Transcript Slide 1

WELD DEFECTS ( WELD DISCONTINUITIES ),
DISTORTION
AND
ITS CONTROLS
Ramanatha C.
Scientist ‘F’ (Retd.)
Gas Turbine Research Establishment
Bangalore
Types of Welding
Fusion Welding
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Manual Metal Arc Welding
Shielded Metal Arc welding
Submerged Arc Welding
Gas Tungsten Arc Welding (GTAW) or (TIG)
Gas Metal Arc Welding (GMAW) or (MIG)
Flux Cored Arc Welding
Resistance Welding- Spot Welding and Seam Welding
Electrogas Welding (EGW)
Electron Beam Welding (EBW)
Laser Beam Welding
High Frequency Welding
Non – Fusion or Solid State Welding
 Diffusion Welding
 Ultrasonic Welding
 Friction Welding
Definition
The word “Defect “ should be used carefully, as it implies that
a weld is defective and requires corrective measures or rejection, In
some cases repairs may be made unnecessarily and solely by
implication without a critical engineering assessment. Consequently
the engineering community has recently begun to use the word
“Discontinuity” or “Flaw” instead of “Defect”, Discontinuity may be
defined as interruptions in the desirable physical structure of a
weld.
The significance of a weld discontinuity should be viewed in
the context of the fitness-for-purpose of the welded construction.
Fitness-for-purpose is a concept of weld evaluation that seeks a
balance between quality, reliability and economy of welding
procedure. It is not a constant varies depending in the service
requirements of a particular welded structure as well as on the
properties of the material involved.
Classification of the Causes of Discontinuities
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Design
related
• Structural
Details
• Choice of the
wrong type of
weld joint For a
given
application
• Undesirable
changes in cross
section
• Metallurgical
• Welding process
Related
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Undercut
Slag inclusions
Porosity
Tungsten inclusion
Backing piece left
on
Shrinkage voids
Oxide inclusions
Lack of fusion
(LOF)
Lack of
penetration (LOP)
Craters
Melt – through
Spatter
Arc Strikes (Arc
burns)
Underfill
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Cracks
Fissures
Fisheye
Segregation
Lamellar
Tearing
Design Related Discontinuity
Misalignment (hi-lo)
Amount of a joint is out of alignment at the root.
Causes:
1. Carelessness.
2. Due to joining of different thicknesses
(Transition thickness)
Prevention:
1. Workmanship
2. Transition angles not to exceed 2.5 to 1
Welding Process Related Discontinuities
Discontinuities resulting from welding process include:
 Undercut: A groove melted into the base metal adjacent to the toe
or root of a weld and left unfilled by weld metal.
 Slag inclusion: Non metallic solid material entrapped in weld or
between weld metal and base metal.
 Porosity: Cavity-type discontinuities formed by gas entrapment
during solidification.
 Tungsten inclusions: Particles from Tungsten electrodes. Which
results from improper gas Tungsten arc welding procedures.
 Backing piece left on: Failure to remove material placed at the root
of a weld joint to support molten weld metal.
 Shrinkage voids: Cavity type discontinuities normally formed by
shrinkage during solidification.
 Oxide inclusions: Particles of surface oxides which have not melted
and are mixed into the weld metal.
Welding Process Related Discontinuities
Contd….
 Lack of Fusion: A condition in which fusion is less than complete.
 Lack of Penetration: A condition in which joint penetration is less than
that specified.
 Craters: Depression at the termination of a weld head or in the
molten weld pool
 Melt-through: A condition resulting when the arc melts through the
bottom of a joint welded from one side.
 Spatter: Metal particles expelled during welding which do not form a
part of the weld.
 Arc strikes (arc burns): Discontinuities consisting of any localized
remelted metal, or change in the surface profile of any part of a weld
or base metal resulting from an arc.
 Underfill: A depression on the face of the weld or root surface of the
adjacent base metal.
INCOMPLETE PENETRATION
This type of defect is found in any of three ways:
1) When the weld bead does not penetrate the entire thickness of the base plate.
2) When two opposing weld beads do not interpenetrate.
3) When the weld bead does not penetrate the toe of a fillet weld but only bridges
across it.
