Welding processes and Duties of a Welding Inspector (CSWIP )
Duties of a Welding
Inspector
General:
My duties are as a welding inspector to check that all welding and
associated activities are being carried out at the job site in accordance with
the requirement of the approved welding procedure specification and through my
qualities of honesty, integrity and knowledge. I also apply my knowledge in
observing, measuring, and recording; before, during and after welding.
Prior to welding
Before
assembly:
I check;
Applicable
codes, standards and project specification.
All applicable
documents such as QIP, procedures, inspection formats and any special safety
requirements.
Approved WPS
with supporting WPAR as per code
Welder
qualification and identification as per WPS
Material;
composition, condition, ratting and storage handling as procedure, MTC’s and
drawings
Consumables;
composition, type, size and baking requirement as per WPS
Surface
preparation method and finish as WPS and to good workmanship
Welding machine
validity as suitable to workmanship standard.
Line-up clamp
type and condition as WPS
After
assembly:
I check:
Cleanliness of
surface to good workmanship
Pre-heat as
procedure
Purging dump to
avoid oxidation as WPS
Weather
condition as suitable to site/field
During welding.
I check:
Consumable and
control as WPS and to good workmanship
Welding process
as WPS
Line-up clamp;
remove after completion of root pass as WPS
Laps timing
between root and hot pass as WPS
Welding
parameters such as voltage, amperage and polarity, welding technique,
weld-direction and run sequence.
Inter-pass
cleaning to good workmanship
Inter-pass
temperature minimum and maximum as WPS
Speed of travel
as WPS
After welding
I check;
Compliance to
WPS.
Weld and welder
ID marked as drawing
Any modification
or as-built add to drawing
Visual inspection
as code or spec.
Post-heat to
good workmanship and as WPS
Monitor post
weld heat-treatment as procedure
Monitor NDT as
code and specs
In case repair;
as procedure
Again after
48hrs to confirm hydrogen cracking
All relevant
reports are available
Pressure/load
test and calibration of gauges as procedure
Collect and collets all documents and keep
for life time record
Welding processes
GTAW (tig) BS 3019
Drooping characteristics arc process
(constant current).
Type of
operation:
Normally manual but can be mechanized.
Mode of
operation:
An arc is maintained between the end of
tungsten electrode and parent metal. The current is controlled by the of power
source. Operator must control arc length and feed the filler wire for correct
welding. Normally argon gas is used for arc shielding to arc and weld pool to
prevent from atmospheric gases. No fluxes are used with the process. The arc is
unstable at low current. Special provision is made for starting.
Power
source:
Generator, transformer and rectifier.
Current:
Normally operate at dc–ve but can be used
with ac for light alloys.
Consumables
metal:
Wires are according to BS 2901 Pt 1-5.
Gases according to BS 4105 & BS 4365
Shielding
gases:
Normally argon
gas is used for shielding but helium and nitrogen also can be used.
Tungsten
electrode:
Unactivated/Plain: rarely used. Suitable for lower
quality welds. May cause of tungsten inclusion.
Activated 1% thoriated: used with dc–ve for all materials
except light alloys.
Activated 2% thoriated: used
where arc stability required with low amperage also used with dc–ve for all
materials except light alloys.
Activated zirconiated: specifically used with ac on the
light alloys.
Direct current
1-3.2mm alternative current
1-6.4mm
Equipment:
·
Power source (dropping – constant
current)
·
Welding torch
·
Gas cylinder
·
Welding cables
·
Welding helmet
Defect
associated with this process:
·
Tungsten inclusion
·
Lack of fusion
·
Lack of penetration
·
Under cut
·
Burn through
·
Porosity
·
Excess penetration
·
Oxide inclusions.
Advantages:
·
Good quality of the weld
·
Good for thin material
·
Wide range of material
·
Clean weld, no slag, and no
smoke.
·
Can be achieved high mechanical
and metallurgical properties.
·
High root run quality.
Disadvantage:
·
High skill required for pipe
welder
·
Very expensive as compare to other
manual metal arc welding
·
Good surface cleaning required
·
Relatively slow process
GMAW (m.i.g. /m.a.g.)
Flat characteristics (constant voltage) arc
process. It may be considered together because same welding equipment and power
source uses. Shielding gases and filler wire may be differing to another.
Type of
operation:
Manual, mechanized or automatic
Mode of
operation:
An arc is maintained between the end of
consumable electrode or wire, and work piece. Wire is continuously fed from a
coil through a special designed gun. The wire is fed at a constant speed
selected to give required current. An arc length is control by power source
setting. An arc length is not considerable for operator but depositing of the
weld metal is considerable. Arc and weld pool shielded by the shielding gas. No
fluxes are used with this process.
