Welding Inspection
Presenter
Steve Cave
Senior P & EW Instructor
WELDING PRESENTATION
By
Steve Cave
The Certified Welding Inspector (CWI) plays an important role
during any welded construction activities ensuring the required
specifications and standards are followed. Due to the numerous
materials and processes associated with metal joining (welding)
THIS PRESENTATION SHALL SHOW ONLY THE BASIC
WELDING PROCESSES AND EXAMINATION METHODS (NDE).
National and International Codes and Specifications along with
measuring devices are the Inspector’s tools. Hopefully the following
presentation shall give an insight into basic welding inspection.
How To Prevent Weld
Failure
Weld Failures
Today welding is the most common method used for joining
steel fabrications largely because of the speed at which joints
can be made and the reliability of these joints in service.
However because most welding operations are now relatively
simple to perform it is all too easy to forget the complexity of
the chemical and metallurgical actions that are taking place
when the weld is being deposited. Therefore not surprisingly
welds occasionally fail.
Overload
Before applying the various design formulas, the problem itself
must be analyzed and clearly stated. This is not always
obvious, and trying to solve the wrong problem can quickly
lead to insufficient design stresses. When a load is placed on a
member, stress and strain result. Stress is the internal
resistance to the applied force. Strain is the amount of "give or
deformation caused by the stress, such as deflection in
bending, elongation in tension, contraction in compression,
and angular twist in torsion.
Overload
For example of this is a lifting lug on a pressure vessel. If the
vessel is lifted by a spreader beam the loading condition on
the lug consists of a simple vertical force putting the
attachment welds either in tension or shear. However if the
vessel is lifted with a rope sling the loading condition
becomes more complex because there is now a horizontal
component of the force to consider as well a the vertical one,
which effectively increases the loading on the welds.
Joint Design
A welded joint should be designed such that the welder can
easily manipulate the electrode to ensure good fusion,
particularly in the root of the joint. The profile of each run
should be roughly as wide as it is deep; wide shallow weld
beads and particularly deep narrow beads are both ideal
candidates for hot cracking.
Hot Cracking
This type of cracking occurs when the weld is starting to
solidify, in the pasty state, as it posses very little strength and
therefore any residual loading is likely to cause it to break
before it has fully solidified. The problem can be
compounded by impurities that are forced out of the
solidifying weld, becoming trapped in the center of the weld
during final solidification. Hot cracking can occur where their
is a high degree of restraint in the structure of the fabrication
or where the structure moves slightly as the weld solidifies.
Hot Cracking
A good example of this type of failure is on the weld used to
secure the small plug in the mandrill hole of a spun dished
head on a pressure vessel, a weld that many people do not
take seriously because of its size. As the weld cools it
contracts causing the plug to move , if the weld at the other
side of the plug is still solidifying it could easily fail. This is
because of the very high contraction stresses generated by
the plug as the weld starts to solidify.
Bad Welding Methods
It is very important when carrying out any welding to ensure
that it is done correctly. Consideration has to be given to all
aspects of the process and also the environment. Often
welding has to be carried out under site conditions, the
welding is often carried out in situation so that small general
purpose electrodes are used resulting in low weld heat input
which when combined with no preheat gives very rapid heat
dissipation Which can create a hard micro structure
particularly in the location of the heat affected zone.
Bad Welding Methods
This along with high levels of residual stress will create the
ideal condition for hydrogen induced cracking, which although
normally associated with high strength steels can occur in low
carbon steels if the conditions are right. The resulting crack
may not occur immediately the weld cools down but some time
afterward, therefore if this type of failure is expected non
destructive examination should be delayed by at least 48
hours after welding.
Metallurgical failure
Materials that are to be welded have to tolerate severe
thermal transients created by the welding process without
suffering deterioration of their mechanical properties or
adverse phase changes. The metallurgical composition or
temper conditions of certain types of metal may make them
unsuitable to weld or may require special controls to be
imposed during the welding operation. For example some
steels that are easy to machine may contain high levels of
sulphur that may result in cracking of any attaching weld.
Therefore this type of material should not be used on load
bearing fabricated items such as the eye bolts that are often
found holding down man way covers on pressure vessels.
Weld Defects
They can usually be attributed to the welders inability to set up
and manipulate the welding equipment; although bad joint
design and faulty welding equipment can also be responsible.
The most significant defects are cracks and those that
resemble cracks such as lack of fusion, cold overlap etc. This
is because of the risk that the crack may become unstable and
propagate when loaded causing a dramatic failure often by
brittle fracture
Weld Defects
Porosity seldom causes weld failure in multi-run welds
however it is a sign that something has gone wrong with
welding operation and can often be caused by other defects
that may not have been detected such as lack of side wall
fusion. Weld profile can also cause failure, if the weld size is
too small because the joint is underfilled with weld then its
load carrying capability will be reduced, if the joint contains
excessive weld metal this can create a notch effect which
can lead to failure by fatigue if the loading condition
fluctuates.
