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NON-DESTRUCTIVE TESTING IN

THE OFFSHORE INDUSTRY

Lecture 02

07 March 2015

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1.Introduction in Nondestructive testing.

2.Discontinuities , origin and classification3.Visual testing.

4.Liquid penetrant inspection method.

5.Process control and interpretation of liquid penetrant inspection

6.Magnetic particle testing theory.

7.Magnetic particle inspection applications and interpretations8.Ultrasonic inspection method.

9.Ultrasonic inspection equipments and materials.

10.Ultrasonic inspections process control and safety.

11.Eddy current inspection method.12.Application on eddy current inspection

13.Acoustic emission testing.

14.Thermal infrared testing.

Topic

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2. Discontinuities , origin and

classification

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Introduction

Nondestructive testing (NDT) or Nondestructive

evaluation (NDE) refers to techniques, which areused to detect, locate and assess defects or flaws inmaterials or structures or fabricated componentswithout affecting in any way their continued

usefulness or serviceability.The defects may either be intrinsically present as aresult of manufacturing process or may result fromstress, corrosion etc. to which a material or acomponent may be subjected during actual use.

It is evident that techniques to detect critical flawsbefore they have grown unacceptably large are ofvital importance in the industry for in-serviceinspection, quality control and failure analysis.

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An inspection procedure may be prepared with

two purposes:

1. Detecting defects with any dimension, ordetecting defects within specific dimension, or

even detecting a specific type of defect;

2. Ratify the inspected part is free of defect, or if

the inspected part is free of defects larger than

specific dimension, or even if the part is free ofspecific type of defect.

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Discontinuity

An interruption of the typical structure of a material,

such as a lack of homogeneity in its mechanical,

metallurgical, or physical characteristics. A

discontinuity is not necessarily a defect.

Defect

A flaw or flaws that by nature or accumulated

effect render a part or product unable to meet

minimum applicable acceptance standards or

specifications. The term designates rejectability.

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Levels of observations

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Definitions

1. Completed weld A weld that is completed and is readyfor final visual inspection.

2. DefectAny harmful discontinuity that must be repairedto be acceptable.3. DiscontinuityAny imperfection in the normal structureor configuration of a weld or the base material that may ormay not need to be repaired.

4. Final visual inspection of welds Visual inspectionperformed on a completed weld in the final surfacecondition and heat-treated condition.

5. Finished weld A weld that has received final inspectionand has been accepted.

6. In-process visual inspection of welds - Visual inspectionperformed on intermediate passes of multipass welds.

7. Sound metal Metal that contains no defects.

8. Skewed fillet welds - Fillet or fillet reinforced weld that is

less than 85or greater than 95.

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CRACKS

Definition A tear, fracture or fissure in the weld or

base metal appearing as a broken, jagged orstraight line.

Cracks in a weldment are probably the most

dreaded of all the weld discontinuities. Because somany materials and applications are used inwelding, cracking is a complex subject.

The base material's crack sensitivity may beassociated with its chemistry and its susceptibility tothe formation of elements that reduce its ductility.Excessive stresses in the weld joint, particularly if

the material is in a crack-sensitive condition, cancause cracking to occur.

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The welding operation itself can produce stresses inand around the weld, introducing extreme localizedheating, expansion, and contraction.

Cracking often is caused by stress concentrationnear discontinuities in welds and base metal andnear mechanical notches in the weldment design.

Hydrogen embrittlement, a condition that causes aloss of ductility and exists in weld metal because of

hydrogen absorption, can contribute to crackformation in some materials.

NOTE: Cracks are the mos t ser ious defect!!!!!

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C C

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Hot and Cold Cracks

Cracks are classified as one of two types: hot or cold.

Hot cracks develop at elevated temperatures, propagatebetween the grains of a material, and commonly formduring solidification of weld metal.

Cold cracks develop after solidification of the weld, as aresult of stresses, and propagate both between grains andthrough grains. Cold cracks in steel sometimes are called

delayed cracks and often are associated with hydrogenembrittlement.

Hot cracks and cold cracks can be further categorized as

base material cracks and weld metal cracks.

H t ff t d (HAZ) ki t ft

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Heat-affected zone (HAZ) cracking most oftenoccurs with base material that can be hardened.High hardness and low ductility in a HAZ often arethe result of a metallurgical response to welding

thermal cycles. In ferritic steels, hardness increasesand ductility decreases with an increase in carboncontent and a faster cooling rate.

The HAZ hardness depends on the base material'sability to be hardened, which in turn depends on thebase material's chemical composition. Carbon has apredominant effect on steel's hardenability. For

instance, cast iron contains between 2 and 4.5percent carbon, which gives the alloy high hardnessand low ductility. Welding this material withoutseriously considering cooling rates and residualstress invariably will result in base material cracking.

