ISIJ International, Vol. 30 (1990), No. lO, pp. 854--861

Assessmentof Remaining Life of

Miniature Creep Rupture Test

Fossil PowerPlant Parts by meansof a

YOShikuni KADOYA,Toru GOTO,ShingO DATE,TakayOShiYAMAUCHl,TOmikaneSAIDAand Tetsuro

SADA1)

TakasagoResearch &DevelopmentCenter, Mitsubishi Heavy Industries, Ltd., Shinhama, Arai-cho, Takasago, Hyogo-ken, 676 Japan.

1)KobeShipyard &Machinery Works, Mitsubishi Heavy Industries, Ltd., Wadasaki-cho, Hyogo-ku, Kobe, Hyogo-ken, 652 Japan.

(Received on December28, 1989, accepted in the final form on April 20, 1990)

In order to validate a procedure for creep rupture tests using miniature specimens, a sampling technique and a methodof making the miniature specimenswere investigated. Finally, the use of the iso-stress creep rupture test on the minia-

ture specimens to predict the remaining creep life of Cr-Mo-V, 21/4Cr-Mo and 11/4-1/2Mo steel wasexamined.

Thecreep curve, rupture life and ductility of the miniature specimenstested in argon were similar to those of conven-tional sized specimensin air. Thefracture modeof the miniature specimenswasalso consistent with that of conventional

sized specimens.

Ona graph showing tremperature Tvs. Iog time to rupture t., the creep rupture data were represented with an iso-stress

line, respectively. Each iso-stress line shifted parallel with the stress level.

Moreover, the sampling device that wasdeveloped wasapplied to the operating plant components. Sampleshave

been successfully removedfrom it and were obtained without affecting the material properties around the core hole.

KEYWORDS:creep; miniature specimen; stress-rupture test; iso-stress; Iow alloy steels; sampling technique; remaininglife.

1. Introductron

Judging from the recent trend in powerdemand,itis expected that presently operating_ power plants, orso-called aging power plants, in particular, whoseop-erating periods are being extended, will have to bear

even heavier loads than ever before as middle-powerand peak-power supply plants. In addition, these

aging power plants are supposed to carry out theplant life assessment in order to cope with the exten-sion of the period between the two regular mainte-

nance inspections.

Under these circumstances, in order to maintainand operate these aging power plants efriciently,

while ensuring their safety and reliability, the exten-sion of plant life is desired nowmore than ever. Akey ingredient in plant life extension is the remaining-life-assessment technology.

For the remaining-life-assessment, the history-basedcalculation methodhas cometo be widely used. Thismethod, however, often fails to provide suflicient ac-

curacy becauseof the uncertainty of various importantinformation such as operating conditions and materialproperties specific to the component. Further studies

to improve the accuracy andconfldence of remaining-life-assessment need to be made.

To solve this problem, the NonDestructive Evalua-tion (NDE) method is considered effective. This

methodis able to assess the remaining life by directly

observing or inspecting the deterioration anddamageof the material without being destructive to it. A

variety of the NDEmethods, therefore, have been de-

veloped and are being applied actively to operatingplant components.1,2)

A destructive test done by sampling is another

method of assessment. This method is somewhatadvantageous in that the remainiig life can be direct-

ly obtained by actually rupturmg the specrmensThrough the development of a sampling technique

and a miniaturization technique, this method is ex-pected to greatly supplement the NDEmethod in

estimates of remaining life. For example, application

of this method to boiler componentsis nowbeing in-

vestigated,3-5) because of' the easiness of sampling andrecovering such as weld repair.

Weare nowexamining to apply this methodto the

materials of turbine componentsas well as boiler com-po_nents.6) So far, however, further development of

the sampling technique as well as accumulation of

various data is necessary before this method can beefficiently applied to operating plant components.

This study aims to investigate a methodof asscssing

the remaining life of' materials used ih fossil powerplant componentsby meansof a miniature creep rup-ture test. Asampling device wasdeveloped and data

was prepared covering a long time period. The va-lidity of this methodis discussed.

2. Materials and Experimental Procedure

2. I .Materials

The materials were Cr-Mo-V steel, 21/4Cr-Mo

854 C1990 ISIT

ISIJ International, Vol. 30 (1990), No. lO

Table 1. Chemical composition and heat treatment of test materials.