Power
source:
Generator, transformer and rectifier.
Current:
Normally dc +ve (constant voltage)
Shielding
gases:
Argon, hydrogen, nitrogen, helium for non-ferrous metal
Argon + oxygen (1-5) stainless steel
Argon + Co2 (2-25) carbon
and low alloy steel
Consumable:
Solid wire, cored wire, self shielded wire,
rutile cored wire, basic cored wire, metal cored wire (Æ 0.8 mm ~ 1.6 mm). According to BS 2901.
Gases according to BS 4365 & BS 4105
Mode of
metal transfer:
Spray or
free flight: give high
deposition rates and deep penetration welds. Suited to thick materials except
light alloys.
Dip transfer: used
for thinner section and for all positional welding including v/down.
Globular transfer: intermediate
range between spray and dip transfer mode, no manual application and success on
mechanized and automatic.
Pulsed: arc
is modified form of spray mode. Give high deposition rate and for all
positional welding. No lack of root runs, regular penetration. No spatter, good
profile, high quality welds.
Typical
defect:
Lack of fusion occurs is dip transfer mode.
Centerline cracking in spray mode transfer
Porosity, under cut, incomplete
penetration, excess penetration and excessive spatters.
Advantages:
·
High quality of the weld
·
Very clean, no inter pass
cleaning required.
·
No slag with solid wire
·
Less skill required
·
Minimal wastage of electrode
·
Heavier weld bead produced
·
Faster welding process
Disadvantages:
·
Costly equipment
·
More maintenance required of
equipment
·
Not portable
·
Increase the risk of porosity
due to shield gases
·
High risk of lack fusion
SAW (SA)
Flat arc process (constant) voltage
Type of
operation:
Mechanized, automatic or semiautomatic.
Mode of
operation:
The arc is maintained between the end of
electrode bare wire and the work piece. As the electrode is melted, it is fed
in to the arc a set of rolls, driven by a governed motor. Wire feed speed is
automatically controlled to the equal the rate at which electrode is melted,
thus arc length is constant. Arc is under the granular flux. Some of flux is
melt to provide a protective blanket (slag) over the weld pool. Flux is
unaffected and can be recycled.
Current:
Dc +ve for
best penetration.
Dc –ve fast
burn of rate. For lower dilution
Ac for
multy wire. To avoid arc blow
Consumable:
Solid wire is used, Æ 1.6 mm to 6 mm according to BS 4165, BS 5465, and AWS A5.17
Fused and agglomerated according to BS
4165, BS 5465, and AWS A5.17.
Defect:
Slag inclusion, under cut.
Advantages:
·
Good productivity
·
Good quality of welding
·
Good for thick material
·
Less skill required
·
Very clean
·
No spatter is sticks smoke.
·
No visible arc (no need eye
protection)
Disadvantages:
·
Very costly equipment
·
Limited position of welding
·
Need accurate fit-up
·
Poor portability
·
No good for thin material
Manual Metal Arc Welding
Drooping characteristics (constant current)
Type of
operation:
Manual
Mode of
operation:
Arc is created between the tip of electrode
and work piece. Arc is formed by momentarily touching the tip of electrode on
the parent plate and then lift the electrode to give a gap 3 ~ 6mm between the
tip of electrode and the plate. Arc melts parent metal and electrode; the
molten metal so formed is transferred as small globules across the arc gap.
Welder controls arc length and electrodes feed rate by the hand movement. The
slag must be removing after each deposited layer. Normally a small degree of
penetration, plat edge preparation required.
Power
source:
Generator, transformer and rectifier.
Current:
Normally dc +ve but also use dc –ve or ac.
Dc +ve best
penetration
Dc –ve deposit
is high
Defect:
Porosity, slag inclusion, under cut,
excessive penetration and spatters.
Equipment:
·
Power source
·
Welding cables
·
Electrode holders
·
Earth return
·
Welding shielding helmet
Consumable:
BS EN 499, AWS A5.1, ISO 2560, BS 2926, BS
2493
Basic: E7018,
E7015, E7016
Rutile: E6013,
E38 2R
Cellulasic: E7010,
E8010
Advantages:
·
Low cost equipment
·
Easy to operate at site
·
Easy shift to site
·
Easy and more deposit thickness
·
Wide range of material
·
Welding all position
Disadvantages:
·
Low production due cleaning
required after every layer
·
Dirty and smoke production
·
Not sufficient to weld all
material successfully.