Welding current has the greatest effect on penetration. Incomplete penetration is
usually caused by the use of too low a welding current and can be eliminated by simply
increasing the amperage. Other causes can be the use of too slow a travel speed and an
incorrect torch angle. Both will allow the molten weld metal to roll in front of the arc,
acting as a cushion to prevent penetration. The arc must be kept on the leading edge of
the weld puddle.
LACK OF FUSION
Lack of fusion, also called cold lapping or cold shuts, occurs when there is
no fusion between the weld metal and the surfaces of the base plate.
Causes:
1. Poor welding technique.
2. Use of a very wide weld joint.
3. Very low travel speed and attempting to make too large a weld in
a single pass.
4. Low welding voltage.
5. Presence of Oxide layer.
INCOMPLETE PENETRATION
&
EXCESSIVE PENETRATION
Lack of Fusion
UNDERCUTTING
Undercutting is a defect that appears as a groove in the parent
metal directly along the edges of the weld. It is most common in lap
fillet welds, but can also be encountered in fillet and butt joints.
Causes:
1. Improper welding parameters; particularly the travel speed
and arc voltage.
2. Excessive welding currents.
Undercutting
Porosity
Porosity is gas pores found in the solidified weld bead. These pores may vary in size and
are generally distributed in a random manner. However, it is possible that porosity can
only be found at the weld centre. Pores can occur either under or on the weld surface.
The most common causes of porosity are atmosphere contamination, excessively
oxidized work piece surfaces, inadequate deoxidizing alloys in the wire and the presence
of foreign matter.
Atmospheric contamination can be caused by:
1) Inadequate shielding gas flow.
2) Excessive shielding gas flow. This can cause aspiration of air into the gas stream.
3) Severely clogged gas nozzle or damaged gas supply system (leaking hoses, fittings,
etc.)
4) An excessive wind in the welding area. This can blow away the gas shield.
Porosity
Weld Defects, Their Possible causes and corrective action
POROSITY
Possible Causes
A. Inadequate shielding gas coverage
Corrective Actions
 Increase the shielding gas flow to
displace all air from the weld area
 Decrease the shielding gas flow to
avoid turbulence and entrapment of
air in the gas
 Remove Spatter from the interior of
the gas nozzle.
 Eliminate drafts (from fans, open
doors, etc.) blowing into the welding
arc.
 Use a slower travel speed.
 Reduce the nozzle-to-work distance.
 Hold the gun at the end of the weld
until the molten crater solidifiers
Weld Defects, Their Possible causes and corrective action
POROSITY ( Contd..)
Possible Causes
Corrective actions
B. Electrode contamination
 Use only clean and dry electrode
wire
 Eliminate pickup of lubricant on
electrode in the wire feeder or
conduit
D. Arc Voltage too high
 Remove all grease, oil, rust, paint
and dirt from work surfaces
before welding.
 Use a more highly deoxidizing
electrode.
 Reduce operating Voltage
E. Excess nozzle -to- work distance
 Reduce electrode extension.
C. Workpiece contamination
Weld Defects, Their Possible causes and corrective action
Lack of penetration
Possible Causes
Corrective actions
A.
Improper joint preparation
b. Improper welding Technique
c.
Inadequate heat input
 Joint design must be adequate to
provide access to the bottom of
the grove while maintaining
proper nozzle-to-work distance
and arc characteristics
 Reduce root face height
 Provide or increase the root
opening in butt joints
 Position the electrode at the
proper travel angle to achieve
maximum penetration
 Keep the arc on the leading edge
of the weld pool
 Increase electrode feed to obtain
higher welding currents
 Maintain proper nozzle-to-work
distance
Weld Defects, Their Possible causes and corrective action
Excessive melt through
Possible Causes
Corrective actions
A.
Excessive heat input
A.
Improper joint preparations
 Reduce the electrode feed rate and
arc voltage
 Increase the travel speed
 Reduce excessive root opening
 Increase root face height
Overlap
Under Fill
SPATTER
EXCESSIVE CONVEXITY
EXCESSIVE CONCAVITY
EXCESSIVE WELD REINFORCEMENT
UNACCEPTABLE
WELD PROFILES
Metallurgical Discontinuities
 Cracks: Fracture type discontinuities characterized by a sharp
tip and high ratio of length and width to opening displacement.
 Fissures: Small crack-like discontinuities with only a slight
separation (opening displacement) of the fracture surfaces.
 Fisheye: A discontinuity found on the fracture surface of a weld
in steel that consists of a small pore or inclusion surrounded by
a bright, round area.