No. MaterialsChemical composition (wto/o)

C Si Mn s Ni Mo VCrHeat treatment

(1) Cr-Mo-V steel O30 O25 O75 O008 O008 O35 1 23 1.20 O.26970'C x22h- Fan cool

665'C x67 h~Furnace cool

(2) 21/4Cr-Mo steel O12 0.23 0.55 0.009 0.008 O. 14 2.22 O96 O. Ol 1920"Cx 8h- Air cool

705'CX15 h- Furnace cool

(3) 11/ Cr /Mosteel O10 O60 O55 0.015 0.013 O14 1 20 O59 0.007920'CX1.5h -Air cool

720'C > 3h- Furnace cool

steel and 11/4Cr-1/2Mo steel, all of which are usedin the key components of power plants. Table 1showsthe chemical composition and heat treatment ofthe test materials. Material (1) was sampled fromthe periphery of a turbine rotor which hadbeenusedfor 100 OOOh. Material (2) was sampled along therolling direction with a thickness of 150 mm. Mate-rial (3) wassampled at a right angle to the directionof rolling with a thickness of 40 mm. This steel wasalso submitted to welded joint tests in addition to thetest materials.

2.2. Preparation of Miniature Specimens

The miniature specimens were produced basically

through a machining process from the sample to their

final forms. First of all, using the conventional sized

specimens with gage diameters of 8 and 10mmasreference, small size specimenswith gagediameters of

2and 4mmwere produced. Further, a creep rup-ture test on the optimumspecimens was conductedand the rupture within the gauge length was con-flrmed.

In addition, the potential for using miniaturizedsamples has beenexplored. Inview of the limitations

of the size and geometry of the sample, welding-typeminiature specimens in which a specimen grip is

welded to the specimen, were also attempted. Elec-

tron BeamWelding (EBW)was employedbecause inthis welding method, temperature control at the timeof welding is easy and because the heat influence areais smaller.

2.3. Creep Rupture Test by Miniature Specimens

The iso-stress creep rupture test was adopted be-

cause it. was considered the best for the acceleratedtest condition.4) This testing method, based on theextrapolation methods of Manson-Succopparame-ter7) or, of Manson-Haferdparameter8) amongvariousextrapolation methods of creep rupture life, utilizes

the fact that log time to rupture t. and temperatureTare in linear relation with each other in cases wherestress is constant. Based on this relation, by extra-polation of the results to service temperature an esti-

mate of the remaining life is made.A typical value was selected for testing stress (1

within a suitable Jfange for assessment of operating

plant components. The values also varied accordingto the type of steel. The testing temperature Twasdivided into 3stages aboveservice temperature withinthe range, in which temperature acceleration wascon-sidered possible.* Also, the creep rupture test wasconducted within a maximurnrupture time t. of upto 3> 103 h.

Thesamecreep rupture test wasconducted on con-ventional sized specimens and under the samecondi-tions as that of thc miniature specimens. Fromthis

test, whether or not the data obtained from the iso-

stress creep rupture test wereexpressed linearly on thelog time to rupture t. vs. temperature Tdiagram wasexamined. The reliability of the miniature speci-

menswas also examined.

3. Results andDiscussion

3. 1. Preparation of Miniature Specimens

Various types of miniature specimens, whosemini-

mumdiameter was 2mm,were produced using botha machining process and welding process by EBW.The size was madeas small as possible to meet theconditions for applicability that the specimendid rup-ture within the gauge length. The miniature speci-

menswere also necessary in order to judge the sound-

ness o_f the microstructure within the gauge length.

In all cases, a size of 2mmdiameter and 30 mmlongfor the miniature specimenswasapplicable as shownin Fig. l.

3. 2. Developmentof a Miniature Creep Test Machine

In general, it is a well knownfact that the creeprupture test in air conducted on a miniature specimenmadeof' a low alloy steel is strongly affected by oxida-tion. Therefore, it is better to conduct the creep test

of miniature specimens in an atmosphereof an inert

gas such as argon.5)

A simple-structured miniaturized creep test ma-chine was developed taking this fact into due con-sideration. Fig_

. 2shows the appearance and schemaof' the miniature creep test machine. This creep ma-chine is I .8 min height and 0.6 mboth in width andin depth. Having a maximumloading capacity of250 kg, it also features miniaturization and conve-nience in handling. The characteristics of this creep

* The testing temperature range was set above the temperature in which each type of steels is used in the operating plant.

set below the tempering temperature.

