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Defects and Failures

Defects and their significance with respect to

failures is a complex subject.

Brittle Failures - Crack propagation

without appreciable plastic deformation

Ductile Failures - Plastic deformationwith gradual propagation of cracks

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Classification of Service Failures

Design (structural, design notches, jointlocation or welding configuration) related

Materials (selection and handling) related

Base Metal Defects (introduced during

raw material manufacture and shaping)

Fabrication Defects

In - service Defects

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Design Considerations

Flexibility for Thermal Fatigue CrackingResistance

Avoid sharp corners and notches for Fatigue

Resistance and gradual taper (4 : 1)

Proper Stress Concentration factors

Proper Weld Joint design

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Material Selection & Handling Considerations

Graphitisation and Embrittlement

Proper choice of steels especially stainless steels

Proper choice of welding consumable

Minimise abrupt material transitions (DMW)

Choice against temper embrittlement

J-Factor = (Si + Mn)x(P + Sn)x 104 (to be less than 160,

elements in weight %)

X-Factor = ( 10P + 5 Sb + 4 Sn + As) (to be less than 15, elements

in ppm)

Step Cooling Cycle, Impact test at sub-zero temperatures

Combination Heat Treatment Feasibility

Material Handling and surface protection in storage

Hydrogen Induced Cracking and Sulphide Stress Cracking

Resistance for sour service applications

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Base Metal Defects

In Wrought and forged Products

Mechanical Notches :

Laminations - Severe inclusions aligned parallel to surface

Laps - Surface defect in rolling, parallel to length, at an angle

Scabs - Scale rolled into surface

Slivers - Metal surface ruptures, rolled into the surface

Bark - Intergranular penetration of oxides and scale

Seams - Surface defect, parallel to rolling, linear fissures

Metallurgical Notches :

Hot Shortness Surface Carburisation / Decarburisation

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Casting Defects

Mechanical Notches

Hot Tears and Cracking Gas and Blow Holes

Unfused chaplets

Inclusions

Internal Shrinkage

Metallurgical Notches

Hot Shortness

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Fabrication Defects

Cold Bending (Excessive Thinning,

Ovality)

Hot Bending (Reduced Hot Ductility

problem)

Weld Defects

Improper Heat Treatment

Surface Cleanliness (for SS, for welds

before welding etc.)

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Weld Defects Improper Fit up

Root Oxidation in SS welds

Burn Through & Porosity Hot Cracks & Cold Cracks

Concavity

Slag Inclusions

Crater Cracking

Undercut Lack of Fusion

Stray Arcing

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In-service Defects

Boiler Tube Failures

Will be covered

as

Failure Analysis

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Boiler Tube Failures

Boiler Tube Failures - main cause offorced outages in electric utility steam

generating boilers

Single tube Failure in a 500 MW Rs. 5

to 6 Crores (replacement power charges

for 3-4 days to repair) besides affectingPlant Morale.

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Boiler Tube Failures (22 Primary Mechanisms)Stress Rupture

Short Term Overheating High Temperature Creep Dissimilar Metal Welds

Fatigue

Vibration Thermal Corrosion

Water-side Corrosion Caustic Corrosion Hydrogen Damage Pitting Stress Corrosion Cracking

Erosion Fly Ash Falling Slag Soot Blower Coal Particle

Fire-side Corrosion

Low Temperature Waterwall - Coal Ash - Oil Ash

Lack of Quality Control

Maintenance cleanindamage

Chemical Excursion damage Material Defects Welding Defects

- indicates that such problems have not been reported in India

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Short Term Overheating

Steam / Water cooled

tubes

Plugged by debris,scale etc.

High Heat Transfer /

Improper firing

Low water/steam

flow due to poorcirculation /

upstream leak

Corrective Action

Prevent Blockage

Maintain Drum level

Assure Coolant flow

Reduce over firing Redesign tubing to

promote flow

Relocation of horiz. /

inclined tubes to

avoid film boiling

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SHORT TERM OVERHEATING

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SHORT TERM OVERHEATING TRANSFORMED MARTENSITE

SHORT TERM OVERHEATING ORIGINAL STRUCTURE

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High Temperature Creep

Typical Locations

Steam cooled Tubes

Partially choked

Radiant Heat Zone

Gas Blockage

Incorrect Material

Material Transition

Higher stress due to weld attachment

Corrective Action

RLA

Fluid flushing

Material up-grades

Tube shielding

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LONG TERM OVERHEATING RANDOM GRAPHITISATION

LONG TERM OVERHEATING OXIDE NOTCHES

LONG TERM OVERHEATING EYEBROW GRAPHITISATION

LONG TERM OVERHEATING CREEP MICROCRACKS

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LONG TERM OVERHEATING

OVERHEATING, CREEP INCORRECT MATERIAL

OVERHEATING BULGING, SATELLITE SCALE CRACKING

OVERHEATING WATERSIDE DEPOSITS

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LONG TERM OVERHEATING

WATERSIDE DEPOSTS &

DAMAGE DUE TO TUBE INSIDE TUBE

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Dissimilar Metal Welds

Corrective Action

Repair/Replacement

Relocating the weld Use of Ni-base filler

Frequent inspection

Typical Locations

At SH / RH dissimilar

weld joints : Temperature / Stress

excursions

Mechanism : 1. The formation of carbon depleted zone on the ferritic side of the

transition from the ferritic to austenitic structure is the initial step and any

treatment which enhances the formation of this zone will enhance the failure

probability.

2. The carbon depleted soft feerritic zone is constrained by the sorrounding harderand stronger material and is subjected to strains induced by thermal expansion

mismatch, bending, vibration and pressure.

3. The strain accumulation in the carbon-depleted zone is relieved by creep at

elevated temperature.

4. Creep damage in the form of cavitation, grain boundary sliding and tearing

results in cracking in the carbon depleted zone along and adjacent to the weldinterface

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DISSIMILAR METAL WELD FAILURE

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Cont to Tube Fail2