Heat Affected Zone And Weld quality

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Heat Affected Zone And Weld quality
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2D and 3D heat loss in welding
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Heat Affected Zone

• Is within the metal itself
• Has a microstructure different from the base
  metal because of elevated temperatures for a
  lengthy time

Fusion zone (weld metal)
HAZ
Base metal
Molten weld metal
Melt point of base metal
Temp at which base metal microstructure is
affected
Original temp of base metal
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• Heat applied during weld recrystallizes the
  elongated grains of the base metal
• Grains in the HAZ grow due to elevated
  temperatures
• Results in a region that is softer and has less
  strength

• Inter-granular corrosion of a weld in Ferritic
  stainless steel welded tube after exposure to a
  caustic solution
• Weld line is at center of graph

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Effect of welding on workpiece

• Single pass low carbon steel weld (e.g. 0.30)
• The microstructure has six zones
• weld deposit (columnar grains)
• Fusion zone ( course grains)
• Fully austenized zone
• Transformation zone
• Austenite and ferrite zone
• Less than critical temperature zone

Schematic illustration of various regions in a
fusion weld zone and the corresponding phase
diagram for 0.30 Carbon steel
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Properties of HAZ

• Depend on
• Rate of heat input and cooling
• The temperature to which the zone was raised
• Metallurgical properties ( grain size, grain
  orientation etc)
• Specific heat
• Degree of prior cold work

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For metals previously subjected to cold work

• Grain growth may precede recrystallization which
  can result in softening
• Parts previously precipitation hardened also
  soften during fusion welding
• Electron beam welding / laser welding etc can be
  used to minimize HAZ

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Weld characteristics

• Reaction of gas from flame or arc play important
  role in weld characteristics
• Gas entrapment and chemical reaction
• If product is soluble there is no major problem
  except embrittlement
• Insoluble reaction can cause physical
  interference to weld

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• Degasification while cooling can lead to bubbles
  being trapped
• Results in poor quality
• Surface deoxidants in the filler
• Diffusion of fusion weld
• For high temperature cycle diffusion of hydrogen
  into heat affected zone may cause embrittlement

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Hydrogen Stability

• For metals with high solubility for hydrogen
  (e.g. Aluminum) there is a chance of porosity
• Other metals (e.g. Nickel) are not affected most
  often as gases evolve out

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Dilution and uniformity

• When filler metal is added and they differ from
  the parent metal
• Dilution (Weight of parent melt /total
  weight of fused) X 100
• Mixing of metal
• Segmentation can also affect
• Alloy distribution

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Weld quality

• Porosity
• Slag inclusions
• Cracks
• Residual stresses
• Incomplete fusion
• Incomplete penetration

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Porosity

• Caused by trapped gases
• released during melting of weld area
• chemical reactions during welding
• Generally spherical or oblong
• May be concentrated in some region only
• Avoided by
• Proper selection of electrodes and fillers
• Improving welding techniques
• Preheating
• Increase heat input
• Proper cleaning
• Slowing weld speed for gases to escape

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Slag inclusions

• Compounds, fluxes, electrode coating etc.
• If shielding gases are not effective
• Atmospheric contaminant
• In principle molten slag should float to the top
• Remedy
• Clean weld before each layer of weld is applied
• Adequate shielding gas
• Proper selection of welding electrode

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Incomplete fusion and penetration

• Incomplete fusion can be avoided
• using good welding practice (same as for slag
  inclusions)
• Penetration can be avoided by
• Increasing heat input
• Lowering travel speed
• Redesign joint

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Cracks

• Various locations and orientation
• Caused by
• Temperature variation causing thermal stress
• Variation is composition of weld zone causing
  differing contractions
• Embrittlement of grain boundaries by segregation
  of elements
• Hot cracks ( when joint is still at elevated
  temp.) cold cracks (after solidification)

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• Remedies to cracking
• Weld design to minimize stresses during cooling
• Control weld parameters
• Preheat components before welding
• Avoid rapid cooling
• Lamellar tearing
• When you weld pre-rolled plates at certain
  positions
• Change position of weld bead (diagram)
• Surface defects
• Electrode marks, metal sputters, carburized area

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Residual Stresses

• Because of localized heating or cooling of joint
• Result in distortion, warping, buckling
• Stress corrosion cracking
• Reduced fatigue life
• (Figure)
• Can be avoided by preheating
• Reduced with plastic deformation and stress
  relieving.

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Residual stresses
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Contraction

• During cooling contraction takes place this can
  lead to problems such as piping and cracking
• Factors affecting are
• Direction of heat loss
• Type of joint

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Testing Welded Joints

• Destructive
• Tension test
• Bend test
• Tension-Shear test
• Fracture toughness ( Charpy V notch)
• Corrosion and creep test
• Non destructive
• Similar to those used for casting defects

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Testing welded joints

• Destructive testing
• Tension testing specimen removed from actual
  welded area
• Tension shear tests- to find performance under
  working conditions
• Bend test ductility and strength of the weld
  beads
• Fracture toughness test- impacts strength
• Creep and corrosion tests because of the
  composition and micro-structural variation the
  welded region we might get variations or
  preferential corrosion

Bend test
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• Nondestructive testing
• ultrasound
• Magnetic particles
• X ray techniques
• Dye penetrant test
• Important for
• Functional life testing (creep, fatigue) and
  design requirement testing

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Summary

• HAZ property changes relationship to Iron
  Carbon diagram grain structure etc
• Weld characteristics
• Hydrogen Stability
• Weld quality
• Cracks, incomplete fusion, residual stresses
• Destructive and non destructive testing