It wasalso

855

ISIJ International, Vol. 30 (1990), No. lO

:aLu,

aOOLOFig. l. Optimized miniature test specimen dimensions.

15

10

5

O

Air, 2mmtr =55lhr

650"C&5kglmm2o : Miniature (2~mm)o : Conventional (10cmm)

el'rel

oc~:~e~co

e,

el

,/,

.~lel'

Airl lomm Jtr=1076hr

e

loel~Ar atmosphere2mm,tr=1250hr

1~~ Cr- 112Mosteel

Miniaturetest specimen

\lYr'fEulre:ctric

furnace

Ar atmosphere

O

Fig. 3.

800::~ 60~• 40

20O

80o\e 60

ee 4020O

Fig. 2. Appearancemachine.

and schema of miniature creep test

machine, such as the degree of axial bending, shield-

ing method of the atmosphencgas and its influence

on load, are already reported in the previous report.6)

Considerably accurate measurementof creep strain ofthe specimens was madepossible by measuring thedistance between the pull-rods, bccause the creepstrain of the pull-rod itself was minimized by the useof a superalloy for the pull-rod material.

Figs. 3and 4 show the results of the test in whichthis creep machinewasused. Fig. 3shows the resultof the creep curves for various test-piece diameters inair and in an argon gas atmosphere. As seen in an-other paper,5) the rupture time in air is shortenedbecause of the influence of oxidation. Onthe otherhand, the rupture time in an argon gas atmosphere is

almost the same as that of the conventional sizedspecimens ((J510mm) in air. The creep curves arealso similar to each other. Moreover, the results ofthe creep rupture data; time to rupture, fractureelongation and reduction of area, are shown in Fig.4. There is no difference between I)oth the data ofthe miniature specimens and the data of' the conven-tional sized specimens. Fromthese facts, evaluationof the creep curve, rupture life and ductility of minia-ture creep specimens was madepossible using this

creep machine.

f~ 20

EE\o'~~k:),

IP

u;8oS:6

cb

4

200 400 600 800 1000

Time t (hr)

Creep curves for various tcst-piece diameters in air

and Ar atmosphcre.

oeo ~'oo

eoo Cf~oo

Reduction of area

Fracture elongation

ee

oO~) oo,

'~o

e: Miniature (2#mm)in Ar

1: Miniature (4~mm)in Ar

o: Conventional (10;mm) in Air

O\..

l~;Cr-112Mo steel o\

16

Fig. 4*

17 18 20 23l9 21 22

Larson-Miller parameter T (20+10g t)

Creep rupture data fbr var[ous test-piece diametersin air and Ar atmosphere.

Onthe other hand, Fig. 5 shows the relation be-

tween time to rupture and steady-state strain rate.

The data of creep rate of the miniature specimensby measuring distance between the pull-rods corre-sponded well to that of the conventional sized speci-

mens.

3.3. Developmentof a Sampling Device

Several sampling techniques have been used thattake advantage of particular characteristics of the

componentand the sample requirements. There arecurrently two rnethods,3) which include the core sam-pling technique and the boat sampling technique.

856

ISIJ International, Vol. 30 (1990), No, lO

1(ro

=\

'y'10

1,

f5

l~u,

a,

Ce

u'l 10

)~1;c!a,

u,

lc'3

O : Miniature (2**mm) in ArMiniature (4~mm)in ArConventional (10#mm)in Air

_ 12MOsteet11/4 Cr ll

o\):\

O\a\..,

,.- \~\o'~_"\

Fig. 5.

10' I03 I04102

Time to rupture t, (hr)

Relation between time to rupture and steady-state

strain rate for various tcst-piece diameters in air andAr atmosphere.

The former is conducted by core drilling and the later

by a slightly concavc circular sawwhcc_1. Both meth-ods use trepanning tools and their tools are very hcavyin weight and are not portable. In addition, these

techniques require substantial clearance for sampling.

A portable, Ii~"htweight, and user-friendly samplingdevice needs to be developed. Moreover, the cutting

methodneeds to permit removal of the sample with-

out plastically deforming it.