 Segregation: A non-uniform distribution or concentration of
impurities or alloying elements which arises during the
solidification of the weld.
 Lamellar tearing: A type of cracking that occurs in the base
metal or heat affected zone (HAZ) of restrained weld joints that
is the result of inadequate ductility in through-thickness
direction of steel plate.
Cracking
This can occur due just to thermal shrinkage or due to a combination of
strain accompanying phase change and thermal shrinkage.
In the case of welded stiff frames, a combination of poor design and
inappropriate procedure may result in high residual stresses and cracking.
Where alloy steels or steels with a carbon content greater than about 0.2%
are being welded, self cooling may be rapid enough to cause some (brittle)
martensite to form. This will easily develop cracks.
To prevent these problems a process of pre-heating in stages may be needed
and after welding a slow controlled post cooling in stages will be required.
This can greatly increase the cost of welded joins, but for high strength
steels, such as those used in petrochemical plant and piping, there may well
be no alternative.
Factors Promoting Hot Cracking
 Welding Current Density (High Levels promote cracking)
 Heat Distribution (Joint design)
 Restraint
 Crack sensitivity of electrode material
 Dilution of weld metal
 Impurities (Eg. Sulphur & Phosphorous)
 Preheating (Increase liability to cracking)
 Welding Procedures (High Speeds, long arcs increase sensitivity)
Types of Cracking
Solidification Cracking
This is also called centreline or hot cracking. They are called hot cracks
because they occur immediately after welds are completed and sometimes
while the welds are being made. These defects, which are often caused by
sulphur and phosphorus, are more likely to occur in higher carbon steels.
A schematic diagram of a centreline crack is shown below:
Types of cracking (Contd..)
Hydrogen induced cracking (HIC)
It is also referred to as hydrogen cracking or hydrogen assisted cracking, can
occur in steels during manufacture, during fabrication or during service.
When HIC occurs as a result of welding, the cracks are in the heat affected
zone (HAZ) or in the weld metal itself.
Four requirements for HIC to occur are:
a) Hydrogen be present, this may come from moisture in any flux or from
other sources. It is absorbed by the weld pool and diffuses into the HAZ.
b) A HAZ microstructure susceptible to hydrogen cracking.
c) Tensile stresses act on the weld
d) The assembly has cooled to close to ambient - less than 150oC
HIC in the HAZ is often at the weld toe, but can be under the weld bead or at
the weld root. In fillet welds cracks are normally parallel to the weld run but
in butt welds cracks can be transverse to the welding direction.
Reheat Cracking
 Mo-V and Mo-B steels susceptible
 Due to high temperature embrittlement of the heat-affected zone and
the presence of residual stress
 Coarse-grained region near fusion line most susceptible
 Prevention by
 Low heat input welding
 Intermediate stress relief of partially completed welds
 Design to avoid high restraint
 Restrict vanadium additions to 0.1% in steels
 Dress the weld toe region to remove possible areas of stress
concentration
Lamellar Tearing
 Occurs in thick plate subjected to high transverse welding stress
 Related to elongated non-metallic inclusions, sulfides and silicates,
lying parallel to plate surface and producing regions of reduced
ductility
 Prevention by
 Low sulfur steel
 Specify minimum ductility levels in transverse direction
 Avoid designs with heavy through-thickness direction stress
Residual Stress & Distortion
Residual Stresses also referred to as internal stresses , initial stresses ,
inherent stresses reaction stresses and locked in stresses, arc stresses that
continue to exist in a body even after the removal of all the external loads.
Thermal stresses occur in parts during welding due to the localized
application of heat. Residual stresses and distortion remain after the welding
process is completed. Thermal stresses, residual stresses and distortion
sometimes cause cracking and mismatching of joints. Correcting
unacceptable distortion is costly and in some cases impossible.
Distortion in a weld results from the expansion and contraction of the weld
metal and adjacent base metal during the heating and cooling cycle of the
welding process.
Causes of residual Stresses
Residual stresses in metal structures occur for many reasons during various manufacturing stages.
They may occur during forming and shaping of metal parts by processes such as shearing, bending,
machining and grinding. They also occur during fabrication processes such as welding.
Residual stresses are classified into two types:
1. Residual stresses produced by mismatch.
When bars of different lengths are forcibly connected , tensile stresses are produced in the shorter
bar and compressive stresses are produced in the longer bars.
