copy of welddefects & quality control

7/28/2019 Copy of Welddefects & Quality Control

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VARIOUS WELD DEFECTSAND QUALITY CONTROL


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SCOPE

WELD DEFECTS TYPES

THEIR ORIGIN

HOW TO AVOID THEM

QUALITY CONTROL


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TYPES OF WELD DEFECTS

Group 1 - Root defects

Incomplete penetration

Lack of root fusion

Execess penetration

Root concavity

Shrinkage groove

Burn through


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GROUP 2 - CONTOUR DEFECTS

Incompletely filled groove

Bulbous contour

Unequal legs


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GROUP 3 - SURFACE IRREGULARITIES

Undercut

Overlap

Gas pore

Crater pipe


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GROUP 4 - SURFACE CRACKS

Longitudinal/Centre-line

Parent metal/HAZ

Transverse

Crater


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GROUP 5 - MISCELLANEOUS

Stray flash/arc

burn/arc strike

Spatter


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Overlap Excess weld metal


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Excess penetration Root concavity


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Weld defects: Their effect

Indications with major dimensions greater than 1.6 mmare termed as relevant indications (ASME Sec VIII)

Cracks Pose the danger of

growing under stress during

service Lack of penetration-reduces

the load carrying cross-

section, corresponds

geometrically to a crack Lack of fusion corresponds

geometrically to a crack

Having a length > than 3

times the width

All relevant linear indicationsare unacceptable

No tungsten inclusions are

acceptable


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Porosities/Slag inclusion reduce the load carryingcross-section

Undercut

create notch

effect at weld toe/trap slag

- up to 0.8 mmacceptable if it lies parallelto the applied force

- up to 0.25 mmacceptable if it liestransverse to the appliedforce.

Rounded indication (circular

or elliptical with length =< 3

times width

Up to 4.8 mm are acceptable

4 or more rounded

indications in a line

separated by 1.6 mm or less

edge to edge distance areunacceptable


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Excess reinforcement

Stress concentration at the

weld toe

Root side concavity

Permitted when the weld

thickness is at least equal

to the thickness of the

thinner member of the twosections and

Contour of the concavity

is smooth

Arc strikes/Spatters

Cause hardened

spots/may become

crack initiation spotsduring service

In DSS, spatters can

lead to inbalance in

Ferrite/austenite ratio


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Cracks in welds How to avoid

Cold cracks

Ferritic steels

Proper preheat/ postheat- Use of thoroughly baked

electrodes /fluxes


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Hot cracks

ASS

Use of Weldingconsumables with

adequate ferrite content

Use of low heat input

Ensuring high degree ofcleanliness during welding

Proper W/D ratio


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Effect of weld shape on cracking tendency:a) W:D = 1, sound weld

b) W:D = 1.4, sound weld

c) W:D = 0.7, weld tends to crack

d) W:D = 2.0, weld tends to crack


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Effect of shape of root run:

a) Incorrect, top concave

b) Correct, flat or slightly convex


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Effect of weld shape in multipass welds:

a & b) Concave with tendency to crack

c) Slightly convex weld beads


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Cracking tendency of deeppenetration weld:

a) Incorrect shape

b) Correct shape

Bridging large gap

gives concave weld

Cracking of deep

penetration fillet weld


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Control joint fit-up to reduce gaps.

Clean off all contaminants from the material

Welding sequence will not lead to a build-up

of thermally induced stresses. Select parameters/technique to produce a

weld bead with an adequate D to W ratio, orwith sufficient throat thickness (fillet weld)

(recommend a depth to width ratio of at least0.5:1).


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Too large a D to W ratio which will encouragesegregation and excessive transverse strainsin restrained joints. As a general rule, weldbeads whose D to W ratio exceeds 2:1 will beprone to solidification cracking.

Avoid high welding speeds (at high currentlevels) which increase the amount ofsegregation and the stress level across theweld bead.

At the run stop, ensure adequate filling of thecrater to avoid an unfavourable concaveshape.


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Porosity in welds How to avoid

Types of porosity

distributed

surface breaking pores wormhole

crater pipes


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Nitrogen, oxygen and hydrogen

absorption due to poor gas shielding

As little as 1% air entrainment in theshielding gas will cause distributed

porosity and greater than 1.5% results

in gross surface breaking pores.


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Air entrainment

- Seal any air leak

- Avoid weld pool turbulence

- Use filler with adequate level of deoxidants

- Reduce excessively high gas flow

- Avoid draughts

Hydrogen- Dry the electrode and flux

- Clean and degrease the workpiece surface


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Crater pipe

- Sudden drop of welding current &/or

stopping wire addition during termination ofwelding cause crater pipe formation

- Can be avoided by using down slope in

P/S and with adequate wire addition


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Lamellar tearing

Transverse strain - the

shrinkage strains on welding

must act in the short

direction of the plate ie

through the plate thickness Weld orientation - the fusion

boundary will be roughly

parallel to the plane of the

inclusions

Material susceptibility - the

plate must have poor

ductility in the through-

thickness direction


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Designs to avoid lamellar tearing


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Welding quality control

Availability of approved Welding

Procedures complying with Codes/

Specifications Verification of Welder Performance

Qualification Records

Familiarity with workmanship standardsand all phases of good shop practice


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Review materials/welding consumables

to be used

Review storage and issue procedures ofwelding consumables

Check condition of Power Sources and

Calibration Records


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Check fit-up and alignment of weld joints

as per the Drawing Requirement

Check adequacy of preheating/postheating arrangements


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Proper included angle sufficient for reaching root of

joint

to ensure fusion to side walls Proper root opening/root

face To ensure proper

penetration

Too large a root face

no

penetration

Too small a root face burnthrough


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Proper Alignment

Cleanliness of the joint

Free from dirt, rust, scale, grease, oil and

moisture

Proper size of electrode/current

To reach the root

To get good fusion


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Proper Tack welds TACKING SHALL BE 50 mm (MIN) LENGTH FOR