Consideratin~" these problems, a portable and sim-ple-structured sampling device was developed. Thisdevice adopts Electro-Discharge Machining (EDM)methodwith a core-shaped electrode. Setting is easybecause machining resistance is designed as low aspossible. The device is operated remotely and canbe used externally on any surface for which there is

at least 15-30 cmof working clearance.Sampleshave been successfully removedfrom the

operating plant components. Fig. 6~a) shows field

sampling condition performed using this device onthe operating plant components. As shown in this

flgure, sampling space is very small. In addition,recovering work after sampling was madeeasier be-

cause of the smoothsurface of the core hole as shownin Fig. 6(b). Fig. 6(c) showsan example ofthe coresamples removedusing this device. It wasconfirmedfrom the inspection conducted after sampling thatthe machining using this device itself'did not aflbct thematerial properties around the core hole. After the

core samples havc been removed, the componentscanbe weld repaired by welding onto it a dummyover-size stub.

Although not described in this paper, other mate-rial properties tests and metallographic investigationis also possible usin~'_ these core samples, in additionto the creep rupture test.

3.4. Results of Creep Rupture Test z~)ith Miniature andConventional Sized Specimens

Fig. 7(a) shows the results of the creep rupture test

Fig. 6. (*)

(b )(c)

Ficld sampling condition performed using the

portable sampling device at the operatingplant components.Appearanceof core hole after sampling per-formed using the devic-e shownin Fig. 6(a).

Core samples remove.d from elevated tem-perature parts.

of miniature specimens and conventional sized speci-

mensof Cr-Mo-Vsteel. It was confirmed from this

figure that the log time to rupture t. vs. temperatureT diag_ram shows linearity and that the rupture timeof the adopted miniature specimens in an argon gasatmosphere is almost the samcas that of the conven-tional sized specimens (c8 mm)in air. The tem-perature and stress conditions are, of course, the samefor both types of specimens. Fig. 7(b) shows the re-sults of the creep rupture test fbr miniature specimensand conventional sized specimcns of 2l/2Cr-Mo steel.

The results show the same trend as in Fig. 7(a).

Fig. 7(c) shows the results of the creep rupture testfor miniature specimens and conventional sized speci-

mensof I l/4Cr1/2Mo steel. This figure shows theresults oi' the base material and welded joint. For

,

i;

857

ISIJ International, Vol. 30 (1990), No. lO

104

~' I03

J~

oE~5

2a~

102

101

o

\o

8

e:O:

Cr-Mo-Vsteel :

(;=30kg/mm2

Miniature (2cmm)in Ar

Conventional (8cmm) in Air

g

d=20kglmm2

e\

550 600 650

Temperature T ('O

Temperature vs. Iog time to rupture tT at iso-

stress testing on basc material for Cr-Mo-Vsteel.

b,

OE~:

OL:a:,'E

104

103

102

\\

e : Miniature (2~mm)in ArBase O : Miniature (4#mm)in Armetal

O: COnventional (10~mm)in Air

I : Miniature (2cmm)in ArHAZ[] : COnventional (10~mm)in Air

a

HAZ Base

~': ~:

(\~~~ta:

~

'

'~l" :'/S,

_~ \\ \I~ ~' ~~~:_' 8\~-":' ;11

~'~~~I

_ o \~.

1;j(14 Cr- 112Mosteel

600

Fig.

104

7(a).

ldiniature (2cmm)in ArMiniature (4~mm)in ArConventional (10~mm)in Air

,~EE~,,

J,

~bcou'

S~co

Fig.

30

- I03

,

d,

E~:

oL:

a:,E I02

e

7(c).

101

e:o:o:

e

c

C=13kglmm2

(r =8.5kglmmeo

21./4Cr-Mo steel

8

550

65O 700 750

Temperature T ('OTemperature vs. Iog time to rupture tT at iso-

stress testing on base material and welded _joint

for 11/4Cr1j2Mo steel.

20

IO

8

6

4

2

Base e : Miniature (2~mm)in Armetal O: Conventional (10~mm)in Air

I : Miniature C2~mm)in ArHAZ [] : Convenlional (10~mm)in Air~,

[~]~(:1!

"\C]

~]

lr• Basemetalti

HAJ~/1:1'~

1/4 Cr- 112Mosteel

C~CI

600 650 700Temperature T ("O

Temperature vs. Iog time to rupture tT at iso-

stress testing on base material for 21/4Cr-Mosteel.

