at least 20minutes per cm of thickest section
  is necessary.
• Hypereutectoid steels always annealed from above A1
  temperature and never annealed from above Acm temperature
  because of the following reason:
  1.)If slowly cooled from above Acm temperature, proeutectoid
  cementite separates along the grain boudaries of austenite and
  completely envelopes the austenite grains which transform to
  pearlite at A1 temperature.The microstructure of such a steel at
  room temperature shows a continuous network of cementitic
  areas all around the pearlite regions.Due to this the dislocations
  get blocked at cementitic regions and are not able to move from
  one pearlitic region to another.This increases the brittleness of
  steel, departing from the aim of annealing.
2.)Acm temperature is high and therefore,
heating to above Acm results in more
oxidation and decarburization of steel.
3.)Heating grain coarsening of austenite
occurs above Acm temperature.This is
because the last part of cementite
dissolves at Acm and grain boundary
migration occurs without any hinderance
above Acm temperature.This leads to
deterioration of mechanical properties
because coarse grained structures are
inferior to fine grained structure in respect
of mechanical properties
             Bright annealing
• Annealing of steel components is carried out
  using some protective medium to prevent
  oxidation and surface discolouration.
• Such a type of annealing keeps the surface
  bright and hence is called bright annealing.
• The surface protection is obtained by the use of
  an inert gas such as argon or nitrogen or by
  using reducing atmospheres.
• A typical composition of a reducing gas is 15%
  H2,10% CO,5% CO2,1.5% CH4 and remainder
  N2.
             Box annealing
• Here annealing is carried out in a sealed
  container under conditions that minimises
  oxidation.The components are packed
  with cast iron chips,charcoal or clean sand
  and annealed in a way similar to full
  annealing.It is also called black annealing,
  close annealing or pot annealing.
        Isothermal(cycle) annealing
• In this process, the components are heated as for ordinary
  annealing and then cooled comparatively rapidly to a temperature
  below A1, held at this temperature for certain period of time to
  provide for complete austenite decompositon.This is followed by
  comparatively rapid cooling.
• Advantage of isothermal annealing is that it reduces the time
  required for heat treatment of steel.It is especially true for alloy
  steels which must be cooled very slowly to obtain the required
  reduction in hardness.
• Because of equalization of temperature,transformation occurs at the
  same time throughout the cross-section.This leads to more
  homogeneity in structure.
• Isothermal annealing produces good results in treating relatively
  small charges of rolled stock or small forgings.
• It shows improved machinability, improved surface finish after
  machining and less warping during subsequent hardening process.
• Increase in surface finish is due to the formation of spheroidised
  structure.
         Sub-critical annealing
• This process involves heating the steel to
  a temperature just below the A1
  temperature,holding it at that temperature
  for a while and then allowing the material
  to cool in air.This is a faster rate of cooling
  than that employed with full annealing
  where the material is cooled in the
  furnace.
• They are used after cold working of steels
  to relieve the internal stresses or to reduce
  the hardness or to refine the structure.
• Stress-relief annealing:
            In this process, cold worked steel is
  heated to a temperature between 500 to 5500C
  which is below its recrystallization temperature
  for 1-2 hours and cooled room temperature in
  air.Due to this internal stresses are partly
  relieved without loss of strength and hardness
  i.e. without change of microstructure.It reduces
  the risk of distortion in machining, and also
  increases corrosion resistance.Since only low
  carbon steels can be cold worked,the process is
  applicable only for hypoeutectoid steels
  containing less than 0.4% carbon.
• Recrystallization annealing:This done
  below A1 temperature i.e. at temperature
  between 6250 and 6750C.The cold worked
  ferrite recrystallizes and cementite tries to
  spheroidise during this annealing
  process.Not only internal stresses are
  eliminated but also steel becomes soft
  and ductile.Refinement in grain size is also
  possible by control of degree of cold work
  prior to annealing or by control of
  annealing temperature and time.
• Process annealing(Intermediate annealing): In
  this method,cold worked metal is heated to
  above its recrystallization temperature.This is
  also accomplished by the formation of strain free
  equiaxed grains.This is given to metals to soften
  them during mechanical processing so as to
  continue the cold working process without
  cracking of metals.It may or may not involve full
  recrystallization of the cold worked metal.In
  principle,process annealing and recrystallization
  annealing are same and both the process
  involve formation of stress free equiaxed grains
  from strained and distorted cold worked grains.
         Spheroidise annealing
• This process is performed by heating the steel
  slightly above the critical temperature A1(730 to
  7700C) with subsequent holding at this
  temperature followed by slow cooling, at a rate
  of 25 to 300C/hr, to 6000C.
• This heat treatment is given to high carbon and
  air hardening alloy steels to soften them and to
  increase machinability.The microstructure,
  typical of this heat treatment, shows globules of
  cementite or carbide in the matrix of
  ferrite.Hence this process is called as
  spheroidise annealing.
Methods producing spheroidise structure
1.) Hardening and high temperature tempering:Due to
   tempering of hardened steels at 650-7000C for a long
   time,cementite globules are formed in the matrix of ferrite
   from martensite.
Martensitecementite(globules)+ferrite
2.) Holding at just below A1:Due to holding for a long time
   just below the lower critical temperature, cementite from
   pearlite globularises.The process is very slow and requires
   more time for obtaining spheroidised structure.It can be
   accelerated by prior cold working of steel.However ,this
   may not be possible if the steel is brittle.(e.g. alloy steels
   and high carbon steels cannot be worked much in the
   annealed or normalised conditions without cracking)
3.) Thermal cycling around A1:Due to this thermal cycling in
   a narrow temperature interval around A1,lamellae from
   pearlite become spheroidal.During heating above A1
   cementite or carbide try to dissolve and during cooling
   they try to form.This repeated action spheroidises the
   carbide particles.
• Spheroidised structures are softer than annealed structures and
  have excellent machinability.
• For plain carbon and high alloy structural steels,optimum
  machinability corresponds to 50% spheroidised and 50% lamellar
  carbides.
• For high alloy steels of air hardening type, it corresponds to 100%
  spheroidised carbides.
• The time of hardening is greatly reduced if the cementite is in fine
  spheroidised form and uniformly distributed because less time is
  required to dissolve spheroidised cementite than the lamellar
  cementite to obtain homogeneous austenite.
• This reduces soaking time during hardening subsequently reducing
  the oxidation and decarburisation.
• This is used in safety razors and needles to reduce decarburization
  and increase hardness.
• Finally they have lower hardness and tensile strength and a
  correspondingly relative elongation and reduction of area than steel
  subject to normal annealing.

				
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posted:8/18/2010
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