ML20107L413

Draft Catawba Socket Weld Evaluation

ML20107L413
Person / Time
Site:	Catawba
Issue date:	07/31/1984
From:	Czajkowski C BROOKHAVEN NATIONAL LABORATORY
To:	NRC
Shared Package
ML20107L149	List: ML20107L394 ML20107L413
References
FOIA-84-722 NUDOCS 8502280643
Download: ML20107L413 (17)

Text

.

DRAFT

.i CATAWBA SOQ2T GEELD EVALUATION

• Carl J. Caajkowski Brookhaven National Laboratory Department of Nuclear Energy Upton, New York 11973 JULY 1984 1

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. *Research carried out under the auspices of the U. S. Nuclear Regulatory Commission, Region II.

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8502280643 841016 PDR FOIA BELL 84-722 PDR Nx. -

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I NTHODiltT I ON Region 11 (Atlanta, Ca.) of the U. S. Nuclear Regulatory Com=ission had received various allegations regarding possible welding procedure violations on the off shifts at the Catawba nuclear site. These allegations intimated that certain stainless steel socket weld joints may have been assembled and welded with no regard for the interpass temperature restrictions imposed by the site's procedures and specifications. Since the non-judicious use of too much heat input could possibly cause significant sensitization of the Type 304 stain-less steel being welded potentially causing susceptibility to intergranular stress corrosion cracking (IGSCC), Region 11 requested Brookhaven National Laboratory's (BNL) assistance in -the development and implementation of field inspection techniques which could provide useful and reasonably accurate inforn.ation regarding a given weld's degree of sensitization.

PROCEDURE DETERMINATION In order to provide a gage of the developed procedure's effectiveness, Region II had eight samples of stainless steel socket veld coupons sent to BNL. Figure 1 is a photograph of the outside surface of the eight socket welds. In addition to the eight half-sectioned socket welds, a cross-sectional slice of each socket weld was also sent. A photograph (Figure 2) of the inside surfac'e of these eight socket velda disclosed that there was significantly more t heat tinting (wider band of oxidized metal) in coupons 1, 3, 5 and 7. This ob-servation was in conformance with the information received from Duke Power

(' Company regarding the welding procedures used for each socket weld coupon:

l1 l} Coupon No.1 - 2* Sch. 40 SS pipe welded into 'a 2* - 3000f SS full coupling l1 I;

'i Weld position 2F .

Voltage 10V Asperage 150A

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DCSP (Direct. Current Straight Polarity)

CTAW Process (Cas Tungsten Arc Welding)

Filler Material ER308 Filler Haterial Diameter 3/32" - 1st Pass 1/8" - 2nd Pass 3/32' - 3rd Pass

The weld was made using no interpass temperature control. The only stops made in the weld were during filler metal and position changes. Duke Power reported that the base material was raised to a temperature of approximately 750*F after welding.

Coupon No.

2- Identical to coupon No. I except that a maximum interpass temperature restriction of 350*F was imposed.

Coupon No. 3 - 1" Sch. 40 SS pipe welded into a 1" - 300f SS full coupling Weld position 2F _

Voltage 10V Amperage 138A DCSP (Direct Current Straight Polarity)

CTAW Process (Cas Tungsten Arc Welding)

Filler Material ER308 Filler Material Diameter 3/32" - 1st Pass 3/32" - 2nd Pass This weld was made using no interpass temperature control. The only stops made in the weld were during filler metal and' position changes. Duke Power reported that ihe base material was raised to a temperature of approximately 750*F af ter welding.

l Coupon No. 4 - 1,dentical to coupon No. 3 except that a maximum interpass temperature restriction of 350'F was imposed.

Coupon No. 5 - 1* Sch.160 SS pipe welded into a 1" - 6000f SS full coupling Wald position 2F Voltage 12V i Asperage , 180A DCSP (Direct Current Straight Polarity)

CTAW Process (Cas Tungsten Arc Welding)

Filler Material ER308 Filler Material Diameter 1/8" - all 5 passes

This weld was made using no interpass temperature control. The only stops sade in the weld were durinE filler metal and position changes. Duke Power reported that the base material was raised to as temperature of approximately 1000*F after welding.

Coupon No. 6 -

Identical to coupon No. 5 except that a maximum interpass temperature of 350*F was imposed.