Welding process defects
1 - Weld decay
Weld
decay?
Weld decay is corrosion between grains.
Causes?
The reduction of the chromium from grains
is main cause of the weld decay. When material is over heated 600° C~800° C,
chromium comes out from the grains, joins with carbon and becomes chromium
carbide on the grain boundaries. It happens in the heat-affected zone. Chromium
is retard corrosion. Corrosion occurs between the grains as chromium is
reduced.
Avoidance?
We can avoid weld decay
·
By using the low carbon content
material such as 316L & 304L instead of 304 & 316.
·
By using the stabilized
stainless steel such as 321 & 347.
·
By adding the titanium and
niobium. Titanium and niobium join the carbon and become titanium and niobium
carbide. Titanium and niobium give stabilization to chromium.
·
By the keeping of appropriate
heat input.
·
By the keeping of appropriate
inter pass temperature.
2 - Solidification cracking
Solidification
cracking?
Solidification cracking is hot cracking,
hot shortness, and centerline cracking in the weld metal
Causes?
·
Sulphur, stress and joint
design are the main causes of the solidification cracking.
·
Sulphur comes from parent
metal, joins with the iron and becomes iron sulphied. Iron sulphied has low
solidify temperature than steel. Iron sulphied becomes a thin film in liquid
form between the solidify grains on the center of the weld. Iron sulphied
possess a very little tensile strength. Any stress makes a solidification crack
at this moment.
Avoidance?
We can avoid solidification cracking
·
By the using low sulphur
content material
·
By changing of joint design.
·
By good cleaning of joint.
·
By the adding of manganese to
join the Sulphur to become manganese sulphied. Manganese sulphied has a same
temperature to solidify with steel. Manganese reduces Sulphur content and
discrete the iron sulphied. Therefore, less chance to crack.
3 - Lamellar tearing
Lamellar
tearing?
Lamellar tearing has characteristics step
like crack.
Occurrence?
It occurs in the thick section in the T, Y,
K, and corner joints, wrought plate, in the HAZ of steels, where fusion
boundary of the weld and only in the rolled direction of parent material.
Causes?
Poor through thickness ductility, stress
and high Sulphur contents are main causes of lamellar tearing. Although, others
non-metallic inclusions may also play a part. The presence of the hydrogen
increases steel’s susceptibility to lamellar tearing quite significantly.
Avoidance?
Lamellar tearing can be avoided:
·
By reducing the size of the
weld, try to use fillet weld instead of butt weld.
·
By changing the joint design,
where stress moving in the roll direction.
·
By using the low Sulphur
content steel.
·
By buttering layer
·
By using z quality plate that
has been stra tested.
Assessed?
Assess by short tensile test according to
BS 5135.
4 - Hydrogen cracking
Hydrogen
cracking?
Hydrogen cracking is known as cold
cracking, hydrogen induced cracking (HICC) and delayed crack.
Cause?
Hydrogen cracking can be occurred when:
·
Hydrogen is exceeds 15ml/100gms
of the weld metal.
·
Stress exceeds ½ yield stress
·
Temperature is less than 350° C
·
Hardness exceeds 400VPN (Vickers
pyramid hardness)
·
Oil/greasy surface
·
Moisturized flux
Avoidance?
Hydrogen cracking can be avoided by:
·
Appropriate baking of flux
covered electrode.
·
Appropriate pre-heating
·
Adequate inter pass temperature
·
Appropriate heat in put.
·
Post-heating to defuse hydrogen
and reduce the residuals stresses.
·
Using the hydrogen controlled
electrode
Consumables
Welding consumables are the electrodes,
wires, fluxes and gases. Each consumable is critical in respect to
specification/supplier, condition and treatment (if any).
Many codes are covered the various
consumables.
Covered
electrodes:
BS EN 499, AWS A5.1, ISO 2560, BS 2493, BS
2926.
Gas shield wires:
BS 2901 Part 1-5
Gases:
BS 4365, BS 4105
Fluxes & wires (SAW)
BS 4165 (CS), BS 5465 (A/SS), AWS A5.17
(CS)
Function
of shielding gases (TIG/MIG/MAG):
·
It provides a suitable, ionize
able atmosphere for the electric arc.
·
It protects the weld pool from
the atmospheric contamination.
Argon: provides a
smooth arc at low arc voltage with dc-ve also gives cleaning action with ac for
light alloys.
In addition of hydrogen, provides
a high arc voltage and gives deeper penetration also increase speed on
stainless steel.
