ME 328'3 E5 Welding Metallurgy

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ME 328.3 E5 - Welding Metallurgy
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Purpose

• To become more familiar with the welding process
  and its effects on the material
• To look at the changes in microstructure and the
  hardness in the Heat Affected Zone (HAZ)
• Welding defects, their cause and preventative
  measures
• Industrial radiography techniques

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Definitions

• Welding is the joining of multiple pieces of
  metal by the use of heat and or pressure. A union
  of the parts is created by fusion or
  recrystallization across the metal interface.
  Welding can involve the use of filler material,
  or it can involve no filler.

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What commercial and technological importance does
welding have?

• Provides a permanent joint
• Weld joint can be stronger than parent material
• If the filler material has superior strength
  characteristics and proper techniques are used
• Usually the most economical way to join
  components
• Can be done in the field away from a factory

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Limitations?

• Expensive in terms of labour cost
• Most welding processes involve the use high
  energy, are inherently dangerous
• Welds are permanent bonds, not allowing for
  convenient disassembly
• The welded joint can suffer from certain quality
  defects that are difficult to detect, these
  defects can reduce the quality of the joint

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Types

• Arc Welding
• A fusion welding process in which the coalescence
  of the metals is achieved by the heat from an
  electric arc between an electrode and the work

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• Shielded Metal Arc Welding (SMAW)
• An arc welding process that uses a consumable
  electrode consisting of a filler metal rod coated
  with chemicals that provide flux and shielding

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• Gas Metal Arc Welding (GMAW)
• Arc welding process in which the electrode is a
  consumable bare metal wire and shielding is
  accomplished by flooding the area with gas

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• Submerged Arc Welding
• Arc welding process that uses a continuous,
  consumable bare wire electrode, arc shielding is
  provided by a cover of granular flux

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• Resistance Welding
• A fusion welding process that utilizes a
  combination of heat and pressure to accomplish
  coalescence, the heat being generated by
  electrical resistance to current flow at the
  junction to be welded

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• Oxyacetylene Welding
• A fusion welding process performed by a
  high-temperature flame from a combustion of
  acetylene and oxygen

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Fusion Weld Joint

• Fusion Zone
• A mixture of filler metal and base metal that has
  completely melted
• High degree of homogeneity among the component
  metals that have been melted during welding
• The mixing of these components is motivated
  largely by convection in the molten weld pool

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• Weld Interface
• The narrow boundary that separates the fusion
  zone and the heat affected zone
• This interface consists of a thin band of base
  metal that was melted or partially melted
  (localized melting within the grains) during the
  welding process, but immediately solidified
  before any mixing could take place
• Heat Affected Zone (HAZ)
• The metal in this region has experienced
  temperature below its melting point, but high
  enough to change the microstructure
• This metal consists of the base metal which has
  undergone a heat treatment due to the welding
  temperatures, so that its properties have been
  altered.
• The amount of metallurgical damage in the HAZ
  depends on the amount of heat input, peak temp
  reached, distance from fusion zone, time at
  elevated temp, cooling rate, and the metals
  thermal properties

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• Heat Affected Zone (HAZ) contd
• The effect on the mechanical properties is
  usually negative, and it is most often the region
  of the weld joint where failure occurs
• Unaffected Base Metal Zone
• Where no metallurgical change has occurred
• The base metal surrounding the HAZ is likely to
  be in a state of high residual stress, due to the
  shrinkage in the fusion zone

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Weld Defects

• Cracks
• Detection
• Surface Visual examination, magnetic particle,
  dye or fluorescent penetrant inspection
• Internal Ultrasonic flaw detection, radiography

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Solidification Cracking

• Causes
• Large depth/width ratio of weld bead
• High arc energy and/or preheat
• Sulphur, phosphorus or niobium pick-up from
  parent metal

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Hydrogen Induced HAZ Cracking

• Causes
• Hardened HAZ coupled with the presence of
  hydrogen diffused from weld metal
• Susceptibility increases with the increasing
  thickness of section especially in steels with
  high carbon equivalent composition
• Can also occur in weld metal
• Increase welding heat beneficial
• Preheating sometimes necessary
• Control of moisture in consumables and
  cleanliness of weld prep desirable

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Lamellar Tearing

• Causes
• Poor ductility in through-thickness direction in
  rolled plate due to non-metallic inclusions
• Occurs mainly in joints having weld metal
  deposited on plate surfaces
• Prior buttering of surface beneficial for
  susceptible plate

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Reheat Cracking

• Occurs in creep resisting and some thick section
  structural low alloy steels during post weld heat
  treatment
• Causes
• Poor creep ductility in HAZ coupled with thermal
  stress
• Accentuated by severe notches such as preexisting
  cracks, or tears at weld toes, or unfused root of
  partial penetration weld
• Heat treatment may need to include low
  temperature soaking
• Grinding or peening weld toes after welding can
  be beneficial

