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WTIA-Panel 1 – Guidance Note 6 PWHT – February 2003 Disclaimer applies of 10

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GUIDANCE NOTE 6

POST WELD HEAT TREATMENT OF WELDED STRUCTURES

INTRODUCTION

During the fabrication process, welding isthe most commonly used method of joiningitems together. The welding processgenerally involves melting and subsequentcooling, and the result of this thermal cycleis distortion if the welded item is free tomove, or residual stress if the item issecurely held. There comes a point when theamount of residual stress can create potentialproblems, either immediately or during thelife of the welded structure, and it needs tobe reduced or removed. Post weld heattreatment is the most widely used form ofstress relieving on completion of fabricationof welded structures. The principle is that asthe temperature is raised, the yield stress andthe elastic modulus of the material fall. Apoint is reached when the yield stress nolonger supports the residual stresses andsome localised plastic deformation occurs.

The objective of this Guidance Note is todescribe the various post weld heattreatment techniques, when it has to be usedand the circumstances where it canlegitimately be avoided. The Note is alsointended to assist the Fabrication Industry inproviding information on where to look forguidance on post weld heat treatmentheating rates, cooling rates, holdingtemperatures, holding times, thermocoupleplacement, thermocouple attachment, andqualification of welding procedures. It isintended for use by Welding Supervisors,Welding Inspectors, fabrication personneland Engineers involved in both thefabrication industry and typicalOwner/Operator industries.

SCOPE

Post weld heat treatment in the contextdescribed here refers to the process ofreheating a weld to below the lowertransformation temperature at a controlledrate, holding for a specific time and coolingat a controlled rate. No consideration hasbeen given to normalising of welds incarbon-manganese and low alloy steels, tosolution annealing of stainless steels or toany precipitation hardening treatments toother alloy materials. Therefore, almost byexclusion, the materials involved will be C-Manganese steels, C-Mo, 1¼Cr½Mo,2¼Cr1Mo, 5Cr1Mo, 9Cr1Mo and12Cr1MoV steels.

WHY THE NEED FOR POST WELDHEAT TREATMENT?

Residual Stress

The development of residual stressesapproaching or even exceeding the yieldstress is possible when welding thicksections. For certain industry sectors, eg.Petrochemical, Chemical, Oil and Gas, etc.the existence of residual stress of thismagnitude is completely unacceptable. Withpressure equipment operating at 2000 C andbelow a variety of stress corrosion crackingmechanisms under the general term“environmental cracking” become prevalent.There is also the problem of fatigue to beconsidered and the effect that residualtensile stresses have in that regard.


Other industries such as Power Generationhave different considerations, and the stressdeveloped due to thermal expansion ofpipework can take on a far greatersignificance than residual stress due towelding. It is interesting to note that it canbe shown (1) by the use of the Larson-Millerparameter that residual stresses in non-postweld heat treated welds are almost fullyrelaxed after approximately 20,000 hours ata service temperature of 5400 C.

Tempering Effect

Post weld heat treatment will generallyresult in a modification of the microstructureof both the weld metal and heat affectedzone. With the exception of the 9Cr1Mo and12Cr1MoV materials, the microstructure ofall other materials should contain a mixtureof ferrite and iron or alloy carbide. Theeffect of short-term (1 to 2 hours) post weldheat treatment on the carbide is generallybeneficial, whereas longer times result in areduction in toughness due to spheroidisingeffects. The normal microstructure for theparent, weld and HAZ for the 9Cr1Mo and12Cr1MoV materials is martensite, and postweld heat treatment is absolutely essential inthese materials to temper the martensitephase.

Effect on Mechanical Properties

As a series of very general statements, thefollowing are the consequences of post weldheat treatment compared with the as-weldedcondition:

Yield strength is decreased slightly, theeffect falling off fairly rapidly with time.

The tensile strength is decreased. The ductility is increased. Hardness levels are reduced. Toughness is slightly reduced at short

times but the effect can be significant overlonger times.