Helium: less than
argon therefore high flow rate (2~2.5 times) required to achieve same
effectiveness with argon. Produces high arc voltage and heat suitable for thick
section. More cost expensive than argon.
Nitrogen: inert gas in
the room but becomes active with oxygen therefore unsuitable for majority of
material but gives good result on the copper. More cost effectiveness than
argon or helium.
Function
of fluxes:
·
Provides a gas shield to
protect the weld pool and arc from atmospheric oxygen and nitrogen.
·
Provides a slag, which gives
additional shielding to the weld pool and assists in manipulation during the
welding.
·
Improves the physical
properties of the arc (arc initiation and stability)
·
Introduces weld metal alloys
such as iron powder, de-oxidant etc.
·
Improves metallurgical
properties to lowering oxygen and nitrogen levels.
·
Increasing deposition factors
and over all efficiency
Types of fluxes (MMA):
Rutile, cellulose and basic fluxes are the
common types of covering s for MMA electrodes; others include acid and
oxidizing coverings.
Rutile:
·
Constituents; titanium dioxide,
clay and sodium silicate.
·
Medium weight of titanium
dioxide plus fluorspar.
·
Used for general purpose such
as ms fabrication, low pressure pipe work, supports, structure and bracket etc.
·
Gives fluid fast freezing slag,
suitable to easy use in all positions but not for vertical down.
·
Should be kept dry but never
baked.
Cellulose:
·
Constituent’s cellulose (wood
pulp), titanium dioxide, sodium silicate.
·
High cellulose content produces
a large volume of gas around the arc.
·
Shield gas consist on H2,
Co, CO2 & H2o
·
Most important gas is hydrogen,
which increases the arc voltage and corresponding in power, which cause for
deep penetration.
·
Rapid, rate of burn.
·
Produces fast freezing weld
pool and thin slag, suitable for vertical down and overhead work.
·
Main use on stovepipe welding
of high strength large diameter pipelines and storage tanks.
·
Good quality of penetration
bead.
·
Rough appearance and uneven
ripples of completed weld.
·
Spatter content higher than
other electrode.
·
Hydroscopic flux designed to
hold between 4-7% moisture.
·
Must be kept dry but never
baked.
Basic:
·
High limestone and fluorspar
content to produce weld metal with low hydrogen content.
·
Limestone has good stabilizing
and produces carbon dioxide gas shield.
·
May be bake up to 500C
approximately or depending on the manufacturer.
·
Ability to weld low alloy, high
& medium tensile strength with high sulphur content without any cracking,
also reduce the possibility of the hydrogen induced cracking in the weld metal
and heat-affected zone, but dependant upon the properly dried.
·
BS 639 certifies for MMA as
hydrogen controlled. Must be less than 15ml/100gms of the weld metal but can be
reduced less than 5 ml/100gms of the weld metal with proper control.
·
Need higher degree of skill and
must be used vertical up technique.
·
Productive and expensive.
·
Constituents; limestone
(calcium carbonate) gas former, Co2 secondary ionizer, fluorspar slag former,
sodium/potassium silicate main ionizer.
Filler wire & Fluxes (SAW):
- Solid wire is used, Æ 1.6 mm to 6 mm according to BS 4165.
- Fused and agglomerated according to BS 4165.
Fused
flux (granular):
·
Manufactured at high
temperature, glassy appearance like crystal.
·
Good chemical mix achieved.
·
Do not attract moisture.
·
Good handling, storage, used
and weld ability
·
Each removal of impurities and
fine particle during recycling
Agglomerated
flux:
·
Dry mixed and then bonded with
either potassium or sodium silicate manufactured at high temperature.
·
Absorb moisture and limited
recycling.
·
Weld appearance not good.
Advantages and disadvantages
Radiography Testing
Advantages:
·
Permanent record.
·
Good for thin material.
·
Wide range of material.
·
No skill required for gamma
rays.
·
No surface cleaning required.
·
No power required for gamma
rays.
Disadvantages:
·
Dangerous for health.
·
Lamination and lack of sidewall
fusion cannot detect.
·
Expensive film.
·
Affected all other activities.
·
Power source required for
X-rays.
·
Not good for thick material.
·
High skill required for X-rays.
Ultrasonic Testing
Advantages:
·
Give accurate location and
depth of defect.
·
No health hazard.
·
No need power supply.
·
Not affected other activities.
·
Good for thick material.
·
Lamination and lack of sidewall
fusion can detect.
·
Portable.
Disadvantages:
·
No permanent record.