C k t bl di ti iti d d t i t l t

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Cracks are unacceptable discontinuities and are detrimental toweld performance. A crack, by its nature, is sharp at itsextremities, so it acts as a stress concentration. The stressconcentration effect of a crack is greater than that of most otherdiscontinuities.

Cracks have a tendency to propagate, contributing to weld failureunder stress. Regardless of their size, cracks aren't permitted inweldments governed by most fabrication codes. They must beremoved by grinding or gouging, and the excavation filled with

sound weld metal.

Successful welding procedures incorporate the controls that arenecessary to overcome the tendency for crack formation. Suchcontrols are preheating temperature, interpass temperature,

consumable type and preparation, and postweld heat treatment.

Welding inspectors are responsible for evaluating theseprocedural controls during inspections to ensure welding isperformed to minimize the possibility of weld cracking.

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CRACKS TYPES

Longitudinal

Transverse

Crater

ThroatToe

Root

Underbead and Heat-affected zone

Hot

Cold or delayed

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Longitudinal Crack - A crack running in the

direction of the weld axis. May be found in the

weld or base metal.

Transverse CrackA crack running into or inside

a weld, transverse to the weld axis direction

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Crater Crack

A crack, generally in the shape of an Xwhich is

found in a crater. Crater cracks are hot cracks.

Throat Crack

A longitudinal crack located in the weld throat

area.

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Toe Crack

A crack in the base metal beginning at the toe of

the weld.

Root Crack

A crack in the weld at the weld root.

C

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Underbead CrackA crack in the unmelted parent

metal of the HAZ.

Hot Crack - A crack in the weld that occurs

during solidification.

Cold Crack - A crack that occurs after the metal

has completely solidified

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Preventive Action

1. Remove contaminants from the joint (rust,grease, moisture, etc.) prior to welding.

2. Apply and maintain required preheat.

3. Do not allow the base material to cool tooquickly.

4. Maintain filler metal control requirements.

5. Use correct filler metal type for the joint.

6. Apply proper bead size and sequencing to

eliminate excessive distortion and/or stress in thebase material.

Corrective Action

Repair in accordance with local procedures.

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UNDERFILL

Definition

The amount of weld that is below a straight line

drawn from the edges of the joint preparation of a

groove weld, with the exception of allowableundercut.

G f

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Gauge for

inspection for

underfill defect

Preventive Action

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Preventive Action

1. During welding of the reinforcement, gauge beadheights in any location that appears to be lower than

the rest of the bead.2. Adjust ampers intensity or volts.

3. Slow travel speed to allow increased weld puddlevolume.

4. Sequence weld passes so that the toes of thebeads sufficiently cover one another, minimizingvalleys.

Corrective Action

Apply additional weld beads until the joint isadecquatly filled.

INCOMPLETE FUSION

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INCOMPLETE FUSION

DefinitionA situation where the weld metal does

not fuse or completely bond with the base metalor previously deposited weld metal.

P ti A ti

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Preventive Action

1. Increase ampers intensity or voltages.

2. Decrease travel speed.3. Maintain appropriate arc length/wire stickout.

4. Adjust torch/rod angle.

5. Ensure previous beads are free of overlap(bead roll-over) and slag prior to welding

additional passes.

Corrective Action

1. Grind or carbon arc the weld to sound metal.

2. Weld repair the affected area

INCOMPLETE PENETRATION

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INCOMPLETE PENETRATIONDefinition A situation where the weld metal does

not penetrate as deeply as required.

Preventive Action

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Preventive Action

1. Increase the bevel angle and/or root opening of the fit-up. Do not exceed the requirements of the joint design.

2. Ensure bevel edges of both members are lined upproperly.3. Increase amps/volts.

4. Decrease travel speed.

5. Maintain appropriate arc length/wire stickout.


Corrective Action


Note: Backgouge 2nd side to full penetration (removefusion lines). Often, more than one fusion line will bepresent during back gouging.

2. Weld repair the affected area.

OVERLAP

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OVERLAPDefinitionA condition where the weld metal rolls over formingan angle less than 90. Sometimes referred to as weld beadrollover. Overlap is defined as a protrusion of weld metalbeyond the weld toe, or weld root. This condition occurs in filletwelds and butt joints and produces notches at the toe of theweld that are undesirable because of their resultant stressconcentration under load.

