Heating and Hot Working of Metals

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Heating and Hot -Working of Metals

• Chapter 3

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Cold Working Wire

• When you draw wire, you strengthen in the
  longitudinal direction
• It is not strengthened axially
• This makes it easy to cut, but hard to break by
  pulling on it!!

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Problem

• Propose a series of steps to reduce a rod of
  copper-zinc alloy from 1 diameter to
  .1diameter.
• The maximum cold work allowable for copper is
  85.
• You will have to draw the copper in several steps
  due to the loss of ductility, then anneal it
  several times.

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Annealing is to soften

• You cant just heat up a piece of metal to undo
  the strain hardening
• Its a temperature dependent process

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• On the next pages well explore how properties
  change during the annealing process
• The whole process depends not only on the
  temperature, but on how long you keep the metal
  hot.

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Definition

• Annealing is heating to an elevated temperature
  for an extended time period and then slowly
  cooling

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Steps for all anneals

• 1. Heating
• 2. Holding or soaking
• 3. Cooling
• Time is important for all 3 steps

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Purposes of annealing

• Relieve stresses
• Soften increase ductility
• Produce a specific microstructure

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Stored Energy

• More CW you do less energy is stored!

energy stored
Energy stored (abs)
Tensile strain
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Power required to heat a worked metal
P O W E R, mW
Recovery stage
Recrystallization Stage
Grain growth stage
200
300
400
Temperature, deg.C
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• We cannot give an exact range of temperatures for
  each process.
• Temperature overlaps!

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Annealing 3 stages

• Recovery (stress-relief anneals)
• Recrystallization (process anneals)
• Grain Growth

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Recovery (Stress-relief)

• If you only add a small amount of thermal energy
  (heat it up at little) the dislocations rearrange
  themselves into networks to relieve residual
  stresses
• Ductility is improved
• Strength does not change

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TS and elongation
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Stresses may result from

• Plastic deformation (cold work, machining)
• Non-uniform heating (ex. welding)
• Phase transformation (quenching)

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Stress-relief

• Is held at fairly low temperature
• Is held for a fairly short time
• So that recrystallization does not occur

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Hardness of CW nickel vs. annealing temperature
H A R D N E S S
200
600
400
Temperature, deg.C
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Recrystallization

• Add more heat and wait some more time, and new
  grains start to grow at the grain boundaries.
• The new grains have not been strain hardened
• The recrystallized metal is ductile and has low
  strength

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How much time to wait?

• Incubation period time needed to accumulate
  stored energy from the lattice strain and heat
  energy
• Then lattice starts to recrystallize
• At first fast (lots of nucleation sites)
• Slower at the end

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How hot is hot?

• Most metals have a recrystallization temperature
  equal to about 40 of the melting point

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• Higher is the temperature less amount of CW is
  needed to start recrystallization
• Critical CW the amount when recrystallization
  cannot happen
• Higher is amount of CW- smaller is grain size, no
  matter what was the temperature

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Minor factors for recrystallization

• Pure metal
• If an alloy host atom solvent
• foreign atom solute
• Solute atoms inhibit dislocations motion, higher
  temperature is needed
• Insoluble impurities (oxides and gases) become
  nucleation sites and refine grains
• Smaller initial grain size will recrystallize
  easier at less temperature and time

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Grain Growth

• If you keep the metal hot too long, or heat it up
  too much, the grains become large
• Usually not good
• Low strength

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Size of grains vs. temperature
G R A I N S I Z E

200
600
400
Temperature, deg.C
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Microscope images show  
Cold rolled steel 90 reduction
recrystallized after 2 min.at 830C
Grain growth after 2min _at_ 930C.
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Grain-Growth is not recommended mainly because

• Energy consumption
• Need of expensive equipment
• Large grain metals get surface distortion under
  tensile forces

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Sometimes annealing happens by itself!!

• Is cold working a good way to strengthen a metal
  used at high temperatures?
• What about a tungsten filament in a light bulb?

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Light Bulb
Some materials are annealed in use. As the bulb
is fabricated , the wire in a light bulb filament
is drawn through a die and then bent into shape,
resulting in considerable cold working . The wire
is very strong but quite brittle. It is easily
broken by a sharp impact (you can break the
filament in a new bulb without necessarily
breaking the glass).
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When the wire is first turned on and heated up,
it quickly anneals. The process of recovery and
then recrystallization of the metal produces a
fine-grain structure with equiaxed grains. In
this state, the wire is still reasonably strong
and very much more ductile. It will bend without
breaking and is durable in service.
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• Eventually, continued grain growth at high
  temperature produces a coarse-grained structure
  in which the grains are as large as the wire. The
  wire becomes weak and easily bent. Also, the
  surface of the wire becomes irregular,
  corresponding to the individual faceted grains.
  It typically burns out when one grain boundary
  along the wire becomes smaller than the remainder
  which produces a higher current density at that
  boundary. This raises the temperature so that the
  wire melts. Because the electrical resistance of
  the wire is greatest when it is cold, this
  generally happens at the moment the bulb is
  turned on.

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What should you do if cold working isnt
applicable?

• Try solid solution strengthening (alloying)
• Try hot working

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Hot Working

• Shape the metal while it is hot.
• Blacksmiths use a combination of hot work and
  cold work.
• Can not fine tune the final properties this way
• Dimensional control is hard
• Surface finishes may be hard to produce

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• Hot working is defined as deforming the material
  at a temperature above the recrystallization
  temperature

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