Weld Zone Cracks in Repaired 2 1/4 Cr-1 Mo Ammonia Converter Leaks occurred in ammonia converters in two separate plants with one converter leak- ing after four years of service and the other leaking after eight years. The converter that leaked after four years was repaired by completely removing the failed weld and rewelding. This repair weld developed cracks after nine months of service. G.R. Prescott Metallurgical Consultant, Newport Beach, CA 92663 B.J. Grotz Brown and Root Petroleum and Chemicals, Alhambra, CA 91803 Background After four years of service, a hot-wall type ammonia converter in France leaked at the closing seam between the top head and shell. The results of an investigation into this problem was reported at the 1991 AIChE Ammonia Safety Symposium and published in the 1992 AIChE Technical Manual, Vol. An almost identical failure occurred in an ammonia converter with the same design and service conditions at a German plant after eight years of service. The results of an investigation of that problem by BASF was also published in the 1992 AIChE Technical Manual, Vol. 32.(2) The French converter was sent to a fabricating shop where the failed weldseam was completely removed and rewelded. After nine months of service, cracks were detected in the repaired weld. All of the aforementioned converter welds, both the original and repair, were made with the same agglomerated flux combined with a low temperature dehydrogenation heat treatment, LTDHT. The inadequacy of the LTDHT was established in the previous paper (1) based on a thorough review of the development work by Kobe (5). This welding procedure is the common denominator unking these converters together. It should be noted that C F Braun prepared the specifications for the original vessels but was not involved in procurement and was not involved in the metallurgical investigation of the closing seam weld failures until after the repairs were completed. This combination of flux and a LTDHT leaves a high residual amount of hydrogen in 116
Transcript
Weld Zone Cracks in Repaired2 1/4 Cr-1 Mo Ammonia Converter
Leaks occurred in ammonia converters in two separate plants with one converter leak-ing after four years of service and the other leaking after eight years. The converterthat leaked after four years was repaired by completely removing the failed weld and
rewelding. This repair weld developed cracks after nine months of service.
G.R. PrescottMetallurgical Consultant, Newport Beach, CA 92663
B.J. GrotzBrown and Root Petroleum and Chemicals, Alhambra, CA 91803
Background
After four years of service, a hot-walltype ammonia converter in France leaked atthe closing seam between the top head andshell. The results of an investigation into thisproblem was reported at the 1991 AIChEAmmonia Safety Symposium and publishedin the 1992 AIChE Technical Manual, Vol.
An almost identical failure occurred inan ammonia converter with the same designand service conditions at a German plantafter eight years of service. The results of aninvestigation of that problem by BASF wasalso published in the 1992 AIChE TechnicalManual, Vol. 32.(2)
The French converter was sent to afabricating shop where the failed weldseamwas completely removed and rewelded.
After nine months of service, cracks weredetected in the repaired weld.
All of the aforementioned converterwelds, both the original and repair, weremade with the same agglomerated fluxcombined with a low temperaturedehydrogenation heat treatment, LTDHT.The inadequacy of the LTDHT wasestablished in the previous paper (1) basedon a thorough review of the developmentwork by Kobe (5). This welding procedureis the common denominator unking theseconverters together. It should be noted thatC F Braun prepared the specifications for theoriginal vessels but was not involved inprocurement and was not involved in themetallurgical investigation of the closingseam weld failures until after the repairswere completed.
This combination of flux and a LTDHTleaves a high residual amount of hydrogen in
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the weld. When the weld is cooled the hardheat-affected zone, HAZ, is susceptible tohydrogen induced cracking. In the previouspaper(l), the conclusion was reached thatthe procedure produced a linear array ofmicrocracks in the HAZ that were notdetectable by conventional inspectionmethods. These microcracks, over a periodof time, developed into macrocracks thatultimately propagated through the wall.
This conclusion is supported by thefact that eleven other converters ofessentially indentical configuration andservice conditions have been thoroughlyinspected and no cracks were found. Theseconverters are exposed to ammonia synthesisgas at 400°C (750°F) and the hydrogenpartial pressure is in the range of 115-126bar. The ammonia content in the synthesisgas varies from 11 percent in the lastconverter of a two converter plant to 15percent in a three converter plant. Theservice life of these converters range up to16 years. None of these converters werewelded with this particular hygroscopic fluxcombined with an inadequate LTDHT.