Preventive Action

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Preventive Action

1. Adjust amps/volts.

2. Increase travel speed.3. Maintain appropriate arc length/wire stickout.


Corrective Action


2. Weld repair the affected area, if needed.

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UNDERSIZED FILLET WELD

Definition Any fillet or fillet reinforced weld that

does not meet the minimum size requirementsspecified on applicable fabrication documents.

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CONCAVE FILLET WELD

Definition.A fillet weld that sinks in the center.

Preventive Action

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Preventive Action



3. Maintain appropriate arc length/wire stickout.4. Adjust torch/rod angle.

5. Feed more wire into the puddle when manual TIG

welding.6. Increase stop time (dwell time) on weaved beads.

7. Sequence weld passes so that the toes of thebeads sufficiently cover one another.

Corrective Action

Apply additional weld passes until the concavity

requirements have been met.

CONVEX FILLET WELD

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CONVEX FILLET WELD

Definition

A fillet weld that bulges out in the center.

E i C it E i it

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Excessive Convexity. Excessive convexity can

produce a notch effect in the welded area and,

consequently, concentration of stress under load.

For this reason, some codes and standards

specify the maximum permissible convexity of a

weld profile.

Preventive Action

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Preventive Action


2. Increase travel speed.

3. Maintain proper wire stickout.


5. Feed less wire into the puddle when manual

TIG welding.6. On multiple pass welds, avoid placing beadstoo close together.

Corrective Action

Grind, carbon arc or weld the affected area until

the convexity requirements have been met.

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INSUFFICIENT THROAT

Definition Insufficient throat usually occurs in filletweld and butt joint profiles that are concave. Excess

concavity reduces throat thickness, which

considerably reduces weld strength.

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UNDERCUT

Definition A groove melted into the base metal and

left unfilled by weld metal.

The amount of undercut permitted at the surface ofthe completed weld usually is specified within the

welding code or standard being used. The maximumpermissible undercut requirements for completedwelds should be followed stringently, becauseexcessive undercut can seriously affect theperformance of a weld, particularly if it's subjected to

fatigue loading in service.

The term undercut describes two specific

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The term undercut describes two specificconditions.

The first is the melting away of the base material at

the side wall of a groove weld at the edge of abead, which produces a sharp recess in the sidewall in the area where the next bead is to bedeposited. This type of undercut can entrap

inclusions within the recess, which then may becovered by a subsequent weld bead.

The second condition is reduction of the basemetal's thickness at the line where the weld bead

on the final layer of weld metal ties into the surfaceof the base metal. This position is known as thetoe of the weld. This condition can occur on a filletweld or a butt joint.

Preventive Action

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Preventive Action

1. Decrease amps/volts.





7. Use undercut gauge to verify acceptability.

Corrective Action

1. Grind the toe of the weld until the unacceptableundercut blends smoothly into the base material.


CORNER MELT

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CORNER-MELT

DefinitionA groove melted in a corner of a welded member

that is left unfilled.

Preventive Action

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1. Start welds at end of joint and work inward.

2. Decrease amps/volts.

3. Decrease travel speed.4. Maintain appropriate arc length/wire stickout.




8. Use undercut or bridgecam gauge to verify acceptability.

Corrective Action

1. Grind the toe of the weld until the unacceptable corner-melt blends smoothly into the base material.


ARC STRIKES

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ARC STRIKES

DefinitionA discontinuity consisting of any localized

remelted metal, heat effected metal, or change in

surface profile of a finished weld or base material

surface resulting from an electrical arc.

Note: Arc strikes may develop stress risers, which

could lead to cracking.

Preventive Action

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e e t e ct o1. Prior to energizing welding equipment, replacedamaged welding lines and ground cables.

2. Ensure grounds are properly installed.

3. Hang lines on trees or J-hooks.

4. Keep gas cups free of weld spatter on flux coreprocesses.

5. Do not allow sub-arc tips to contact the basematerial.

6. Be careful when striking an arc.

Corrective Action1. Grind the affected area until the unacceptable arcstrike blends smoothly into the base material or weldface.


POROSITY

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POROSITY

DefinitionOpen holes formed by gas that was trapped when

the weld cooled. Sometimes called pinholes.

Preventive Action

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1. Remove contaminants from the joint (rust, grease,moisture, etc.) prior to welding.



5. Use the largest size gas cup possible and keep it free ofspatter.

6. Position wind screens between the welding operationand any heavy flow of air.

Corrective Action

1. Completely remove porosity from all intermediate weldareas.

2. Grind or carbon arc the affected area until theunacceptable porosity is removed from the weld.


SPATTER

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SPATTER

Definition

The metal particles expelled during welding thatdo not form a part of the weld.

