Welding Inspection NDT Methods
Welding is a process of joining two metals by heating both
the metals. Heating of metals may be carried out various process based on which
welding is broadly classified as
Arc welding – by
creating an electric arc.
Gas welding – by
utilizing flame of burning gas.
Friction Stir –
by heat generated due to friction.
Electric Resistance
– by resistance of material to passage
of current.
Electron Beam –
Heat produced by hitting electron beam on to the metal.
There is possibility of discontinuities in other words
called as “Defects” formed during the process of welding or during cooling for
solidification. The causes for the defects are various few of them are
described below.
1) Undercut – at the weld fusion area/
weld toe on the base metal, may occur on the face side or root side of the
weld.
Cause :- Caused due to excessive heat melting the base
metal probably caused by two reasons.
They are Slowdown of welding speed at that particular location or
increase in the current suddenly with constant speed.
2) Underfill – Occur at face side of the weld,
can be identified as lower surface level than the adjacent base metal.
Cause: Pour of less or insufficient weld
metal during welding required more filling.
3) Excess Penetration- Occurs at root of
the weld, can be identified as excess weld drop at root.
Cause:- Occurs due to excessive root Gap ,
less root face and excess current during these conditions. Avoid too much root
gap and provide adequate root face to support the high heat input during weld.
4) Lack of Penetration : Occurs at the root
side of the weld can be identified as lack of weld metal at the root side and
both the root face edges will be visible 
Cause :- May be due to any of the following
reasons- Less Root Gap, More root face, too small bevel angle, Less current, Too
large electrode diameter, Longer arc length.
5) Incomplete
fusion: Occurs as gap between the weld metal and base metal and can be at any
location, Root, Face or at Sidewall.
Cause:- Due to improper welding skill, incorrect
welding current and voltage, incorrect travel speed, poor inter run cleaning & wrong selection of electrode.
6) Root Concavity: Can be observed as suck
back or concave like depression in the root, even though both the root faces
has fused with weld metal.
Causes:- Insufficient arc power to produce positive bead,
excessive backing pressure ( if purging is done), Lack of welder skills, Slag
flooding in backing bar groove.
7) Excess weld Reinforcement: Occurs in the
face side of the weld as improper weld bead as shown below. It is considered as defect because of formation
of sharp corners at toes.
8) Misalignment: Found as un even level(up and
down ) between two joining base metal.
Cause: may be caused during
pre-welding fit-up, or after welding due to in adequate tack welds and
temporary retainers.
9) Cracks: Cracks may be found as longitudinal
or transverse, and also may be found in the base metal near to weld (HAZ Heat
Affected Zone) as shown in the figure. Hot cracks occur during welding and
cooling or during heat treatment
process. Cold cracks occurs during the service due to loads
Causes: May be due to any or all
of the following reasons. Improper electrode, Improper cooling rate or
quenching, High carbon content steel, High
Sulphur and phosphor content in steel . Certain alloys, welding on coated
steels, Cold cracking mostly starts from HAZ areas due to internal &
external stress, and hydrogen induced.
10 Porosity: Occurs as round or curved
cavities in the weld, may be single, clustered or aligned.
Cause:- Due to absorption of
nitrogen, oxygen and hydrogen in the molten weld metal . The gases are involved
in the weld metal due to moisture in
base metal or welding electrodes, and poor gas shielding.
11) Spatter : Weld metal splashed and sticks to
the surface of the weld or base metal. Few are easily removable and few has to
be grinded down.
Causes: Long welding arc., Short welding arc,
Cleanliness of the base material surface. Selection of gas welding (CO2 welding yields
more), Welding at a wrong angle.
Imbalance between voltage and
amperage, incorrect wire feed speed.
1 Arc Strike: Occurs as a base metal
damage and splash of melted base metal on the base metal surface.
Causes: When Electrode holder
touches the other base metal areas causing an arc between the base metal and
the holder.
13 Slag Inclusions: Occurs as
non-metallic left outs in the weld metal
or between the weld metal and base metal as shown below. These slag inclusions
causes lack of fusion at few locations
Causes: The flux layer formed over the
solidified weld pool if not cleared forms as slag inclusion. The melted holders
in MIG welding form Copper inclusions
on the weld metal. The melted tungsten rod in TIG welding forms Tungsten inclusions on the base metal.
14 Crater / Crater Pipe / Crater Cracks: Craters
occurs as elliptical shaped depression at end of weld or weld stopping point. Crater
pipe occurs as a hole at the center of crater and crater crack occurs as star
shaped crack at the middle of the crater.
Causes: Crater is caused due to improper
weld stopping technique, crater pipe or crack is formed due to low weld filling
at the crater area. Crack is formed during cooling due to insufficient metal.
15 Overlap: Occurs as Weld metal flown
more than the edge of the face 
Causes: Overlap is caused due to wrong
electrode angle, slow welding speed, high current and contamination.
What are
the methods to find welding defects?
Methods of
NDT:
In addition with Visual inspection of
complete weld area (both Face and Root), additional Non Destructive Examination
methods are essential to examine 100 % volume of the weld joint.
Few of the NDT Methods and their uses are
given below:
1)
Ultrasonic
Testing UT- To find Internal Defects (no Surface and below surface defects
found).
2)
Magnetic
Particle Testing MT- To find the Surface and Sub-Surface Defects in
Ferro-magnetic material with major content of ferrous / Nickel/ Cobalt.
3)
Liquid
Penetrant Testing PT- To find surface defects including minute cracks in
any material except material with porous surface.
4)
Radiographic
Testing RT / X-ray – To find the defects at any locations of any material.
5)
Eddy
Current Testing ET – To Find the surface and sub surface defects on any
conductive material , especially thin sections.
6)
Leak
Testing LT – To find discontinuities in a hollow component such as Pipe,
tank, pressure vessel or tubes.
And Lot more…… To know more about How
NDT services are used in industries visit our blog page at
https:// ndtcenter.blogspot.com/
It is essential to utilize reliable testing methods on the
fabricated jobs or raw material to ensure the life of the component and safety
of the component. Further the application of NDT methods saves companies plenty
of money by avoiding rejection of components after machining, fabrication,
processing & transportation.
Apart from NDT methods, the welding process involved and
welders doing the job should be evaluated for the particular process and
particular group of metal. In that case International
Standards requires the companies to
create a Welding Procedure Specification (WPS) for a specific job and qualify
every welders based on the WPS created
for welding at particular positions .
Few of the Welding Procedure Standards are given as below:
1)
ASME BPVC Section IX
2)
ISO 15614
3)
AWS D1.1 Steel
4)
AWS D1.3 Sheet Metal
5)
AWS D1.6 Stainless Steel
Lot
more…..
Also Global Industries & internal Standards require the
documents & inspection to be done by qualified persons as per the standards.
Advanced Quality Centre has engineers qualified for all inspections as per ISO,
API, NACE, BINDT PCN and ASNT standards. To avail NDT service for your jobs
reach us on https://aqcinspection.com/non-destructive-testing/ or call 04224342244, we do third party
inspection services at our facilities or at your premises.