Coupon No. 7 - 2" Sch. 160 SS pipe welded into a 2" - 6000f SS full coupling Weld position 2F Voltage 12V -

Amperage 180A DCSP (Direct Current StraiEht Polarity)

CTAW Process (Gas Tungsten Arc Welding)

Filler Meterial ER308 Filler Material Diameter 1/8" - All 5 passes This weld was made using no interpass temperature control. The only stops made in the weld were during filler metal and position changes. Duke Power reported that the base material was raised to a temperature of approximately 1000*F after welding.

Coupon No. 8 -

Identical to coupon No. 7 except that a maximum interpass i

temperature of 350*F was imposed. ,

2.0 TECHNIQUE _

intergranular corrosion is generally defined as a local attack on the I

grain boundaries of a metal by a corrosive media.

In stainless steels, ausceptibility to intergranular corrosion is greatly enhanced by the sensitisation process. Sensitisation can be described (in austenitic stainless steels) as the formation of chromium carbide precipitates in t he grain boundaries and the resultant depletion of Cr, brought about by heatlng the steel in the temperature range 500-800*C (1,2).

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This temperature range is easily schieved during the welding pruces6 where the normal temperature of welding exceeds 1600*C during fusion welding. There-fore, the welded base material could receive a sensitising hent treatment in the critical range (500 - 800*C) at some point outward from the veld fusion line which would be maintained long enough to precipitate Cr carbide at the grain boundaries. This is not to say that the welding process alone will in-duce a sensitising effect on the base material, as the degree of material sen-sitisation is a cumulation of the material's prior thermal and mechanical treatments, weld cycle history, (f passes, heat input, etc.) material chemis-try, thickness and thermal conductivity, and time at temperature in the sensi-tisation range.

In order to measure a materials' degree of sensitization, the American Society for Testing and Materials (ASTM) has published Standard A-262,

" Standard Recommended Practices for Detecting Susceptibility to Intergranular i

Attack in Stainless Steels." This standard includes a rapid screening test as part of its content; Practice A - Oxalic Acid Etch Test for classification of etch structures of stainless steels. This practice is described in A-262 as:

"1.3 The oxalic acid etch test is a rapid sethod of identifying, by simple l etching, those specimens of certain stainless steel grades which are es-sentia11y free of susceptibility to intergranular attack associated with Cr i carbide precipitates. These specimens will have low corrosion rates in certain corros' ion tests and therefore can be eliminated (screened) from testing as

" acceptable.*"

For purposes of this investigation, each of the socket welds were hadd ground (similar to field conditions) and then polished down to a 1 micron .

finish. The polished specimens were then etched with a current density of 1A/cm for 1.5 min. in a 10% oxalic acid + desineralised water solution. The 2

cathode used for the. etching was a piece of 304 stainless steel while the l;

socket weld specimen was used as the anode. For comparison study, the welded i cross sections were also polished and etched in a similar manner. The welded cross sections were examined on a metallograph while the socket veld specimens were examined on a microscope slaulating field examinations. The results of  ;

the examinations are shown in the photomicrographs (Figures.3-10). It should be noted that these photomicrographs were taken at approximately 250X and were the areas of most pronounced sensitization. In no instance was any structure )

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cunwidered rejectable (ditched structure in accordance with ASTM 262 - Practice A). There were no rejectable structures found on either the socket welds or the socket weld cross sections. The worst case structure was found on coupon

1. 5 which showed a definite dual structure but no one grain was completely encircled by carbide ditching.

One area of cracking was found on socket weld specimen #7 (FiFure 11).

This area appeared to be associated with an are strike which appeared to have cracked on cooling. The cracking was found on the pipe portion of the weldment; outside the socket weld's heat affect'ed none.

3.0 CONCLUSION

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The exanination of the eight socket weld coupons and their associated cross sections have led to the following conclusions:

a) None of the welds examined were considered rejectable if co: pared to ASDi A-262 - Practice A. Even the specimens with no heat input control would be considered acceptable.

J b) The ASTM A-262 - Practice A technique is a viable - method of field metallography for determination of sensitization of stainless steels.

c) Care should be taken to prevent arc strike on these stainless steels.

4.0 REFERENCES

1. Pande, C. S. , Suenaga, M. , Vyas, B., Isaacs, H. S., and Emiling, D. F. ,

Scripta Metallurgica, Vol. II, 1977, pg. 681.