New Investigation
When the repaired weld developedcracks after nine months of service, thecompany ordered a replacement converter.The new replacement converter was weldedwith a revised welding procedure thateleminated the use of a LTDHT. Instead, anintermediate postweld heat treatment,IPWHT, at 593°C (1100°F) for 1 hour wasused to remove the hydrogen. When theoriginal converter was removed from service,an opportunity was presented to investigatethe cause of cracks that developed in therepaired weld.
The extent of cracking on the interiorsurface of the repair weld is shown in Figure1. The sketch shows that cracks wereintermittent and occurred predominately onthe head side with a few short cracks on theshell side. All cracks are located in the heat-affected zone (HAZ). The cracks weredetected using ultrasonic inspection from theoutside surface. After nine months ofservice, the cracks were 12 mm deep. After21 months, the cracks had propagated to 27mm but the growth rate had slowedconsiderably.
The sample obtained for thisexamination was located in a zone free ofdetectable cracks as shown by the sketch inFigure 1. A photograph of the sample isshown in Figure 2. The sample was obtainedas one solid piece and this photo shows somepreliminary cuts made for the purpose ofsubsequent nondestructive testing,metallurgical tests, and metallographicexamination. The cut sections are identifiedas shown in Figure 2.
The overall plan of the investigationincluded the following:
1. A detailed ultrasonic examination of thesolid block and the subsections was madeusing various sensitivity settings in anattempt to detect small internal cracks.
2. A complete metallographic examinationof the weldment was made with specialemphasis on the heat-affected zones. Allultrasonic indications were carefullyexamined.
3. Hardness traverses were made to detectthe presence and extent of nitriding onthe inside surface.
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4. A scanning Auger microprobe was usedto determine the amount of nitrogenpresent at various distances from theinside surface.
ultrasonic
The overall objective of the ultrasonicexamination was to see if the samplecontained any type of detectable defects.More specifically, the purpose was todetermine if microcracks, if they existed inthe heat-affected zones, were detectable byusing various techniques. The base caseutilized the minimum requirements of theASME Code for ultrasonic testing. Nothingwas detected using Code requirements, butseven indications were recorded at a highlevel of sensitivity. Figure 3 shows thelocation of two of the indications, and Figure4 shows the instrument settings and display.
Metallographie
All of the ultrasonic indications wereinvestigated and no cracks were found.Apparently the indications were caused bysome microstructural aberrations such aslarge grains in the HAZ. This is not unusualat high gain settings.
Figure 5 shows a cross-section of theweld between the top head and the shell.Most of the weld was made from the outsidesurface, followed by back chipping andwelding from the inside. The entire weld andHAZ of each cut section was scannedmicroscopically in an effort to find hydrogeninduced microcracks. Cracks were found inthe weld and HAZ near the inside surface.No microcracks were found in the portion ofthe weld or HAZ made from the outside.Microcracks were found in the heat-affectedzone of the last weld passes made from the
inside surface and microcracks were alsofound in the weldmetal.
Figure 6 is a compositephotomicrograph of the weldmetal at theinside surface. This view is perpendicular tothe weld surface after it has been grounddown about 1/16-inch. Figure 6A is a fullyetched view of the weldmetal and somecracks are visible. Repolishing the etchedsurface clearly delineates the microcracks asshown in Figure 6B. The technique ofpolishing, etching, and repolishing is a veryuseful procedure when looking formicrocracks. Figure 7 is a cross-sections ofweld sample E. This view shows the depthand interdendritic nature of the microcracksin the weldmetal. All of these microcracksare below the surface.
Figure 8 is a cross-section of the weldat the inside surface of sample D showingextensive microcracking in the HAZ. Figure9 is a cross-section from another location,sample E, showing a similar group ofmicrocracks in the HAZ of the weld. Asmentioned previously, these microcrackscould not be detected by ultrasonicexamination over a wide range of sensitivitysettings. It is an important observation thatnone of these microcracks are connected tothe surface which disproves the hypothesisby BASF that surface nitrides are a causativefactor(2). In addition, the microcracksextend to a depth well below the nitridelayer. It also should be noted that thissample of the weld did not contain anyvisible or detectable cracks. As shown inFigure 1, the sample provided for thisinvestigation was located betweenmacrocracks visible on the inside surface.This strongly suggests that the actual cracksfound in the vessel were generated in zoneswhere the microcracks existed in greater
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density and extended to greater depths.Even with more extensive microcracks, thenormal non-destructive examinations afterfabrication are not capable of detectingmicro-defects.