16

Fig. 8.

17 18 I9 2120 22

Larson-Miller parameter T (20+10g t)

Master ruptl're curve by Larson-Miller parameteron base material and welded joint for 11/4Cr-

1!2Mosteel.

Fig. 7(b).

both cases, the data plots* have a similar tendency in

relation of rupture time to temperature such as in

Figs. 7(a) and 7(b). The welded .joint specimensruptured at the HAZregions.

Fig. 8 also shows the results of the creep rupture

* Thedata plots of the miniature specimens ruptured at the HAZregions alone showa steeper gradient than that of the real line in the

figure. However, in this figure, these data display as data band, as explained in Sec. 3.5.

858

ISIJ International, Vol. 30 (1990), No. lO

test fbr 11/4Cr-i/2Mo steel. The test was conductedfor various stresses and the data were arranged usingthe Larson-Miller parameter. For both cases of thebase material and welded joint, there is no differencebetween both the results of the miniature specimensand of the conventional sized specimens. The scat-tered data of miniature specimens of a welded joint((T=5 kg/mm2) correspond to the data shownby theband in Fig. 7(c).

Fig. 9 shows the typical micrographs of a longi-tudinal cross section of' typical ruptured specimens.Figs. 9(a) and 9(b) are the cases of 21/4Cr-Mo steel

in comparison with the conventional sized specimen(clO mm)and the miniature specimen (c2 mm)rup-tured under the same test condition. It was foundthat the fracture modeof both specimens were the

sameand showed transgranular fracture. Fig. 9(c)shows the case of welded joint for 11/4Cr-1/2Mosteel. It was found that the fracture modewas anintergranular fracture with cavitation at the HAZregions. Although the fi~"urcs are not shr_)wn in this

paper, similarly, in the case of' base material of Cr-Mo-Vsteel and 1Ij4Cr-1/2Mo steel, the miniaturespecimen and the conventional sized specimen cor-respond well with each other.

Onthe other hand, these data have generally beenlimited to test times oi' Iess than 3X103 h. Oucstionshave also remained as to the effect of creep ductility

and heat to heat variations on the validity of the iso-

stress creep rupture test. To investigate the pos-sibility of linearity on the log time to rupture tr vs.

temperature T diagram when thc iso-stress test is

conducted for longer periods, the samerelation suchas in Fig. 7was confirmed using the creep data sheetof NRIM9-11) of the samesteels. This attempt ap-pears to be very uscful because the data includes creeprupture tests up to 8xl04 h, at the longest. Al-though the flgures are not sho_wnin this paper, it wasconfirmed that, within limits of scatter, the iso-stress

lines (the lines which connect the data under the samestress condition on the log time to rupture t. vs. tem-

perature T diagram) were straight and in parallelwith o_ne another depending on the stress level.

3.5. Remaining Creep Llfe of a Welded Joint UsingMiniature Specimens

In the scatter band as shownin Fig_.

7(c), the dataplots of the miniature specimensruptured at the HAZreg_ ions scattered moreor less in comparison vvith thatof' the conventional sized specimens. Oneof the rea-sons for this is considered to be the difference in creeprupture characteristics of the welded joint, as ex-plained below.

C'.enerally, the welded joint is a composite materialconsisting of the base metal, HAZregions and theweld metal. Mechanical properties can vary fromzone to zone. So, its creep rupture characteristictends to showan unpredictable mannerin comparisonwith that of the base material. Nevertheless, in gen-eral, it is also a fact that in most of the welded joint,

creep cracking is initiated at the HAZregions andthat this crack growth leads to rupture.12) In this

study, creep rupture in welded joint has also beenobserved at the HAZregions described in Sec. 3.4.

Fig. IOshowsthe typical appearanceofcreep cracksat the HAZregions in interrupte.d specimcns for aconventional sized specimen (clOmm) of 11/4Cr-1/2Mo steel (t/t,=approximately 0.5, where t is aninterrupted test period). In this figure, it is rec-ogni7,cd that a creep crack initiates at the coarse-grain HAZregion. The figure showsan area abouthalf the size of the diameter of the conventional sizedspecimen. Theremaining half is occupied mostly bythe fine-grain HAZreg_ ion with no initiation of' cavityand any other creep cracks. It is thought tha,t the

creep rupture life is dominated by the propagation ofthe crack, which, initiating in the coarse-grain HAZreg_ ion, finally reaches the fine-grain HAZregion.Thus, it seemsthat, as in the case of a low ductilitymaterial of intergranular fracture such as the HAZregions, crack initiation and propagation are the mainprocess of creep rupture.