X 35
X 200
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• Cavities
• Detection
• Surface Visual inspection
• Internal Ultrasonic flaw detection, radiography

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Worm Holes

• Resulting from the entrapment of gas between the
  solidifying dendrites of weld metal, often
  showing herringbone array ( B )
• Causes
• The gas may arise from contamination of surfaces
  to be welded, or be prevented from escaping from
  beneath the weld by joint crevices

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Uniformly Distributed Porosity

• Resulting from the entrapment of gas in
  solidified weld metal
• Causes
• Gas may originate from dampness or grease on
  consumables or workpiece, or by nitrogen
  contamination from the atmosphere
• If the weld wire used contains insufficient
  deoxidant it is also possible for carbon monoxide
  to cause porosity

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Restart Porosity

• Causes
• Unstable arc conditions at weld start, where weld
  pool protection may be incomplete and temperature
  gradients have not had time to equilibrate,
  coupled with inadequate manipulative technique to
  allow for this instability

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Surface Porosity

• Causes
• Excessive contamination from grease, dampness, or
  atmosphere entrainment
• Occasionally caused by excessive sulphur in
  consumables or parent metal

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Crater Pipes

• Resulting from shrinkage at the end crater of a
  weld run
• Causes
• Incorrect manipulative technique or current decay
  to allow for crater shrinkage

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Solid Inclusions Detection - normally
revealed by radiography

• Linear Slag Inclusions
• Cause
• Incomplete removal of slag in multi-pass welds
  often associated with the presence of undercut or
  irregular surfaces in underlying passes

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Isolated Slag Inclusions

• Causes
• Normally by the presence of mill scale and/or
  rust on prepared surfaces, or electrodes with
  cracked or damaged coverings
• Can also arise from isolated undercut in
  underlying passes of multi-pass welds

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• Lack of Fusion and Penetration
• Detection
• This type of defect tends to be sub surface and
  is therefore detectable only by ultrasonics or
  X-ray methods
• Lack of side wall fusion which penetrates the
  surface may be detected using magnetic particle,
  dye or fluorescent penetrant inspection
• Cause
• Incorrect weld conditions (eg. low current)
  and/or incorrect weld preparation (eg. root face
  too large)
• Both cause the weld pool to freeze too rapidly

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Lack of side-wall fusion Lack of root
fusion Lack of inter-run fusion
Lack of penetration
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Imperfect Shape Detection - all shape
defects can be determined by visual inspections

• Linear Misalignment
• Cause
• Incorrect assembly or distortion during
  fabrication

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Excessive Reinforcement

• Causes
• Deposition of too much weld metal, often
  associated with in adequate weld preparation
• Incorrect welding parameters
• Too large of an electrode for the joint in
  question

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Overlap

• Causes
• Poor manipulative technique
• Too cold a welding conditions (current and
  voltage too low)

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Undercut

• Results from the washing away of edge preparation
  when molten
• Causes
• Poor welding technique
• Imbalance in welding conditions

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Undercut

• Results from the washing away of edge preparation
  when molten
• Causes
• Poor welding technique
• Imbalance in welding conditions

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Excessive Penetration

• Causes
• Incorrect edge preparation providing insufficient
  support at the weld root
• Incorrect welding conditions (too high of
  current)
• The provision of a backing bar can alleviate this
  problem in difficult circumstances

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Root Concavity

• Causes
• Shrinkage of molten pool at weld root, due to
  incorrect root preparation or too cold of
  conditions
• May also be caused by incorrect welding technique

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Miscellaneous Faults

• Arc Strikes
• Cause
• Accidental contact of an electrode or welding
  torch with a plate surface remote from the weld
• Usually result in small hard spots just beneath
  the surface which may contain cracks, and are
  thus to be avoided

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Spatter

• Causes
• Incorrect welding conditions and/or contaminated
  consumables or preparations, giving rise to
  explosions within the arc and weld pool
• Globules of molten metal are thrown out, and
  adhere to the parent metal remote from the weld

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Copper Pick-Up

• Causes
• Melting of copper contact tube in MIG welding due
  to incorrect welding conditions

X 275
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PROCEDURE

• Students are provided with weldments of
  approximately 0.4 C steel. The first weldment
  was prepared without preheat treatment. The
  electrode used produces a large amount of
  hydrogen which diffuses into the weld metal. The
  second was preheated to 150C. An electrode with
  relatively low hydrogen content was used. For
  each of these samples
• Examine the microstructure of the weldments in a
  traverse from weld metal to parent metal,
  sketching about five different areas. Using the
  Fe-C diagram and your knowledge of the phase
  transformations in steel, comment on the
  microstructures describing the time-temperature
  history and how this history resulted in the
  observed structure.
• Conduct a microhardness traverse across the HAZ
  and correlate the hardness with the
  microstructure observed in (a).
• 2. Some radiographs of weld defects are provided.
  Examine these radiographs and describe the
  defects responsible, citing ways of avoiding the
  problem.

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Radiographs