8/4/2019 Analysis & Prevention of Weld Failure

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ANALYSIS & PREVENTION OF WELD FAILURE

By

P.K. GHOSH

Professor

Department of Metallurgical & Materials EngineeringIndian Institute of Technology Roorkee

Roorkee 247 667

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Failure of a part or assembly is defined as :

(i) When it becomes completely inoperable.

(ii) When it is still operable but is no longer able toperform its intended function satisfactorily.

(iii) When serious deterioration has made it unreliableor unsafe for continued use, thus necessitating its

immediate removal from service for repair orreplacement.

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Failures of weld joint are mostly caused by static

overloading, impact loading, dynamic loading, wear,corrosive attack and irradiation damage.

Failure investigation of weld joint is a quite complicated

affair and it is practically impossible to generalise the

cause of failure.

Failure of welded structure involves several interactingphenomena of scientific and technological factors related

to the material, stress distribution, interacting service

conditions, structural safety analysis etc.

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Under these circumstances a critical analysis of various

aspects contributing to the failure mechanism is very

much essential to identify the primary and secondarycauses of failure.

The investigator must have interdisciplinary view and

knowledge to co-ordinate the investigation and if necessary the investigator may have to take opinion of the experts of different Specialisation.

An investigator has to follow step by step the standardprocedure of investigation with a bias free analyticaloutlook. The fundamental sources of weldment failure

are primarily accounted as follows.

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PRIMARY SOURCES OF WELD FAILURE

Design deficienciesDeficiencies in Selection Weldable of Material & ConsumablesImperfections in MaterialsDeficiencies in Weld Fabrication & Processing

Aggressive Improper Service ConditionsImproper Maintenance

Design Deficiencies

The presence of mechanical notches at points of highstresses in any component or a member of a structurewhich is especially subjected to bending or torsionalloading.

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The changes or modification in design of an existingstructure, made without adequate consideration of possibleintroduction of stress raisers.

May also result from an impossibility of making reliablestress calculations for complex parts and from insufficientinformation about the types and magnitude of the serviceloads.

Gross deficiencies in design may arise from reliance on static

tensile properties without taking into account the possibilityof failure by the mechanisms as brittle fracture, low cyclefatigue, stress corrosion and interacting service conditionslike corrosion fatigue and creep fatigue interaction etc.

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In this regard careful consideration of fracture mechanicsdata, like critical plain strain stress intensity factor (K Ic),

critical J-integral factor (J Ic), critical stress intensity factorfor stress corrosion cracking (K Iscc ) etc., along with theconventional design criteria may be very much useful.

Deficiencies in Selection of Weldable Material &

ConsumableThe material selection must be made in relation todimensional and geometrical aspects of design and by keepinga close look to service conditions.

The tensile data are only partially indicative of inherentmechanical resistance to specific service conditions.

The mechanical and chemical properties of a base materialmay change to a great extent in the HAZ under weld thermal

cycle.

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The material must be carefully checked by considering itsweldability under any prescribed welding process andprocedure.

No generalisation can be made that will be valid for allmaterial selection problems.

However, on the basis of previous knowledge andexperiences a guide to criteria useful for selection of material in relation to possible failure mechanism has beenprescribed [Table I].

The most troublesome areas of material selection are thoserelated to mechanical behaviour in which properties of amaterial are influenced by time in service under wear,elevated temperature, corrosion, stress corrosion, corrosionfatigue and irradiation

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Table - IGuide to General Criteria for Selection of Material in Relation to Possible Failure Mechanisms

Failure Types of loading Types of stress Operating Temp. Criteria for selection

mechanisms St. Dy. Imp. Ten. Com. Sh. Low Amb. High of materials

Brittle fracture X X X X - - X X - Charpy V-notch Tc,notch toughness,

KIc toughness

Ductile fracture X - - X - X - X X Tensile strength,shear yield strength.

High-cycle - X - X - X X X X Fatigue strength for fatigue expected life in

presence of typicalstress raisers

Low-cycle - X - X - X X X X Static ductility & peak cyclicfatigue plastic strain at stress raisers

during prescribed life.

Corrosion - X - X - X - X X Corrosion - fatigue strengthfatigue of the metal and contaminant

for similar time.

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Failure Types of loading Types of stress Operating Temp. Criteria for selectionmechanisms St. Dy. Imp. Ten. Com. Sh. Low Amb. High of materials

Buckling X - X - X - X X X Modulus of elasticity andcompressive yield strength.

Gross yield X - - X X X X X X Yield strength.

Creep X - - X X X - - X Creep rate or sustainedstress-rupture strength for thetemperature & expected life.

Caustic or- X - - X - - - X X Stability under simultaneoushydrogen stress & hydrogen or other embrittlement chemical environment.

Stress corr- X - - X - X - X X Residual &/or imposedosion cracking stress & corrosion resistance

to environment. KIscc value.

St. : Static Ten. : Tension Amb. : Ambient Sh. : Shear Dy. : Dynamic Com. : Compression Imp. : Impact

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Imperfections in Materials

Failures of weld joint often originate from the imperfection ordefects in weld deposit as well as HAZ.

The imperfections of weld metal, contributing to its failure,are primarily known as inclusions, micro-segregation, centreline segregation, shrinkage porosity, voids and micro cracks.

The presence of certain inclusions or second phase at the

surface of base material or weld metal may serve aspreferential sites for pitting or intergranular corrosion undercorrosive service environment, causing early failure of acomponent.

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The lamination in base material along with elongatedinclusions (iron sulphide), especially when they are present in

HAZ of a weld joint, severely damages the service life of aweld structure by initiating failure by lamellar tearing.

The microstructure of HAZ also contribute to weldmentfailure by formation of metallurgical notch creating localisedproblems and potential sites inviting corrosion attack.

The grain growth at HAZ close to fusion line makes theregion prone to stress corrosion cracking.

The grain boundary precipitation and coarsening of precipitates in HAZ, depending upon weld thermal cycle,also make the region more susceptible to corrosion andinitiate failure.

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Deficiencies in Weld Fabrication and Processing

Processing or fabrication by welding can lead to failure by avariety of mechanisms unless suitable precautions are taken inworkmanship, fixture for welding, selection of weldingparameters and pre and post weld cleaning.

Welding under severe restraint, such as joining of large partswith improper fixturing, contraction of the weld metal andHAZ during cooling induces residual tensile stress in certainregion of the weldment that can cause cracking at once or

promote cracking in service.

Selection of unsuitable filler metal or use of excessive heatinput during welding can cause similar effects.

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Unless it is done by carefully developed and closely controlledprocedures, a build-up of undersize or worn parts like large

shafts (particularly of high strength steel) by weld depositionis likely to cause cracking immediately or to reduce fatigueendurance.

During welding fabrication an improper heat treatmentoccurring in variety of forms, like over heating, under-tempering, excessive temperature gradient and unsuitablepost weld heating, may cause failure to a specific material.

The welding operations with a high heat input, can render ametal susceptible to stress corrosion cracking. In othersituations, stress relief after welding may act as a sensitisingfactor leading to failure.

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Aggressive Service Conditions

Abnormally severe conditions of speed, loading, temperatureand corrosive environment, or without regular maintenance,inspection and monitoring often impart major contributionto the occurrence of service failure of weld joint.

Inspection maintenance and monitoring procedures must bebased on a thorough consideration of the variousmechanisms of failure that may be possible for the part inquestion.

Inspection maintenance and monitoring procedures shouldbe capable of detecting significant deterioration duringnormal inspection and maintenance operations at scheduledintervals.

Improper Maintenance

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Thank You