Before welding;
Free state
After welding;
Stressed state
2. Residual stresses produced by uneven distribution of non elastic strains.
When materials are heated uniformly, thermal stress is not produced, as they also expand uniformly.
However, residual stress is expected when materials are not heated uniformly. Also, residual
stresses are produced when unevenly distributed non elastic strains, or plastic strains exist.
Residual Stresses developed during welding
Fig : Residual stresses developed during welding of a butt joint.
Distortion after Welding
Fig : Distortion of parts after welding : (a) butt joints; (b) fillet welds. Distortion is caused by differential
thermal expansion and contraction of different parts of the welded assembly.
Prevention or Minimization of distortion
Several ways can be used to minimize distortion caused by shrinkage:
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Do not overweld
Control fitup.
Use intermittent Welding
Use as few weld passes as possible
Place welds near the neutral axis
Balance welds around the neutral axis
Use backstep welding
Anticipate the shrinkage forces
Plan the welding sequence
Remove shrinkage forces after welding
Minimize welding time
Use the smallest leg size permissible when fillet welding.
Prevention or Minimization of distortion
(Contd..)
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For groove welds, use joints that will minimize the volume of weld metal. Consider
double-sided joints instead of single-sided joints.
Weld alternately on either side of the joint when possible with multiple-pass welds.
Use low heat input procedures. This generally means high deposition rates and
higher travel speeds.
Use welding positioners to achieve the maximum amount of flat-position welding.
The flat position permits the use of large- diameter electrodes and highdeposition-rate welding procedures.
Weld toward the unrestrained part of the member.
Use clamps, fixtures, and strongbacks to maintain fitup and alignment.
Prebend the members or preset the joints to let shrinkage pull them back into
alignment.
Sequence subassemblies and final assemblies so that the welds being made
continually balance each other around the neutral axis of the section.
Prevention or Minimization of distortion
(Contd..)
Prevention or Minimization of distortion
(Contd..)
Other Techniques for Distortion Control
Water-Cooled Jig
Various techniques have been developed to control distortion on specific
weldments. In sheet-metal welding, for example, a water-cooled jig is
useful to carry heat away from the welded components. Copper tubes are
brazed or soldered to copper holding clamps, and the water is circulated
through the tubes during welding. The restraint of the clamps also helps
minimize distortion.
Other Techniques for Distortion Control
(Contd..)
Strongback
The "strongback" is another useful technique for distortion control during butt welding of
plates. Clips are welded to the edge of one plate and wedges are driven under the clips to
force the edges into alignment and to hold them during welding.
Other Techniques for Distortion Control (Contd..)
Thermal Stress Relieving (Heat Treatment)
Except in special situations, stress relief by heating is not used for
correcting distortion. There are occasions, however, when stress relief is
necessary to prevent further distortion from occurring before the weldment is
finished.
Preheat
Preheat reduces the temperature differential between the weld region and
the base metal
 Reduces the cooling rate, which reduces the chance of forming
martensite in steels
 Reduces distortion and shrinkage stress
 Reduces the danger of weld cracking
 Allows hydrogen to escape
Interaction of Preheat and Composition
CE = %C + %Mn/6 + %(Cr+Mo+V)/5 + %(Si+Ni+Cu)/15
 Carbon equivalent (CE) measures ability to form
martensite, which is necessary for hydrogen
cracking
– CE < 0.35
treatment
– 0.35 < CE < 0.55
– 0.55 < CE
treatment
no preheat or postweld heat
preheat
preheat and postweld heat
 Preheat temp. as CE and plate thickness
Other Techniques for Distortion Control (Contd..)
Post-Weld Heat Treatment
 The fast cooling rates associated with welding often produce martensite
 During postweld heat treatment, martensite is tempered (transforms to
ferrite and carbides)
– Reduces hardness
– Reduces strength
– Increases ductility
– Increases toughness
 Residual stress is also reduced by the postweld heat treatment
Postweld Heat Treatment and Hydrogen Cracking
 Postweld heat treatment (~ 1200°F) tempers any martensite that may have
formed
– Increase in ductility and toughness
– Reduction in strength and hardness
 Residual stress is decreased by postweld heat treatment
 Rule of thumb: hold at temperature for 1 hour per inch of plate thickness;
minimum hold of 30 minutes
Thank You