Weld Defects
Bad fit up excessive root penetration on single sided welds
can create defects in the root of the weld such as wormholes
and even cracking. Distortion of welded joints can cause
failure by buckling if the welded member is subjected to
compressive loads.
Conclusion
To minimize these problems the following points should be
considered
1
. Design of the weld based on the loading condition(s) the joint will carry
2
. Accessibility to enable ease of welding
3
. Control of distortion
4
. Careful consideration of the welding environment
5
. Matching welding process with materials
6 A factor of safety applied to the design stress of the weld which should be based on
. the consequence of weld failure and the level of non destructive testing that is to be carried
out.
Welding Inspection
Duties of a Welding Inspector
Duties Prior to Welding
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Obtain all relevant documentation…
Relevant specifications.
Relevant procedures.
Copies of welders test test certificates.
Copies of drawings.
Duties Prior to Welding
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Obtain all relevant documentation…
Ensure welder qualification.
Correct material type condition etc.
Correct equipment with certificates.
Correct consumables type condition,
size.
Correct pre heat.
Duties Prior to Welding
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Assess / measure fit up…
Root face.
Bevel angle.
Root gap.
Alignment.
Joint cleanliness.
Ensure no undue stress is applied to the
joint.
Duties During welding
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Check amperage, voltage, polarity.
Ensure correct welding technique.
Ensure correct welding direction.
Check welding time.
Ensure adequate cleaning between
passes.
Correct interpass temperature.
Duties During welding
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Check root internally.
Check all back gouged welds.
Duties After welding
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Ensure welds are post cleaned.
Visual inspection of welds for defects.
Visually check for arc strikes.
Check weld contour and weld width.
Ensure joint is covered to retard cooling
rate.
Ensure monitor post weld heat
treatment.
Duties After welding
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Report on weld.
Check NDT reports where needed.
Welding Processes
Shielded Metal Arc Welding
Gas Metal Arc Welding
Tungsten Arc Welding
Submerged Arc Welding
Problems Associated With
Incorrect Weld Joint Fit Up
Root Problems
Gap size too large:
a) excess penetration
c) shrinkage grooves
b) burnthrough
d) gas entrapment
Root Problems
Gap size too small:
1) incomplete penetration
2) incomplete root fusion
4) slag inclusions
3) incomplete side wall fusion
5) root concavity
Root Problems
Root face too large:
a) incomplete root penetration
b) incomplete root fusion
Root Problems
Root face too small:
1) excessive penetration
3) root concavity
2) burnthrough
4) root undercut
Included Angle Too large
a) excess penetration
b) incomplete filled groove
Fillet Welds
Gap size too large:
1) reduced root penetration
2) slag inclusions
3) gas inclusions
4) reduced vertical
leg length size
5) cracking
Included Angle Too Small
1) incomplete root penetration
3) incomplete root fusion
5) excessive cap
2) incomplete interun fusion
4) incomplete sidewall fusion
6) poor toe blend
7) slag inclusions
Visual Inspection of Welds
Butt Weld Size
a) excess weld metal height
b) root penetration
c) weld width
d) root bead width
Fillet welds (size)
Consider:
a) z minimum (and maximum) leg length
size
b) a minimum design throat
thickness
Shape (Butt Welds)
Consider:
Ideally, (a) is the most desirable but
very often it may be difficult to
achieve. Because of this, one should
assess the excess weld height in
conjunction with the weld profile and
perhaps the toe blending.
Shape (Fillet Welds)
Consider:
In normal practice, (a) is the most desirable
but, again, in many instances it is difficult to
achieve. Acceptance levels, therefore, allow
tolerances on weld shape.
Toe Blend (Butt Welds)
For butt welds, consider:
In normal practice, (a) is the most
desirable but, again, in many
instances it is difficult to achieve.
Acceptance levels, therefore, allow
tolerances on weld shape.
Depending on the service conditions
of the product, the toe blend may be
of greater importance than the size
and shape of the weld. A poor toe
blend may reduce service life by a
considerable margin if the product is
under a cyclic load.
Toe Blend (Fillet Welds)
For fillet welds, consider:
In normal practice, (a) is the most
desirable but, again, in many
instances it is difficult to achieve.
Acceptance levels, therefore,
allow tolerances on weld shape.
