Metallography of Rolled Metals

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Metallography of Deformed, Welded and Surface
Hardened Structures MSE 206-Materials
Characterization I Lecture-8
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Metal Fabrication Methods
FORMING
Forging (wrenches, crankshafts)
Rolling (I-beams, rails)
often at elev. T
Extrusion (rods, tubing)
Drawing (rods, wire, tubing)
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Metal Fabrication Methods Forming Methods
1-Forging
A bulk deformation process in which the workpiece
(preform) is squeezed between two opposing dies,
thus the shape of the dies is imparted on the
work part. It can be done cold or hot.
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Metal Fabrication Methods Forming Methods
2-Rolling
A bulk deformation process in which the thickness
of the work part is reduced by compressive forces
exerted by two opposing rolls.
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Metal Fabrication Methods Forming Methods
3-Extrusion

• Extrusion is a bulk deformation process in which
  material is forced to flow through a
  shape-forming die.
• In extrusion, work part is pushed to flow through
  die opening rather than pulling.
• Extrusion can be cold or hot.
• Cross-section of the part must be uniform along
  its extruded length.

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Metal Fabrication Methods Casting
CASTING
Die Casting (high volume, low T alloys)
Sand Casting (large parts, e.g.,auto
engine blocks)
Continuous Casting (simple slab shapes)
Investment Casting (low volume, complex
shapes e.g., jewelry, turbine blades)
plaster die formed around wax prototype
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Metal Fabrication Methods Casting
Process in which molten metal flows by gravity
or other force into a mold where it solidifies
in the shape of the mold cavity. Term casting
also denotes the part made in the process. 1.
Melt the metal 2. Pour the metal in cavity 3.
Let the metal freeze
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Metal Fabrication Methods Joining
JOINING
Welding (when one large part is
impractical)
Heat affected zone (region in which the
microstructure has been changed).
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Metal Fabrication Methods Forming
Temperature of Forming

• When the deformation is achieved at a temperature
  above the recrystallization temperature (?0.5Tm)
• the process is called hot working
• otherwise, it is cold working
• For most forming techniques, both hot and cold
  working procedures are possible.

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Metal Fabrication Methods Cold Working

• Strength is increased considerably due to strain
  hardening.
• Ductility is decreased.
• Quality of the surface is higher when compared to
  hot worked metals.
• Better control of the dimensions of the finished
  piece is possible.

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Effect of Cold Rolling On Grain Shape
Metal Fabrication Methods Cold Working

• As a result of cold working, the equiaxed grains
  of the metal are deformed into elongated grains
• The elongation is along the direction of applied
  stress

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Effect of Reduction by Cold Rolling on
hypoeutectoid Steel
Metal Fabrication Methods Cold Working
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Annealing after cold working

• Annealing is a heat treatment to negate the
  effects of cold work and increase the ductility
  of a strain hardened metal.
• Strain hardening is a result of increased
  dislocation interactions due to increased
  dislocation density.
• Heating of a strain hardened metal for annealing
  results in
• 1-Recovery (dislocation and vacancy annihilation)
• 2-Recrystallization (nucleation and growth of
  strain and dislocation free grains)

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Annealing after cold working
Cold rolled and annealed AISI 1010 steel
(a) (b) (c)

1. Cold rolled, 90 deformation. Grains elongated
   along the rolling axis can not be differentiated
   easily in the optical microscope
2. Annealed at 650ºC for 5 min, recrystallized
   10-40. New dislocation free ferrite grains are
   nucleating and growing.
3. Further annealed, recrystallized 80.
   Recrystallized ferrite grains can be seen clearly.

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Annealed Structures of Non-ferrous Alloys
a) Fully annealed homogeneous hexagonal equiaxed
grains. b) Cold worked metal with flattened
grains. c) Annealed after cold working showing
twinned grains. d) Cold worked again, after
annealing, showing distorted twin lines and
strain lines in the grains.
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Effect of Annealing Temperature
Annealed Structures of Non-ferrous Alloys
a-brass (70Cu-30Zn) -- Cold worked and annealed at
(a) (b)
(c)
450 ºC 550 ºC
750 ºC
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Annealed Structures of Non-ferrous Alloys
Annealing twins in a-brass
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Metal Fabrication Methods Hot Working

• In Hot Working, which is carried out at
  temperatures over the recrystallization
  temperature, Large deformations and successive
  operations are possible because the dislocations
  (hardening) generated by deformation are
  simultaneously annihilated by dynamical recovery
  and recrystallization.

• Deformation energy requirements are less compared
  to those for cold working.
• Surface oxidation may cause loss of material and
  poor surface finish.

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Banding in Hot Worked Alloys
Metal Fabrication Methods Hot Working

• During solidification of steels elemental
  segregation of C, S, P, Mn may occur.
• When such steels are hot-rolled, elongated bands
  of different carbon and mangenese content occurs.
• Bands of ferrite and pearlite forms along the
  direction of rolling.