Effect on Creep Properties

For creep resisting material, post weld heattreatment is required in order to fullydevelop the creep strength. This is especiallytrue for thicker components such as headers.There has been a tendency in recent years toallow waiving of the post weld heattreatment stage for thinner materials usedtypically for superheater and reheater coilsin the Power Generation industry, but avariety of conditions have to be met.

Other benefits

Improving the diffusion of hydrogen outof weld metal

Softening the heat affected zone and thusimproving toughness (although not weldmetal toughness)

Improving dimensional stability duringmachining.

Improving ductility.

Improving the resistance to stresscorrosion cracking.

Reducing the effects of cold work.

WHEN TO POST WELD HEAT TREAT

Within pressure equipment standards, therequirement for post weld heat treatment islargely a function of the material and thethickness. The material (in terms of alloycontent) and the thickness (in relation to thequench effect) control the microstructurethat will be formed. Large sectionthicknesses in alloy steels can result inmartensitic, pearlitic or bainitic structures,depending on the cooling rate, and this isusually controlled by the use of preheat. Inaddition, the thicker the material that iswelded, the greater the amount of residualstress that will be developed on cooling.


For typical carbon-manganese steels, thethickness at which post weld heat treatmentbecomes mandatory is consistent in the 32 –38 mm range for most of the Codes in use inAustralia. The reason for each standardchoosing a specific thickness is not entirelyclear, but little has changed over the past 30years. What is interesting though is thatexperiments conducted in the mid-1970sshowed that fully restrained butt welds incarbon-manganese steels could developresidual stresses in excess of the yield stressat a thickness of approximately 35 mm.

With alloy steels, the thickness at which postweld heat treatment becomes mandatory ismuch less. Typically, the range is 13 – 20mm, and even below 13 mm, a series ofstrict conditions have to be met before postweld heat treatment can be waived. It isclear therefore, that with alloy steels, theremoval of residual stress is not the onlyconsideration for the application of postweld heat treatment.

Post weld heat treatment of structural steelsis almost unheard of. Even in the offshoreindustry, the Nodes and K-joints on theJackets are no longer post weld heat treated.When that industry was in its infancy, postweld heat treatment was mandatory and wasapplied, but very little CTOD informationwas available then, and the materials in usesuffered laminar tearing. That has allchanged now, and very few joints requireany attention today. Similarly, the massivemachines in use in the mining industry arenot subjected to post weld heat treatment,and it is not even addressed in the range ofstructural welding standards, AS 1554 parts1 to 6. Therefore, the topic is essentiallylimited to the pressure equipment industry.

HOW TO PERFORM POST WELDHEAT TREATMENT

General

Most of the requirements for post weld heattreatment can be found in the fabrication

Standard to which the vessel is constructed.In Australia, most fabrication standards nowrefer to AS 4458 for manufacture, and alldetails can be found there. The AmericanStandards still tend to retain their ownrequirements, and ASME I, ASME VIIIDivision 1 and ASME VIII Division 2 areall different. Part 4 of the new EN 13445deals with manufacture, and contains a lot ofdetail on post weld heat treatment.

Fixed Furnace

This method is traditional and well known tomost people in the fabrication industry. Afixed furnace usually consists of arectangular box made from heat resistantmaterials in which are embedded electricresistance elements. Doors open at each endand a travelling bogie allows for loading andunloading of the charge. Furnaces such asthese are often capable of heating to 12000 Cand can normalise and anneal as well asstress relieve. Some furnaces are gas fired,with two or even four nozzles at each end.Fixed furnaces tend to be large andexpensive to operate. They often have fixedthermocouples that measure the furnaceatmosphere temperature and not thetemperature of the article being heat treated.This is usually satisfactory up to around3000 C, but beyond, thermocouplesphysically attached to the article must takeover both temperature control andtemperature measurement. Furnaces such asthese must be equipped with correctlycalibrated temperature controllers/recorderswith at least 12 recording points.

Care must be taken when cooling after postweld heat treatment. Most manufacturingCodes specify a controlled rate of coolinguntil a certain temperature is reached(typically 300 – 4000 C depending on thethickness), so it is normal to control cool inthe furnace before opening the doors.