Nitriding
Hardness traverses and Auger analyseswere used to determine the extent ofnitriding at the inner surface. The questionof nitriding is an important considerationsince both BASF (2) and the French WeldingInstitute (3) concluded that a thin layer ofnitride was instrumental in the initiation andpropagation of the failures of both the BASFconverters and the original failure in theFrench converter.
As a comparison, a hardness traversewas made on a piece of the shell course fromthe original failure of the French converter.Both the French Welding Institute (FWI) (3)and Creusot-Loire (4) reported highhardness values at the inside surface. TheFWI report shows maximum base metalhardnesses on the three French convertersranging from 223 HV to 406 HV.Curiously, the high hardness reading was onR101, the first converter in series, with thelowest ammonia content. Creusot-Loire hada sample from R101 and reported interiorsurface hardnesses up to 420 HV. All ofthese hard layers were about 0.5 mm deep.BASF presented data on their converterfailure (2) that showed nitrogen enrichmentat the inside surface to a depth of 1.5 mm.Hardness readings in this zone were as highas 367 HV.
Our traverse on base metal from theoriginal vessel, shown in Figure 10, confirmsthat the hardness is relatively high to a depthof about 1.0 mm. Similar traverses on basemetal, HAZ, and weldmetal from the most
recent sample, D, are also shown in Figure10. After almost two years of operation, theinside shows a moderate amount of increasedhardness up to a depth of about 0.5 mm.The weldmetal is inherently harder than thebase metal and the inner surface does showsome increase in hardness over the bulksample. The pattern of hardness suggeststhat the hardness gradient might beassociated with a temperature gradientduring the local PWHT.
Creusot-Loire (4) examined a sampletaken from the first converter and reported anitrogen content of 0.18 percent at a depthof 0.1 mm from the inside surface. At 1.25mm the nitrogen content had decreased to0.01 percent which represents the nominalnitrogen content of the steel. BASF (2)reported 0.24 percent nitrogen at a depth of0.1 mm and dropping to about 0.01 at adepth of 1.5 mm.
Auger analysis of the samples receivedfrom the original and repaired vessel isshown in Figure 11. At a depth of 0.1 mm,the nitrogen content is 0.22 percent in theoriginal vessel surface and 0.23 weightpercent nitrogen in the repaired converter.Both cases level off to the nominal nitrogencontent of the steel at depths between 1 and1.5 mm. The depth of nitriding is greater inthe original surface as compared to therewelded surface. This could reflect thelonger time in service or variations in theanalyses, or because the degree of nitridingvaries from one location to another. Asshown in Figure 10, the maximum hardnessvaries widely from one location to another.
Discussion
In the paper presented previously (1).the conclusion was reached that the cause offailure was due to the presence of
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microcracks in the heat-affected zone. Overthe four year period, these microcracks grewand coalesced into macrocracks thatultimately caused leakage. The microcrackswere caused by residual hydrogen which wasthe result of using a hygroscopicagglomerated flux, in conjunction with aninadequate intermediate dehydrogenationheat treatment. In order to. establish theinadequacy of the LTDHT, the originalpapers describing the development ofLTDHT were reviewed in detail and thereview proved conclusively that the LTDHTwas inadequate even for typical conditionsdefined by the manufacturer of the flux. Ifthe flux were not stored properly, or handledproperly on the shop floor, then theconditions would be much worse thantypical.
Conclusions
The results of this investigation showonce again the risk of using an inadequatedehydrogenation heat treatment on a weldmade with a hygroscopic agglomerated flux.The LTDHT used was 280C (536F) for 2hours. The original work (5) done todevelop a satisfactory LTDHT for this weldwould require at least 10 hours at thistemperature. This LTDHT might beadequate for a welding flux that imparts 1 to2 ppm hydrogen content in the weld. It isnot adequate for a flux that typically puts 7to 8 ppm hydrogen in the weld. Fabricatorswho use LTDHT should familiarizethemselves with the correlations developedby Kobe (5) and use the information todesign appropiate dehydrogenation heattreatments. In addition, they should followthe directions of the flux manufacturer withregard to keeping the flux properly driedduring storage.