2. Cowan R. L. and Tedmon, C. S., Jr., Advances in Corrosion Science and Technology, Vol. 3,1973, pg. 293.

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APPENDIX A FIELD TRIP TO CATAWBA SITE t On June '20, 1984, I visited the Catsubs site with Mr. N. Econonos from

! Region II - U. S. Nuclear Regulatory Commission. The purpose of the visit was to review the methods used by Duke Power in determining acceptability of stain-less steel field installed socket welds.

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! An entrance meeting was convened on the morning of June 20, 1984 with the following people in attendance:

1 Attendee Title Company A. R. Hollins Construction Duke

! S. E. Ferdon Engineer Assistant Duke C. J. Czajkowski Research Engineer RNL N. Econonos Region 11 U.S. NRC

, D. H. Llewelyn Assoc. Field Engineer Duke B. Kruse Field Engineer Duke The agenda for the meeting is included (Attachment #1) of this report.

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During discussions with the utility personnel at this meeting, it was de-cided that Mr. Econonos and myself would witness the utility's method of field etching and replica production on three field welds.

j The field; welds reviewed were:  ;

i I 1-33-130-18 - - - --

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• 1-35-130-19'

! 1-33-97-18 f The welds were ground and polished by high-speed stor-tool to 1/4 micron l finish. Af ter polishing, the suspect areas were electrochemical 1y etched using oxalic acid and a portable power source. The voltage density was adjusted to j IA/ca' and 'was maintained for 1.5 minutes. After cleaning with distilled l water, a replica was ande of the area using a silicone based (two part)

[ polymer. After thermal setting of the replica, it was removed and then examined in the veld test booth area using a metallograph.

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Although the quality of the replicas varied somewhat in sensitivity even the least sensitive was capable of replicating the ma t e rial 's structure suf ficiently to ascertain the degree of sensitisation present in the weldaent.

In all three cases, the material condition would have been acceptable to ASTM A-262 Practice A.

In order to verify the sensitivity of the replicating process; the welds

~ themselves were inspected using a portable microscope. The structures observed were consistent with those obtained by this replicating process.

A review of the following field procedures / forms showed these to be consistent with good construction practices:

Patch / Wipe Test Documentation Fort CF-57A, Rev.1 Alteration / Repair Process Control Fora F-10A, Rev. 1 Construction Procedure CP-857 Rev. 9 Additional documentation reviewed included four mill test reports for stainless steel tube / pipe and fittings. Although one lot of material had a carbon content of .0171; all of the other lots of material ran between 0.051 -

0.07% carbon. (The lower the carbon content the less the susceptibility to sensitisation.)

During discussions with utility personnel at an sait critique (Attachment

1. 2) the following points were made by the BNI. employee.

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1) The replicating process and 41 eld etching technique appear ^ fo~ be~~

antisfactory methods of determining the materials' conditions with regards to senaltisation.

2) The carbon content of the materials used for construction would dictate the need for at Isast a sampling of auscept welds for a sensitised condition.

3) The welds which were performed by the welder who stated he did not use interpass control should be considered a separate population from the other welds and should be inspected accordingly.

4) The re-use of oxalic acid for etching by the utility was discouraged.

It was suggested that any used oxalic acid be discarded in a waste bottle for proper disposal.

Although not discussed at the exit critique, the cleaning of the stainless steel after etching should be pe rformed in accordance with ASTM A-262 -

Practice A - Section 4.7 as practicable.

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CATAWBA NUCLEAR STATION ,

MEETING TO REVIEW ELDING INDUCED SENSITIZATION OF STAINLESS STEEL FIPING JUNE 20,1984 AGENDA INTRODUCTION - 0FFICE OF D. N. LLEWELLYN REVIEW OF DUKE IDENTIFICATION OF CRITICAL WELDS REVIEW OF FIELD FORTABLE METALLOCRAPHIC TECHNIQUE USED BY DUKE - CATAWBA ET. LAB.

FIELD TOUR TO UNIT I BEACTOR BLDC. TO EKAMINE APPLICATION OF F10 CRAM - #1 R. B.

EXAMINATION OF METALLOCRAPHIC RESULTS FROM FIELD TOUR - CATAWBA MET. LAB.

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CRITIQUE - 00NF. RDOM 9

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