Temporary Furnace

These are custom-built around a vessel,rather than transport a vessel to a fixed


furnace. The idea is to minimise the airspace between the vessel and furnace walls,and they allow for faster heating andcooling. The basic structure of the furnaceshould be creep resisting piping (if the pipesare to be continually re-used) with heatresistant materials attached to them. Heatingcan be through resistant heating mats placedon a concrete floor or via gas burners placedat each end.In the case of gas burners, care must betaken to avoid direct flame impingement onthe vessel.

Temperature control is again through a 12point recorder/controller, but atmospherethermocouples are not generally used. Thesame heating and cooling rate restrictionsapply as with fixed furnaces.

Internal Firing

Vessels of suitable dimensions andarrangement of openings can be post weldheat treated by gas firing through nozzles ormanways. Manways are large enough toaccommodate the gas burners, but careneeds to be exercised with the diameter andposition of nozzles and expert opinionshould be sought. Care must also be taken toplace deflector plates inside the vessel andopposite the burner entry points to avoiddirect flame impingement on the shell. It isnot advisable to post weld heat treat vesselsthat contain internals in this manner.The outside of the vessel must becompletely encased in insulating material,and again, at least a 12 point temperaturerecorder is advisable.

Local Heating

Circumferential weld seams can be postweld heat treated by heating a band aroundthe weld. Although not specifically stated,such heating is essentially limited toresistance or induction heating, mainlybecause of the controls required on heatedband width, width of insulation andtemperature measurement requirements. Theuse of gas ring burners and other methods

should be carefully documented, proposed tothe Client and accepted before beingcontemplated for use.Typical restrictions on the width of theheated band in both AS 4458 and EN 13445involve the use of a formula, ie. 5(Rts)½

where the weld is in the centre and:

R = inside radius of the shell, in mm ts = nominal shell thickness, in mmThis means that with a vessel of say 2 metreinside diameter and 50mm wall thickness,the minimum width of the heated bandwould be

5 x (1000 x 50)½ = 1118 mmor 560mm each side of the weld.

In addition to this, the width of the insulatedband is recommended to be 10 (Rts)½, whichin the above example would be 2236mm.This is to ensure that the temperaturegradient of the portion next to the heatedband is not harmful. There may be otherrestrictions or different formulae involved,and the relevant Code must be checked.

Partial Heat Treatment

There are occasions, for example with verylong vessels, when the entire vessel will notfit into a fixed furnace. This has beencatered for in most Standards, and it ispermissible to post weld heat treat section ofthe vessel first, then turn the vessel aroundand heat treat the remaining section. As withlocal heating, there are restrictions in thiscase as well over the degree of overlap andthe longitudinal temperature gradient. AS4458 requires an overlap of at least1500mm, and care should be taken to ensurethat this section does not contain a weldwhich may be post weld heat treated twice.Generally, there are better ways to post weldheat treat vessels than to use this partialmethod.

Thermocouples

For the temperature ranges applicable in thisguidance note, ie. up to 7500 C, the normal


chromel-alumel thermocouples perform in asatisfactory manner. They can be purchasedeither as insulated wires or as sheathed units.The wire type are better in that they can beattached directly to the equipment being heattreated by capacitor discharge. The sheathedtypes need to be physically attached, oftenby welding a drilled nut onto the equipmentand bolting the sheath into the nut. There isan argument that says this measures theatmosphere temperature just above the nutand not the metal temperature, but the basisof the argument is tenuous at best.The number and placement ofthermocouples is also governed to a certaindegree by Standard requirements. Forexample, AS 4458 stipulates that themaximum variation in temperature withinthe component shall not exceed 1400 Cwithin any 5 metre length. EN 13445stipulates a maximum variation of 1500 Cwithin a 4500mm length, but restricts this to1000 C when the temperature exceeds 5000

C. Sufficient thermocouples therefore needto be attached to demonstrate that thisrequirement is met. Allowance should alsobe made for breakage and thermocouplemalfunction, so it is normal to attach morethan just the minimum. For internally firedvessels and locally heat treated weld seams,for each thermocouple attached to theoutside, there should be a corresponding oneon the inside. There is also a requirement forlocally heat treated weld seams and partiallyheat treated vessels to measure thetemperature at the edge of the heating bandto ensure it is at least half of the peaktemperature. For vessels where there arediffering material thicknesses,thermocouples should be placed on allthicknesses, irrespective of the 5 metrelength.