Other ammonia converters in fourdifferent plants of essentially the same designand in the same service for up to 16 yearshave been carefully inspected and no crackswere found. These other converters werewelded with either low hydrogen fused fluxand a 593°C (1100°F) IPWHT or were stressrelieved in a furnace. The two sets ofconverters in the French plant and in theGerman plant are the only ones welded withagglomerated flux, LTDHT, and given localstress relief of the closing seam..
The portion of the weld made from theoutside is free of defects in the samplefurnished for this investigation. Thesituation in other areas of the repair weld isobviously much worse as evidenced by thefact that cracks were found as deep as 29mm after two years of service. Due to thelimitations of the sample, it is not possible tospeculate with regard to future crackpropagation over an extended period of time.
Gouging and back welding the insideportion of the weld is quite another matter.Maintaining a preheat of 250C (482F) withwelders inside the vessel is a problem of longstanding. The tendency is to let the preheatcool down below the minimum requirements.The compensatory heating is reducedbecause these are the last weld passes madein a short period of time. The weld is thengiven the LTDHT and cooled to roomtemperature. If the hydrogen content has notbeen sufficiently reduced, the hard weldheat-affected zone is subject to hydrogeninduced cracking on cooldown. Thecracking can be macrocracks and/ormicrocracks. The macrocracks are detectedand repaired. Microcracks are not detectedas we have shown in this investigation, andthese can eventually coalesce intomacrocracks if the density of microcracks issufficient to promote propagation. The
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microcracks found in the HAZ of the lastweld passes are the cause of toe cracks.
A contributing factor to the crackingproblem is the existence of high residualtensile stresses at the ID of the weld andHAZ resulting from the local postweld heattreatment. These residual tensile stresses aresafely dissipated in sound welds by a smallamount of plastic deformation under normaloperating conditions.
The cracks that were detected after 9months of operation propagated fairlyrapidly for about 14 months, thenpropagated at a slow rate for the followingyear. The rate is shown below.
DateAugust 1990
January 1991
July 1991January 1992
August 1992
Crack Depth12 mm - after 9months service
25 mm - after 14months service
27 mm - slow growth28 mm - little or no
growth29 mm - little or no
growth
The above growth rate fits themicrocrack pattern that we observed in ourweld sample. The microcracks are adjacentto the last weld passes made from the insidesurface. The microcracks apparently grewand coalesced into a toe crack. The rate ofpropagation slowed when it reached the endof the zone of microcracks. If the toe crack
intersects a zone of internal microcracks dueto an inadequate LTDHT, the propagationrate increases leading to ultimate failure.
Literature Cited
1. Prescott, G. R., Weld Failure in a 2l/4Cr - IMo Ammonia Converter,AIChE Technical Manual, AmmoniaPlant Safety, Vol. 32, 1992.
2. Wagner, G. H., Heuser, A., and Heinke,G, Hydrogen Attack in 2 l/4Cr - IMoSteel Below Nelson's Curve, AIChETechnical Manual, Ammonia PlantSafety, Vol. 32, 1992.
3. Institute de Soudure, Paris, France,Documents 2 and 3, Rapport Technique,March 26, 1990.
4. Etude Métallurgique De La FissurationDu Reacteur RI01 De La Boucle DeSynthese NH3 Société Chimique DeGrande Paroisse, Rapport Technique No89144C, Creusot Loire Liquidation,Nov. 1989.
5. Takahashi, E. and Iwai, K., Omission ofIntermediate Postweld Heat Treatment(PWHT) by Utilizing Low-TemperaturePWHT for Welds in Pressure Vessels,STP 755 ASTM, 1982.