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Banded Structures in Steel
Metal Fabrication Methods Hot Working
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Surface Hardening
Surface Hardening Used to obtain hard wear
resistant surface without effecting the
relatively soft, tough interior.
Case Hard wear resistant surface Core
Relatively soft, tough inside

• Surface hardening is usefull in parts such as
  cam or ring gear that must have a very hard
  surface to resist wear, along with a tough
  interior to resist the impact.

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Surface Hardening
Surface Hardening of steels
Layer addition Substrate treatment
Hardfacing - Fusion hardfacing - Thermal spray Coatings - Electrochemical plating - Chemical vapor deposition (electroless plating) - Thin films (physical vapor deposition, sputtering, ion plating) - Ion mixing Diffusion Methods - Carburizing - Nitriding - Carbonitriding and cyaniding - Nitrocarburizing - Boriding - Titanium-carbon diffusion - Toyota diffusion process Selective hardening methods - Flame hardening - Induction hardening - Laser hardening - Electron beam hardening - Ion implantation - Selective carburizing and nitriding - Use of arc lamps
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Surface Hardening

• Surface is hardened to
• increase wear resistance
• increase surface strength for load carrying
  capacity (crush resistance)
• impart favorable residual compressive stresses
• improve fatigue resistance
• produce tough core for resistance to impact

Some of the surface hardening methods
Carburizing Nitriding Cyaniding and
carbonitriding Flame hardening Induction
hardening
Diffusion methods Change the chemical composition
Selective hardening methods dont change the
chemical composition. The steel must be capable
of being hardened.
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Carburizing
Surface Hardening

• In the austenic temperatures, carbon is
  introduced to the surface of a low carbon steel
  component by diffusion to increase the surface
  carbon content and produce a hard martensitic
  surface layer after quenching.
• Traditionally, the carbon is supplied from
• (a) hydrocarbon gas atmosphere
• (b) coke packed around
• (c) carburizing salt bath.

Case depth K vDt
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Surface Hardening
Gas Carburizing

• The steel is heated in contact with CO and/or
  hydrocarbon which is readily decomposed at the
  carburizing temperature.

• Methane
• Propane
• Natural Gas
• Vaporized fluid hydrocarbon

Hydrocarbon gas
The commercial practice is to use a carrier gas,
such as obtained from an endothermic generator
and enrich it with one of the hydrocarbon gas.

• High surcafe carbon concentration may be
  decreased by turning of the gas and keeping the
  samples in furnace for a period of time.

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Nitriding
Surface Hardening

• Nitrogen is introduced to the surface of a
  special alloy steel (with Al and Cr) component by
  diffusion to form alloy nitride precipitates to
  produce a hard surface layer. It is done at
  around 525C and since hardening is by
  precipitates rather than martensite quenching is
  not necessary.
• Traditionally, nitrogen is supplied from amonia
  gas atmosphere or cyanide bath.

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Flame Hardening
Surface Hardening

• In Flame hardening a thin surface shell of medium
  carbon steel or cast iron is heated rapidly to
  austenite by an intense high temperature flame of
  propane and oxygen gas mixture, then quickly
  quenched to obtain martensite of high hardness.

• In flame hardening chemical composition of the
  steel doesnt change

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Induction Hardening
Surface Hardening

• Surface of the part is heated very fast by
  induction heating and then quenched. Part is put
  in a work coil composed of several turns of water
  cooled copper. By passing current through this
  coil, an eddy current is induced on the surface
  of the part, which heats the surface. Then the
  part with the austenitized surface is quenched by
  using water jets.

• As in flame hardening, chemical composition of
  the steel doesnt change in induction hardening

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• Instead of carburizing, induction hardening etc.
  sometimes metals are coated with hard ceramics
  such as TiN, TiAlN, CrN to increase the wear
  resistance of the part

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Joining Welding

• A technique for joining metals in which actual
  melting of the pieces to be joined occurs in the
  vicinity of the bond. A filler material may be
  used to facilitate the process.
• Welding is very similar to casting since it also
  consists of melting and solidification of the
  metal

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Joining Welding

• Weld Root Dendritic microstructures may be
  observed.
• HAZ (Heat Affected Zone) Change in grain size
  may be observed
• Parent Material Welding has no effect on the
  microstructure of this region

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Joining Welding
HAZ
After welding
Before welding
Hypoeutectoid steel
(a) Before welding, parent metal consists of a
pearlitic and ferritic matrix. (b) In the regions
exposed to high temperatures (HAZ), pearlite
loses its lamellar morphology, the layers break
up leading to a structure known as divorced
pearlite.
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Joining Welding
HAZ

• Pointed edged widmanstatten ferrites form on
  grain boundaries and grow into the grains. Black
  regions are unresolved pearlitic areas.

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Joining Welding
Fusion Zone

• The upper left part is the parent metal, lower
  right part belongs to the filler material.

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Joining Welding
Weld zone

• Filler materials generally contain very low
  amounts of carbon, therefore only ferrite forms
  upon solidification. The structure is directly
  solidified from the liquid phase, resulting in a
  dendritic morphology.

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Welding Discontinuities
Joining Welding

• Cold Cracking
• Hot Cracking
• Porosity in Weld regions
• Slag and Inclusions