Heating/Cooling Rates

These are specified in most of theconstruction Standards, and are reasonablysimilar. For example, AS 4458 for carbonmanganese steel vessels limits heating andcooling rates to 2000 C/hour for thicknessesup to 25mm, and 50000 C/hour divided by

the thickness for vessels over 25mm. Thereis also a minimum limit of 500 C/hour forvery thick vessels. However, for½Cr½Mo¼V and 2¼Cr1Mo alloys, theheating rate is limited to half that of carbonmanganese steels. For comparison, EN13445 allows 2200 C/hour up to 25mm thick,5500/thickness for 25 to 100mm and 550

C/hour above 100mm thick. There are norestrictions on the alloy steels in EN 13445.By the time the holding temperature hasbeen reached, the variation in temperaturebetween any two thermocouples should notexceed the range specified for the material.In AS 4458, the range is typically 400C (eg.C-Mn steel range is 580 – 6200C), and in EN13445, it is 50 – 700 C. In most practicalsituations, the variation can be limited tomuch less than this.The issues described here are vital to avoidpost weld heat treatment going wrong andthe result being a scrap vessel. It isrecommended to all fabricators to obtain apost weld heat treatment procedure from aheat treatment company, where all of theabove-mentioned restrictions are spelled out,before starting.

Test Pieces

There are two main types of test piece thataccompany pressure vessels through the postweld heat treatment cycle. In most countries,they are the Production Test Plate (PTP) andthe Coupon Plate. The PTP contains a weldseam, and is either a run-off plate from alongitudinal weld, or a weld done in parallelto represent a circumferential weld. TheCoupon Plate is usually an off-cut of parentmaterial, and is used to follow the vesselthrough all heat treatment cycles, includinghot forming, normalising and post weld heattreatment.It is highly unlikely that such test pieceswould be needed where local post weld heattreatment has been applied. Test pieces aretherefore limited to furnace heat treatedvessels. Most Standards require that the testpiece(s) accompany the vessel through theactual heat treatment cycle, and will not


allow a simulated cycle (AS 4458 is anexception).For internally fired post weld heat treatment,the test pieces must be placed inside thevessel, but with furnace methods, they maybe placed inside or outside the vessel. Caremust be exercised with the location, sincethe test piece must be subjected to the sameheating and cooling rate as the vessel. It isalso critical for each test piece to have aseparate thermocouple attached.

Charts and Certificates

No post weld heat treatment should ever beperformed without a chart generated by acorrectly calibrated recorder. The chart isproof that the post weld heat treatment wascarried out correctly, and forms part of theManufacturer's Data Report. In manycountries, the Third Party Inspector isrequired to witness the loading of thefurnace or the set-up in local methods, andactually sign the chart. In Australia, this isnot required, but a senior member of theCompany performing the post weld heattreatment should sign the chart.A Certificate of post weld heat treatment isalso often issued, describing the equipmentand giving details of job number, etc. butthis should be complimentary to the chart,not a replacement for it.