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DISCUSSION
L. Gens, BASF: Mr. Prescott, I think you made itvery clear that you don't agree with the BASF the-ory that was presented by Mr. Wagner during the'91 Ammonia Safety Symposium. After furtherinternal investigations and additional experimentssince '91 by our metallurgical department, we stillstick to our stated theory that owing to nitridingfrom the surface, the material becomes in principlesensitive to hydrogen attack within a relatively thinlayer. If sufficiently high stress is applied, crackingmust be expected. The cracking propagates slowlysince it depends upon the diffusion of the nitrogenin the steel and on the transformation of the car-bides. Commenting on your statement about otherammonia converters in four different plants ofessentially the same design and so on, as far as weknow the other converters were all built accordingto ASME code. The French followed the CODAP,and the BASF converters were built according tothe German code A.D. Regelwerk. This really indi-cates an essential difference, for instance, in wallthickness and stress level.Prescott: Concerning your first comment on thenitride formation and the hydrogen attack, oneproblem that we have with that theory is that it'svery difficult to arrive at a mechanism or a reactionmechanism that limits this attack to just one zonein this entire converter. Why would it only beoccurring in weld metal or in heat-affected zonesand not in base metal? In reference to the codes, ofcourse, the vessels built in the U.S. were built tothe ASME code, yours were built to the Germancode, the French to the French code, and NSM's orNorsk Hydro I believe were built to the Dutchcode.Guns: No, I believe they were all built to ASMEcode, also.Prescott: Well, they also had to meet the Dutchcode. We looked at these differences in terms ofwall thickness, stress levels and so on, and it reallydidn't seem to us to be significant. And it certainlyshouldn't be significant if it were hydrogen attack.Gnus: I think we have just a different opinionthere.Prescott: We certainly do; we'll fight on.
S. Thomas, Pequiven: Mr. Prescott, was a hydro-gen baking applied before the repair of thesewelds? If not, can you explain the reasons?Prescott: A hydrogen baking was applied, definite-ly, and extensively.K. Nassauer, Babcock-Borsig: We have also expe-rienced during fabrication or manufacturing of 21/4 Cr-1 Mo material hydrogen induced cracks,and we have also done extensive tests with differ-ent kinds of sub-arc welding procedures withagglomerated fluxes. We found, for example, thatwith the so-called tandem wire AC current proce-dure, the hydrogen is much higher in the weld thanwith the so-called single wire DC current proce-dure. This is due to the fact that the humidity, thesteam or the humidity, is dissociated into hydrogen,and this is part of the weld. For 21/4 chrome mate-rial we only weld now with the so-called singlewire DC current process to avoid hydrogeninduced cracking during welding. Even if youcheck the hydrogen content of the agglomeratedflux, this is only one possibility and only one point.You also must look very carefully at the weldingprocedure itself. As I mentioned, we did extensivetests with the German "Bundes Anstalt für MaterialPrüfung" or "BAMP" and found this a very impor-tant fact apart from, of course, the very extensiveKobe research work, which did not focus much onthe welding process itself.Preseott: While I appreciate your comment, Ibelieve that the purpose of the submerged-arc flux,of course, is to protect the weld from the environ-ment and the humidity. Secondly, all agglomeratedand bonded fluxes are not alike. Some pick upmoisture very readily or very slowly. Of course,the ultimate flux was the original fused flux. Idescribe in my paper where all the ingredients aremelted and then it's ground up into a flux, which istotally nonhygroscopic. It's only when you get intobonded and agglomerated fluxes that you find thatsome are very or slightly hygroscopic. So, youmust look at flux as one of the principal sources ofthe hydrogen. Other factors can enter into it, Iagree.Appt Your theory does not explain that intensityof damage or number and depth of cracks are
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increasing with the number of converters. It's lessin converter 1 and more in converter 2. And if youhave three, it's more in converter 3. And this corre-sponds exactly to the increasing content of ammo-nia.Prescott: You're absolutely right, and one possibil-ity is that, as you mentioned, we found cracks in allthree converters in the French plant. You foundcracks in both of your converters, and the onelooking at the highest concentration of ammonia isthe one that leaked. And it's certainly possible thatthe ammonia is participating in accelerating thepropagation of a crack once it's formed. That'sanother mechanism that you have to consider, andwe don't have any experimental work to back upeither one of these.AppI: Some work must be done to get a final clari-fication.Prescott: I think so.J.G.MacDonald, ICI Engineering: Some of yourinitial slides showed that different etching charac-teristics were found adjacent to the crack, and alsothe internal surfaces. Have you considered anycontribution to cracking by solid solution strength-ening by nitrogen?Prescott At 400°C, we haven't. In our last slide,the Auger analysis shows the nitrogen dropping offto normal steel values at about one mm in depth.MacDonald: Were they typical of what you wouldexpect for nitrogen levels?Prescott: Yes.S. Thomas: Mr. Prescott, those repair welds devel-oped cracks that propagate fairly rapidly; so, wehave cracks after nine months service. If we aretalking about hydrogen attack, I have my concerns.21/4 chrome is a more resistant material, so I don'tthink that hydrogen attack can develop in such ashort time. Probably, you cannot discard hydrogenembrittlement or the heat treatment applied as therelated cause of this failure.Prescott: I would also like to mention that thesemicrocracks originate at the surface on the samplethat I showed you with no defects detectable bynormal methods. They are all below the surfaceand well below the nitride layer. This tends to sup-port our side of the argument.C. Miola, Snamprogetti: According to your experi-ence on this kind of weld, especially in service, theultrasonic attenuation method can help find outreally if there is hydrogen absorption or not.