Preventive Action1 R t i t f th j i t ( t i t t )

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1. Remove contaminants from the joint (rust, grease, moisture, etc.)prior to welding.


3. Use Refrasil to protect surrounding surfaces from secondary weld

spatter.4. Adjust amps/volts.



7. Use ceramic tape or approved metal backing strap on areas with root

gap.8. Consult local Welding Engineering in cases where the base materialis magnetized.

Corrective Action

1. Completely remove spatter from all intermediate weld areas.2. Remove all loose spatter with a needle gun.

3. Grind all tightly adhering, unacceptable spatter until it blendssmoothly into the base material or weld.

SLAG

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SLAGDefinition The nonmetallic layer that forms on top of the molten metal.

Note: Slag is a byproduct of the welding processthat cannot be prevented. Below are some

actions that make slag removal easier.

Preventive Action

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2. Maintain a consistent travel speed.

3. Maintain an appropriate arc length/wire stickout.4. Adjust torch/rod angle.

5. Sequence weld passes so that the toes of thebeads sufficiently overlap one another, minimizing

valleys.

Corrective Action

1. Completely remove slag from all intermediate weld

areas.2. Remove all loose slag with a needle gun.

3. Grind all tightly adhering, unacceptable slag fromthe surface of the base material or weld.

MELT THROUGH

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MELT THROUGH

Definition Excessive heat and/or penetration that

results in irregularity on the surface of the backingring or strip, fused root or adjacent base material.

Preventive Action

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Preventive Action

1. Reduce amps/volts.

2. Increase travel speed.


4. Use ceramic tape or approved metal backing

strap on areas with root gap.

Corrective Action

1. Grind or carbon arc the weld to sound metal.2. Weld repair the affected area, if needed.

OXIDATION

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OXIDATION

Definition A condition resulting from partial or completelack of purge of a surface which is heated during welding

resulting in formation of oxide on the surface. Thiscondition may range from slight oxidation evidenced by amulticolored or tightly adhering black film to the extreme ofa very rough surface having crystalline appearance

(referred to as sugaring.)

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Preventive Action

1. Ensure adequate pipe purge is maintained

prior to and throughout the welding process.2. Remove contaminants from the joint (rust,

grease, moisture, etc.) prior to welding.

Corrective Action

1. Grind the weld to sound metal.


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CRATER PIT

Definition

A hole extending into the weld resulting from

shrinkage during cooling.

Preventive Action

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1. Remove contaminants from the joint

(rust,grease, moisture, etc.) prior to welding.

2. When breaking the arc for TIG welding, rapidly

pop the trigger several times to avoid sudden

pull-offs. This will provide sufficient post purge of

the weld puddle.

Corrective Action

1. Grind the weld to sound metal.


Detecting and Evaluating Discontinuities

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Detecting and Evaluating Discontinuities

Weld discontinuities often are detected throughvisual inspection. Some are detected with inspectionmethods, such as radiography, ultrasonics, liquidpenetrant, and magnetic particle inspection.

The maximum acceptable limitations for thesediscontinuities depend on the performancerequirements of the welded component and are

specified in the appropriate welding code, standard,or specification. The welding inspector often isrequired to determine the extent of discontinuitiesand to establish their acceptance or rejection basedon the relevant acceptance criteria.

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FINGER GAUGE

TO MEASURESKEWED

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DIFFERENT GAUGE

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Visual Inspection Responsibilities1. When Final visual inspections must be performed:

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a) When the weld is complete, and in the final surface and heat treated conditions

b) Before other NDT (when required)

c) Before being made inaccessible for inspection

2. Where the weld inspection zone:a) All work - Completed weld faces plus inch on both sides of weld shall be visuallyinspected for the entire length of the weld.

b) To gauge weld size:

(1). Gauge where the weld size visually appears to be the smallest

(2). If the weld length is five feet or less, gauge a minimum of once per weld(3). If the weld length is greater than five feet, gauge a minimum of once every five feet

Visual Inspection Technique

1. What In order to perform final visual inspection you need:

a) Adequate lighting (use flashlight as needed)

b) Weld gauges (bridge cam and finger gauges recommended)c) Prescription glasses (as required)

2. How Distance and angle:

a) The inspectorseyes should be within 24 inches of the surface to be inspected and

b) At an angle of at least 30 degrees to the surface being inspected.

Postweld Inspection

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All of the nondestructive testing (NDT) required for

final acceptance of the weld must be completed afterthe weld repair is done.

This means that even if several inspections weresatisfactory before the weld was rejected, all of theinspections must be redone.

Repairs can cause new problems in a weld.

After a repair is made, the entire weldment must be

reinspected.

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end

ndt lecture02 2015

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