Non-destructive testing

EN 13445 requires that all non-destructivetesting be carried out after post weld heattreatment. There is a modification thatallows for material that is not sensitive toheat treatment cracking to be examinedbefore post weld heat treatment. Thesematerials include carbon manganese, carbonmolybdenum and 1¼Cr½Mo steels, but thecustomer should be consulted beforeassuming non-destructive testing may bedone before post weld heat treatment.Of the American Codes, ASME B31.3 isprobably the most widely used in Australia.It is slightly more restrictive than it’sEuropean counterparts in that only carbonmanganese steels can be tested before heat

treatment, and all alloy steels must be testedafterwards. In this regard, ASME 1, VIIIDiv.1 and VIII Div.2 are all similar toB31.3.AS 4458 refers to AS 4037, and this againallows testing before or after post weld heattreatment depending on the alloy.Restrictions to testing after heat treatmentapply mainly to high alloy steels such asmartensitic, austenitic and ferritic types, aswell as the ½Cr½Mo¼V steels. There is afurther restriction for 9%nickel steels andquenched and tempered alloy steels in thatthere has to be a 7 day delay between postweld heat treatment and magnetic particletesting, and that this testing must also bedone after hydrostatic testing.It is strongly recommended that fabricatorsfully understand the requirements of thecontract and the particular requirements ofthe Standard before establishing theirinspection and test plans.

OMISSION OF POST WELD HEATTREATMENT

There are a number of situations wherepressure equipment requires work to be donedue to the service environment, and suchwork often involves welding. Typicalsituations include:

Repairs due to mechanical damage Repairs due to a corrosion mechanism,

including cracking. Repairs due to creep damage Repairs to service-propagated defects. Repairs to original manufacturing defects Modifications to take advantage of the

economic situation. Modifications due to changes in raw

material feed.

In some States in Australia, the use of AS3788 is mandatory, and in the others, it isused extensively as a guideline. Thisstandard, in paragraph 6.2.1 makes a clearstatement that all repairs shall be carried outin accordance with the relevant design andfabrication standards. It goes on to state inparagraph 6.2.2 that minor repairs may be


carried out only for those materials andservice conditions, or both, which do notrequire any post fabrication heat treatment.

Standards in other countries, eg. theNational Board Inspection Code in theUnited States, make similar statements. Atfirst glance therefore, any repair, alterationor modification to an item of pressurisedequipment that was originally post weld heattreated after fabrication needs to be postweld heat treated again after repair. This isnot always possible, and efforts over the past20 years have been aimed at finding waysaround this.

Cost Advantages

Whilst most repairs that are contemplatedcould be post weld heat treated, even if itmeans removing the equipment to aworkshop, the cost of doing so becomesprohibitive in today’s economic climate.There are other situations, notably in theNuclear Power industry, where removal ofequipment is simply not possible. Therefore,in terms of risk management, there has beena strong drive to develop techniques toeliminate the need for post weld heattreatment following an on-site repair.

Technical Advantages

This is quite an emotive issue, and likestatistics, any number of conclusions can bedrawn from sets of data. Of the effects thatpost weld heat treatment has on a weld,residual stress will obviously remain if postweld heat treatment is waived, and for creepresisting materials, the full creep strengthwill not be developed. However, there arewelding techniques that can simulate thetempering effect of post weld heat treatment,and there are some claims that mechanicalproperties in the HAZ can be improvedcompared with conventional post weld heattreatment. The particular property that isinfluenced more than any other whenwelding with these alternative techniques isheat affected zone toughness. Most of theso-called temper bead techniques are

primarily designed to give adequatetoughness in both the weld and the heataffected zone and to produce a satisfactoryhardness profile. Whether the roomtemperature, and sometimes sub-zero impactproperties are the most importantconsideration with alloy steels operating inthe creep range is indeed questionable.After having said that, there is no doubt thatfor materials susceptible to reheat cracking,the absence of post weld heat treatment andthe absence of the coarse-grained heataffected zone are definite advantages. Careneeds to be exercised however, and suffice itto say that the justification for waiving postweld heat treatment on technical groundsshould not be confused with economics.

TECHNIQUES FOR WELD REPAIRWITHOUT POST WELD HEATTREATMENT

Attempts to achieve satisfactory results fromweld repairs where post weld heat treatmenthas been omitted are not new. Much of theoriginal work carried out for the NuclearIndustry began in the early 1980s, and CodeCase N-432 for a limited range of materialsunder ASME III in the United States wasestablished in 1986. Since then, the varioustechniques have been refined and adoptedinto other standards such as ASME B 31.1,B 31.3 and even ASME XI. Some of thebetter known techniques will be examined inmore detail.