Basemetal

HAZ Weldmetal

Fig.

(a) Conventional specirnen (c 10 mm)for 21/4Cr-Mo steel

(b) Miniature specimen (c2 mm)for 21/4Cr-Mo steel

(c) Miniaiure specimen (~;2 mm)of weldcd .joint consisting of the base metal, HAZand weld metal for

11/4Cr-1/2Mo steel

9. Typical micrograph of creep ruptured specimen.

859

ISIJ International, Vol. 30 (1990), No, lO

LlIt c10

11Il

It /lLI ll\\ ll\L ll

--,\ __JL___

Fig.

Base HAZWeldmetal metal

t/tr= 0.5

Coarsegrains

Finegrains

Diameterof

miniature

speamen(2mm)

10. Typical appearanceof creep cracks in interruptcd conventional specimen ((,~lO mm)of welded

joint for 11/4Cr-1/2Mo steel.

In contrast to conventional sized specimen, thecoarse-grain HAZregion which initiated the crackoccupies almost half the diameter of the miniaturespecimen. (The size is shownin the figure.) It is ex-pected that the crack will lead to a rupture through

a relatively rapid growth. As seen in the figure, eventhe HAZregions consist of' a coarse-grain region anda fine-grain region and the both microstructures arenot unifbrm but different. It is considered that, be-

cause of due to this microstructural ununiformity, the

creep crack propagation rate differs accordin~" to the

specimens, eventually the rupture life of the miniaturespecimensmayscatter moreor less.

Onthe other hand, material properties data per*taining to wclded joint are very poor so far.5) In the

absenceofrelevant data, the most convenient practice

has been simply to assume that a weld (the weldmetal and HAZregions) is always weaker than the

base metal. In addition to the fracture often occursat the weakest location such as a weld. From theresults of the above, therefore, it is considered thatthe creep life assessment of the welded jo_ int usingminiature specimcns is effective to evaluate the critical

part (the weakest location) such as a weld. lvfore

specifically, from the iso-stress line which uses the

lower limit line (broken line) of the band shown in

Fig. 7(c), assessment, is to be madefor the critical

part of' the welded joint. This evaluation could beconservative ibr the life assessment.

Another finding is that, as explained in the Sec.

3.4, in the case of the data plots for the miniaturespecimens alone, their gradient is steeper than that of

the real line in the figure. This fact should be recon-sidered after accumulating surricient data in further

study.

4. Conclusions

In order to validate a procedurc of creep rupturetests using miniature specimens, a sampling device

wasdeveloped and a methodof making the miniaturespecimens was investigated. Finally, the use of iso-

stress creep rupture test on the miniature specimensto predict the remaining creep life of Cr-Mo-V,2l/4Cr-Mo and 11/4-1/2Mo steels wasexamined. Theresults of this study are summarizedas follows :

(1) The miniature specimens could be producedby either machining or welding. A miniature speci-

menthat was2mmin diameter and 30 mmlong wasapplicable.

(2) The creep curve, rupture life and ductility of

the miniature specimens tested in argon were similar

to those of' conventional sized specimens in air. Thefracture modeof the miniature specimens was also

consistent with that of conventional sized specimens.(3) Ona graph showing temperature T vs. Iog

time to rupture t., the creep rupture data of Cr-Mo-V, 2Ij4Cr-Mo and 1Ij4Cr-lj2Mo steels were rep-resented with an iso-stress line, respectivcly. Eachiso-stress line shifted parallel with the stress level.

(4) To improve the sampling technique, a sam-pling device vvhich adopted the Electro-Discharge

Machining was developed. This device was applied

to the operating plant components. Samples havebeen successfully removedfrom it and were obtained

without affecting the material properties around the

core hole.

Fromthe results of the above, it wasfound that the

miniature specimen could be used in iso-stress creeprupture test to predict the remaining creep life. How-ever, to apply this methodof life prediction to operat-ing plant componentsactively, further development

860

ISIJ International, Vol.

of a sampling technique involving a weld-repair tech-

nique and accumulation of various data would benecessary.

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861