·
High skill required.
·
Surface defect cannot detect.
·
Smooth surface required.
·
Not good for thin material.
·
Large grain size material
defect cannot detect.
Dye Penetrant Testing
Advantages:
·
Can check all materials.
·
No need power source.
·
Low cost.
·
Direct indicate to defect
location.
·
No high skill required.
Disadvantages:
·
For surface defect.
·
More dwell time as compare to
MT.
Magnetic Particle Inspection
Advantages:
·
Surface and sub-surface defect
can check.
·
Low cost.
·
Less time and more output.
·
Direct indicate to defect
location.
Disadvantages:
·
Cannot use for non-ferrous.
·
May be danger for operator.
·
Power required.
Difference
between microscopic and macroscopic.
Microscopic
·
For grain structure analysis
·
Cross-section view at high
magnification e.g. 100x or 1000x
·
Some defects could also be
detected and assessed.
·
The degree of preparation is
much higher than macroscopic examination, e.g. for ferrite steel; P1200 grade
finished followed by 1um polish using a diamond paste then an etch using 1-5%
nital
·
This test is primarily used by
the metallurgists
Macroscopic
·
For welder/procedure
qualification
·
To view the cross-section view
magnification is required 5x ~ 10x
·
To detect the weld defect and
also to measure the actual defects already detects.
·
Carried out on full thickness
specimen included reinforcement
·
Width of cross-section should
include the heat-affected zone plus some parent material.
·
Same test piece is sometimes
used for hardness testing after macro examination
·
Specimen transversely cut from
the weld
·
Each test specimen is then
ground, polished and etched to the degree required by the specification, e.g.
for ferrite steels P400 grade finish with an acid etch using 10-15% nital
(nitric acid + alcohol)
·
Test specimen examined visually
·
The intent is to disclose any
cracks, lack of fusion, porosity, slag etc.
Steel properties
Chemical
properties of steel
1.
Aluminum;
i.De-oxidizer, grain refiner
for improved toughness
2.
Carbon;
i.Hardening agent
ii.Increases Strength
iii.Reduces weld ability as
increase carbon
3.
Chromium;
i.Creep resistance and resist
oxidation.
ii.Corrosion resistance
iii.Hardening element
iv.Increase the strength of steel
4.
Cobalt;
i.Known as red hardness
ii.Used where high strength,
high hardness at high temperatures are desired
5.
Manganese;
i.De-oxidant, grain refiner,
increase toughness at low temperature
ii.Increase strength by
increasing harden ability
6.
Molybdenum;
i.Creep resistance
ii.Increase hardness also
improve corrosion resistance qualities
7.
Nickel;
i.Grain refiner
ii.Increase harden ability
iii.Improves toughness and
ductility even with strength and hardness
iv.Improve toughness at low
temperatures
v.Increase tensile strength
about 6000psi for each additional 1% of nickel
8.
Phosphors;
i.< 0.015%
ii.Usually found in all steels
iii.Hardened steel
iv.Cause to embrittelment
v.Improves machine ability of
high-carbon steel
vi.Improve strength and
corrosion resistance of low carbon steel
9.
Silicon;
i.De-oxidizer (0.2 – 0.3%)
ii.Promotes fluidity of molten
steel
iii.Also contributes to the
strength of LA steel
10.
Vanadium;
i.Grain refiner
ii.Promotes control of grain
size, grain refine
iii.Increase harden ability
11.
Sulphur;
i.Undesirable impurity (less
than 0.04%)
ii.Cause brittleness and reduce
weld ability
iii.Improve machine ability
12.
Niobium
i.Grain refiner
ii.Increase harden ability
iii.Also known as columbium
Mechanical
properties of metals
Strength
The ability of material to withstand an applied load
Tensile strength shear strength, torsional strength impact strength
and fatigue strength
Tensile: the ability of metal to resist
failure
Yield: that strength level at which the
material’s response to loading changes from “elastic” to “plastic”
If the hardness increased, the tensile strength increases also and
vice versa
If temperature increases, the strength decreases
Ductility
The ability of material to deform, or stretch under load without
failing
Ductility increases as temperature increase
High ductility is referred to as “ductile” and low ductility is referred
to as “brittle”
Hardness
The ability of material to resist indentation or penetration
Hardness increases as strength increases, or vice versa
If hardness is known possible to estimate tensile strength
Toughness
The ability of material to absorb energy
Toughness decreases as the temperature is reduced
Toughness decreases as the hardness is increased
Fatigue
strength
That strength necessary to resist failure under repeated load
applications.
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