Half bead technique

This technique was developed using theshielded metal arc (SMAW) process andwas essentially aimed at providing analternative to the use of post weld heattreatment. The technique was originallydeveloped for use in the nuclear industry,but has since become widely used for repairsto piping, headers and turbine casings inconventional power plant and otherindustries. The SMAW technique usuallyemploys the use of increasing diameter


electrodes, starting with a 2.5 mm, then a 3.2mm and finishing with a 4.0 mm.The area to be repaired is preheated to atemperature commensurate with the materialand thickness, and a buttering techniqueused as a first layer with the 2.5 mmelectrode. The purpose is to produce a small,shallow heat affected zone. The next step isto remove approximately half of thethickness of each run by grinding. Thesecond layer deposited using 3.2 mmelectrodes effectively re-transforms thecoarse-grained heat affected zone and firstlayer, and the third pass with the 4.0 mmelectrode further tempers the heat affectedzone. Each subsequent layer transforms andtempers the layers beneath in the normalway.The major advantage of the technique is thatthe toughness of the heat affected zone isconsiderably improved over conventionalmethods, but the disadvantages include thefact that a lot of grinding is required, andaccurate grinding at that. Not only is thistime consuming, but if too much material isremoved from the first layer, the effects ofthe re- transformation may not be assuccessful as might be expected.

Consistent layer temper bead technique

This technique utilises either the SMAW orthe GTAW process, and was developed byEPRI in the early 1990s to ensure thattoughness properties in both the heataffected zone and the weld metal were atleast equal to the toughness properties of theoriginal base material. The techniqueinvolves depositing weld layers that aresufficiently thick that the subsequent weldlayer only tempers the heat affected zonecaused by the first layer. The temperature isnot intended to exceed the AC1 so no grainrefinement occurs, and the effect isessentially similar to a post weld heattreatment. The technique produces a heataffected zone microstructure that consistspredominantly of tempered martensite withsmall amounts of bainite, resulting in goodtoughness properties.

Alternate temper bead technique.

This technique was also developed under theEPRI program but specifically for carbon-manganese and carbon-molybdenummaterials used in nuclear reactor pressurevessel components. It utilised the automaticGTAW process and was an alternative to thehalf bead technique for use in areas of highradiation exposure. The technique involvespreparing the area to be repaired so that atleast six buttering layers can be performed.A preheat of 1500 C minimum is applied,and the heat input of each layer is controlledto within 10% of that measured in theprocedure qualification test. If more than sixlayers are used, the same control over heatinput is required. The final step is a lowtemperature post weld heat treatment at 230– 2900 C.The aim is to produce tempering usingseveral weld layers so that each subsequentlayer penetrates the previous layer todevelop overlapping temperature profiles.However, the AC1 is exceeded, resulting inre-transformation and grain refinement ofthe heat affected zones. Sometimes,different heat inputs are used for the firstthree layers, starting with a low heat input tominimise the extent of the heat affected zonein the parent material. The heat input for thesecond layer is increased to give a slightlythicker deposit whilst still re-transformingthe parent heat affected zone, and the thirdlayer is still thicker to ensure tempering onlyof the heat affected zone.

Controlled deposition technique.

This technique resulted from special caseswhere creep embrittlement and reheatcracking were potential problems duringrepair, and was aimed at specific materialsused in conventional fossil-fired stations. Itis also a SMAW technique, and uses strictlycontrolled ratios of heat input between oneweld layer and the next. The heat input forthe second layer is 1.3 to 1.8 times higherthan for the first layer, and is designed toproduce grain refinement and tempering inthe original heat affected zone. The ratios


need to be experimentally verified for eachmaterial to be welded. The techniqueeliminates the coarse grained region withinthe heat affected zone where creep crackingand reheat cracking have caused problems inthe past.