PLATE THICKNESS 6mm-32 mm and 75 mm

LENGTH FOR PLATE THICKNESS >32 mm

LOCATION OF TACKS SHALL BE ~75 mm FROM

EITHER END OF THE L-SEAM AND AT AN

INTERVAL OF 400mm IN THE REMAINING LENGTH

NO TACK WELDS ON THE ENDS OF THE LONG

SEAMS

TACKS FOR THE CIR.SEAMS ~500 APART FROM

EACH OTHER


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NO TACKS FOR THE CIR.SEAMS#LONG SEAMS -T-

JOINTS

START AND END POINTS CAN BE GROUND

SMOOTHLY/FEATHERED SO THAT THEY MAY BE

INCORPORATED INTO THE FINAL WELD

WHILE TACK WELDING IN TWO LAYERS, TOP

WELD IS SHORTER IN LENGTH THAN BOTTOM

LAYER


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Proper Preheat and Interpass

Temperatures

Proper sequencing of beads


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Proper travel speed

Too slow Molten

metal & slag run

ahead


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Proper purging in GTAW

Volume of backing gas should be 6 times

the volume of the system Mandatory for Inconel (Ni-base alloys),

Titanium, Aluminium, SS, Duplex, LAS

welds

As a good practice, required for the first 2

passes (5 mm)


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Consumable inserts in GTAW

For better quality root passes

For achieving smooth and uniform rootcontour without root concavity/excess

penetration

Less welder skill is required Best results achieved with gas backing


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Welding quality control Weaving in SMAW

Smooth finish low heat input

welding-high speedIrregular finishHigh heat input Welding-low speed

3*D

D-electrode dia


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Property * Change

Yield Strength reduced by 30%

Tensile Strength reduced by 10%

Percent Elongation increased by 10%

Notch Toughness (CVN) reduced by 50%,

for 1.9 H


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In FCAW/GMAW welding, replace the

gas nozzles/contact tips at regular

intervals - coagulation of spatter, wirewobbling

Use anti-spatter spray for easy removal

of spatter In SAW, replace the contact tips at

regular intervals to avoid wire wobbling


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CLAD JOINTS NO WELD SPATTERS SHOULD FALL IN THE CLAD

MATERIAL

SEPARATION FROM THE BASE MATERIAL AND THE

CLAD SHALL BE CLEAR

ENSURE THAT THE CLAD IS REMOVED BEFORE

WELDING THE BASE MATERIAL BY CuSO4

SOLUTION

PREHEATING TO BE DONE IN SUCH A WAY THAT

DIRECT IMPINGEMENT OF FLAME ON CLAD

MATERIAL IS AVOIDEDA

ENSURE MIN. 50% OVERLAP DURING CLAD-

RESTORATION OR OVERLAY IN SMAW AND FCAW

RATERIAL HE E THAT THE CLAD IS REMOVED BLDING


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Controls specific to Al & Al alloys Pre-clean the surface to remove the tenacious,

refractory oxide by mechanical methods such as

wire brushing, scraping and filing. For brushing

stainless steel wire brushes must be used andthey must be maintained scrupulously clean

Use of He or He+Ar gas mixtures compared to Ar

for welding thick sections


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Controls specific to Nickel alloys Use of a larger groove angle compared to CS/SS to permit

proper manipulation of the filler metal and deposition of

stringer weld beads As Nickel and its alloys are susceptible to embrittlement by

sulphur, phosphorus and low melting point metals such as

lead, zinc and tin, oils, paints, marking crayons, cutting fluids

and shop dust are to be removed.

Oxides should be thoroughly removed from the surfaces tobe welded. Wire brushes used for cleaning should be made

of austenitic stainless steels.

Preheat is not required.


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Controls specific to Titanium During edge preparation, attention must be paid to the scrupulous

removal of all foreign matter like oil, grease, grinding debris, oxide orother compounds etc,. Degreasing with acetone and pickling in asolution containing HCl and HNO3 is usually recommended.

Filler rods must also be so cleaned and must be handled only with apair of clean gloves. It will be a good practice to remove the oxidisedend of the wire prior to welding.

Purity of Argon shall be 99.9978% and use clean nonporous flexiblegas hose (such as Tygon or Vinyl plastic) for carrying gas to the torch(Do not use rubber hose as it is porous and Ar can pick up moisture)

If protected chamber welding is not possible, effective protection can beobtained by providing argon not only with the welding torch but also onthe trailing side through a separate feed-tube and nozzle attached tothe torch. Argon is also supplied on the under side through alongitudinal groove in a copper backing plate. Argon shielding must beprovided till the HAZ has cooled to below about 300 C.

It is generally advisable to use a wider nozzle and get better coverage.


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Controls specific to Stainless steel Avoid the the presence of carbonaceous material

such as grease/oil on b.m/f.m, marking made with agraphite pencil while welding

Ensure complete removal of flux/slag from thecompleted weldments

Do not blend the welds autogenously as this practiceimpairs corrosion resistance

Heat tint or temper colouration as well as a heavier

oxide scale can affect corrosion resistance - removesuch heat tints/oxide scales

Avoid contamination with sulfur, iron, copper


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Cleaning of Weld No slag

No tool impressions or blow marks

Prof i le and Dimens ions o f Weld Complete penetration and full fusion

Complete filling of weld groove

Free from coarse ripples, grooves,overlaps and abrupt ridges and valleys

Consistency in Weld Width


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