Root Defects
Incomplete root penetration
Failure of weld metal to extend into the root of a joint
Lack of root fusion
Lack of union at the root of a joint
Excess penetration bead
Excess weld metal protruding through the root of
a fusion weld made from one side only
Root Defects
Root concavity
(suck-back; underwashing - non-standard terms)
A shallow groove which may occur in the root of a
butt weld, but full fusion is evident
Shrinkage groove
A shallow groove caused by contraction in the metal
along each side of a penetration bead or along the
weld centreline
Burnthrough
(melt through)
A localised collapse of the molten pool due to
excessive penetration, resulting in a hole in the
weld run
Contour Defects
Incompletely filled groove
A continuous or intermittent channel
in the surface of a weld, running
along its length, due to insufficient
weld metal. The channel may be
along the centre or along one or
both edges of the weld
Bulbous Contour
Bulbous contour
A non-standard term used to
describe poor appearance
Unequal Legs
Unequal legs
(non standard term)
Variation of leg length on a fillet weld
Note: Unequal leg lengths may be
specified as part of the design - in which
case they are not imperfections
Undercut
Undercut
An irregular groove at a toe of a run in the
parent metal or in previously deposited
weld metal
The inspector must determine if the
undercut is continuous or intermittent, or
sharp or smooth
Overlap
Overlap
An imperfection at the toe or root of a
weld caused by metal flowing on to the
surface of the parent metal without fusing
to it
Gas Pore
Gas pore
A cavity, generally under
1.5mm in diameter, formed by
trapped gas during the
solidification of molten metal
Porosity
A group of gas pores
Crater Pipe
Crater pipe
A depression due to shrinkage at the end of a
run where the source of heat was removed.
Crater pipes may also lead to micro-cracking
Surface Cracks
Crack
A linear discontinuity produced by fracture
Cracks may be ...
a) ... longitudinal, in the weld metal, i.e. centreline
b) ... longitudinal, in the parent metal or heat affected zone
c) ... transverse
d) Crater crack
(star cracking)
Arc Strike
Stray flash/arc burn/arc strike
(stray arcing)
1. The damage on the parent material
resulting from the accidental striking of an
arc away from the weld
2. The accidental striking of an arc away
from the weld
Note that the same term is used for both
the action and the result
Weld Width
Weld width and consistency of weld width
For butt welds and fillet welds,
consider:
Weld Defects
Lack of Side Wall Fusion
Lack of Inter Run Fusion
Porosity
Slag Inclusions
Incomplete Root Fusion
/Penetration
Solidification Cracking
Overlap
Excess Weld Metal
Excess Penetration
Root Concavity
Slag Inclusion
Surface Breaking Porosity
Cracking
Arc Strikes
Crater Pipe
Using Welding Inspection Tools
Weld Profile Gauge
Scale in
inches or mm
Measuring
Fillet Welds
Measuring Cap
Reinforcement/
Misalignment
Measuring
Bevel Angle
Hi Lo Gage
Measures Hi Lo…
Pipe Thickness…
Bevel Angle…
Using the tools…
Throat Thickness (WPG)
Fillet Weld
Cap Reinforcement (WPG)
Measuring
the height
of the cap
Bevel Angle (WPG)
Readout
Measuring
the pipe
bevel
angle
Misalignment (WPG)
Shown
on scale
Misalignment (Hi Lo)
Pipe Misalignment
Non Destructive Testing
Visual Inspection
Visual inspection is the one NDT method
used extensively to evaluate the condition or
the quality of a weld or component. It is
easily carried out, inexpensive and Visual
inspection is the one NDT method used
extensively to evaluate the condition or the
quality of a weld or component. It is easily
carried out, inexpensive and usually doesn't
require special equipment.
Radiography
X-rays are produced by high voltage x ray
machines whereas gamma rays are
produced from radioactive isotopes such as
Iridium 192 The x-ray or gamma rays are
placed close to the material to bc inspected
and they pass through the material and are
then captured on film This film is then
processed and the image is obtained as a
series of gray shades between black and
white.
Magnetic Particle Inspection
Magnetic particle inspection is a method that can be
used to find surface and near surface flaws in
ferromagnetic materials such as steel and iron.
The technique uses the principle that magnetic lines
of force {flux) will be distorted by the presence of a
flaw in a manner that will reveal it's presence. the
flaw (for example, a crack) is located from the "flux
leakage", following the application of fine iron
particles, to the area under examination. There are
variations in the way the magnetic field is applied.
but they are all dependant on the above principle .
Penetrant Testing Inspection
Liquid penetration inspection is a method that is used
to reveal surface breaking flaws by bleed out of a
colored or fluorescent dye from the flaw.
Ultrasonic Testing
Ultrasonic inspection uses sound waves of short
wavelength and high frequency to detect flaws or
measure material thickness. It is used on aircraft,
the power stations generating plant, or welds in
pressure vessels at an oil refinery or paper mill.