DECISIONS ON REPAIRS

Most decisions regarding repairs topressurised equipment in large, complexplant are made using risk managementprinciples. On the one side of the riskmanagement equation is consequences offailure, and this can be financial,environmental or health and safety related.Once an item of equipment has been foundto contain a flaw of some description, theconsequences of

Doing nothing, Repairing the flaw, Replacing the item,

need to be considered. If the decision is torepair, the consequences of the repairmethod also need to be evaluated. Thedecision to repair must be made by theOwner/Operator, who should also take fullresponsibility for the consequences of thedecision. Decisions on the repair method aremore difficult. This is because in today’sclimate, plant maintenance is oftensubcontracted, and the subcontractor doesnot always have the knowledge to advise onrepair consequences. The issues here tend tobe as much legal as technical, but again, theOwner/Operator must take responsibility forensuring the correct advice is sought. Incertain industries, there are sufficientInstitutions (eg EPRI and UKAEA) andpeople within those Institutions with theknowledge and experience to advise on thelikelihood of success. In other industries,this is not the case, and more is said of thisissue below.On the probability of failure side and theattendant risks, repairs where post weld heattreatment has been omitted is reasonablymature technology. In the power generation

and nuclear industries, there is sufficientevidence to show that carbon-manganese,carbon-molybdenum and chromium-molybdenum steels operating at hightemperatures can be safely repaired. Oneoutstanding problem is the question ofproperties when a defect is found in atemper bead repair and a fracture mechanicsassessment has to be carried out. A reviewof the literature shows that very little K1Ctesting has been carried out. The charpyimpact testing that has been done has beendone on the heat affected zone. If a defect isin the weld repair, this immediately takesany calculation into the realms ofprobabilistic fracture mechanics.

In other industries such as Petrochemical,the technology is probably less mature.There are some instances where the temperbead technique has been used quitesuccessfully in non-sour service. WheneverNACE MR 0175 restrictions apply, repairswithout post weld heat treatment havealmost universally resulted in cracking ofthe repair at a later date. This is quiteobviously the result of residual stress.

QUALIFICATION OF WELDINGPROCEDURES

In just about every world Code or Standard,the application of post weld heat treatment isan essential variable. This means that theapplication of post weld heat treatment to aprocedure or the removal of it from aprocedure requires re-qualification. This isbecause post weld heat treatment affects themechanical properties of the weld, and thisis the whole purpose of procedure testing.There are even more restrictions if heattreatment is above the lower transformationtemperature, but this guidance note has beenlimited to the stress relieving aspect of postweld heat treatment.Caution must also be exercised whenworking with the ASME code. The systemof supplementary essential variables can beconfusing, but basically, these are invokedwhenever notch toughness requirements are


specified. In the case of post weld heattreatment, there is an additional essentialvariable that specifies that the procedurequalification test shall be subjected to postweld heat treatment essentially equivalent tothat to be encountered in production,including at least 80% of the aggregate timeat temperature.Taking a practical example of this, a 50mmthick procedure qualification test platerequiring impact testing and representing a50mm shell plate would normally only have

been held at temperature for 2 hours (ie.1hour/ 25mm). In practice, that same vesselmay have different shell thicknesses or evena tube plate, and the 50mm shell may havereached the holding temperature long beforethe rest of the vessel. Quite often, the shell isat temperature for 5 or 6 hours, so theprocedure qualification test needed to be attemperature for 6 hours x 80%, ie 4 hours 50minutes and not the 2 hours it received.Under these conditions, re-qualificationwould be necessary.

DISCLAIMER

While every effort has been made and all reasonable care taken to ensure the accuracy of the material containedherein, the authors, editors and publishers of this publication shall not be held to be liable or responsible in any waywhatsoever and expressly disclaim any liability or responsibility for any injury or loss of life, any loss or damage costsor expenses, howsoever incurred by any person whether the purchaser of this work or otherwise including but withoutin any way limiting any loss or damage costs or expenses incurred as a result of or in connection with the reliancewhether whole or partial by any person as aforesaid upon any part of the contents of this Guidance Note. Shouldexpert assistance be required, the services of a competent professional person should be sought.

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