Prescott Experts tried every possible setting ontheir instrument to detect microcracks of this sizeon the sample that I showed you, and they wereunable to do so.Miola: The ultrasonic attenuation method is a verysophisticated new method in order to detect hydro-gen absorption.Prescott: You're right. You can detect hydrogenattack by very sophisticated methods, but thesemicrocracks are not detectable even by those meth-ods, I believe. What is really important here is thatthe requirements of the codes are far below thesensitivity and settings attenuation that you're talk-ing about. So, the U.S., German, French, andDutch codes do not detect these microcracks. Theycould have existed at the time these vessels werefabricated and shipped, and that's how we got intoa leaking situation.J. Korkhaus, BASF: You showed us some slideswith macrocracks. I would like to know how youexplain the microstructural change with an obviousdecarburization along the cracks, if the mechanismof macrocrack formation is the linking up of coldcracks?Prescott: All of these cracks, subcracks, and asso-ciated cracks are intergranular in nature. Of course,when this crack opens up and becomes exposed tothe process gas which contains ammonia — this isvery reactive metal -- it nitrides, just like the sur-face of this.Korkhaus: It's not only nitrided, it's also decar-burized.Prescott: Well, it's nitrided. It has to be. However,it's not hydrogen attack; it's just nitride.Korkhaus: That's the point. It's a question of themetallurgical reaction pattern and the velocities ofthe different reactions.Prescott: It's only a surface effect.Korkhaus: I don't believe it.Prescott: Well, I do. So, it goes on and on. Youhave to decide whether you use a low-temperaturedehydrogenation heat treatment or a conventionalintermediate heat treatment.Korkhaus: I have some problems with your rec-ommendation concerning the flux. Some years agoI made some hydrogen measurements on weldmetal molten down using different fluxes includingfused fluxes. During these measurements hydrogencontents up to 10 ppm were found for weld metalproduced with fused fluxes. The hydrogen ingress
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into the weld metal from the fused flux depends onthe way this flux has been produced. The use offused fluxes is not generally a guarantee for hydro-gen contents of 2 to 3 ppm in the weld metal.Prescott: I'm only saying that normal fused fluxesused for Cr-Mo steels normally have 1 to 2 ppm.This has been substantiated and supported by anextensive amount of work done by both Kobe,Hitachi, and Japan Steel. I don't know of any otherpublished paper that comes close.
Korkhaiis: The way of production is decisive forthe hydrogen ingress by fused fluxes into the weldmetal. Fused fluxes sometimes are produced bypouring the melt into water. During this step, theflux picks up moisture which acts as hydrogensource and which you cannot get out.Prescott: That goes back to what I said. Make surethe fabricator knows what he has in the weldbefore he designs his heat treatment. If he doesn'tknow, he can't design one.
G.R. Prescott B.J. Grotz
Con verier R-103 - Inside Surface
Cracks Head
Braun Sample
Figure 1. Location of cracks adjacent tohead-to-shell weld.
Shell
Outside Surface
Head
Shell
Figure 2. Overall view of weld sample.
Scanning SideIndicationSound PathDist. To Ref.DepthLengthAngleFrom Side 2
