ML20128C559

Discusses Discovery of Small Inclusions in Inconel Tubing from Which Reactor Pressure Vessel Stub Tubes Being Fabricated

ML20128C559
Person / Time
Site:	Monticello
Issue date:	07/10/1968
From:	Thornburg H US ATOMIC ENERGY COMMISSION (AEC)
To:	James O'Reilly US ATOMIC ENERGY COMMISSION (AEC)
Shared Package
ML20128C554	List: ML20128C549 ML20128C559
References
NUDOCS 9212040469
Download: ML20128C559 (5)

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'n O- GLEN ELLYN. ILLINOIS 60137 July 10, 1968 J. P. O'Reilly,. Chief, Reactor Inspection and Enforcement Branch Division of Compliance, Headquartero COMPLIANCE INQUIRY EMORANDUM NORTHERN STATES POWER (MONTICELLO)

DOCKET NO. 50-263 STUB TUBE MATERIAL INCLUSIONS AT MONTICELLO During a visit to the subject facility on June 27, Messrs. Jones, Reinmuth, Lofy, and Feierabend were informed by Northern States Power (NSP) personnel that Chicago Bridge and Iron (CB&I) had discovered small inclusions in.the Inconel tubing from which the reactor pressure vessel stub tubes were being fabricated. The inspectors were informed that CB&I and their consultant had performed a series of metallurgical investigations which proved that the material 1: question was acceptable for the intended service. General Electric personnel at the site ag reported to have stated that the material met the code and was 4cceptable in their opinion.

On July 3,1968, Messrs. Thornburg, Jones, and Feierabend visited the CB&I corporate offices in Oak Brook, Illinois, to discuss the matter with their management. Discussions were conducted with:

Mr. Perry Arnold, Vice President, Welding Mr. Ed Varnum, Manager, Quality Control Mr. Oscar Seth, Metallurgist The following information was obtained:

A. Material Specifications The. material in question was produced by the Huntington Alloy Products Division of the International Nickel Company (INCO). The material-is designated as Inconel 600 tubing with an ID of 6" and a vall thickness ofes1". The certified mill analyses for two heats of the material as compared to ASTM specifications are shown in Table I below:

u 9212040469 680724 PDR ADOCK 0S000263 S PDR f

., .4 J. P. O'Reilly July 10 1968 TABLE I 7.C Mn Fe S Si Cu Ni Cr Specification 0.10 1.0 6.10 0.015 0.5 0.5 72 _

Aa.17 Heat No. 6400 0.09 0.19 7.15 0 . 00 .' O.23 0.24 76.80 15.27 Heat No. 6569 0.02 0.13 8.15 0.007 0.20 0.21 76.96 14.27 The material certification report reviewed by the inspectors' contained the following statement:

"Af ter proper surface preparation the naterial will be capable of meeting the applicable ultrasonic and liquid penetrant requirements of Section III Article 3 of the boiler code."

B. Action Taken by CB&I Mr. Varnum indicated that the inclusions were-found while the tubing was being machined to the proper OD in the Birmingham Shop of CB&I. The inclusions appeared as minor surface irregularities. They were not detectable by UT or liquid penetrant. A metallurgical investigation was initiated by CB&I, including a visit to INCO by Mr. Varnum. Mr.

Varnum was informed by INCO personnel that such inclusions were normal in Inconel. According to Mr. Varnum they also_ stated that most users of the material bought the material with an extra thickness of --1/4" which is machined off.to eliminate the inclusions.

Dr. Staehle of Ohio State University, an expert in the field of metal corrosion and metallurgy, was retained to study the problem. EHe recommended a metallurgical investigation which included tensile strength-l_

tests, free bend tests, stressed weldment tests, and corrosions tests.

All tests were to be accompanied by photomicrographic studies.

C. Results of Metallurgical Investigations I

( The inspectors reviewed a special report prepared by CB&I which outlined I the investigations performed, the results obtained, and the conclusions reached. The inspectors observed some of the meta:1urgical specimens which resulted from the investigations. The investigatior.s and their results as reported by CB&I are summarized as follows:

1. The inclusions are small, approximately 1/32" in length and 2 to 3 mils deep. In general, they are oriented _in the direction of rolling'in strings. In the verst case, five or six inclusions were oriented in a group followed by another group several inches removed. The condit*on varied from several groups per 4 square l

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J. P. O'Reilly July 10, 1968 inches of surface to a single inclusion per square foot of surface area. The investigations were carried out using the material with the most inclusions (Heat No. 6400). Single inclusions were not joined.

2. Battelle Memorial Institute performed a microprobe analysis of the inclusions and reported that they were pure carbon (not a metal carbonitrite).

3. ansile strength tests were performed whic' included a number of inclusions in necked area. The test specimens demonstrated tensile properties which significantly exceeded specifications. The samples were taken normal to the direction of rolling, which is the worst case. Photomicrograpns indicated no cracking or failure associated with or originating from the inclusions.

4 Free bend tests were taken to 180 with no failure or cracking observed. The cample specimens were again taken normal to the direc-tion of rolling. Photomicrographs indicated no cracking or failure associated with or originating from the inclusions.

5. Laboratory welding tests were performed in highly constrained gec.ne try . A section of 6" ID tubing from Heat No. 6400 was welded to a section of 18" x 18" A-516-Grade 70 plate 2" thick. Photo-micro;raphs of the area of the inconel tubing which "entained the most inclusions indicated no deleterious effects due to welding in highly constrained geometry, viz, no cracks originated from inclusions, etc. No cracks were observed elsewhere in the veld. In the judge-ment of CB&I personnel present, the welding test was performed in more highly constrained geometry than that of the vessel weld.

6. A second constrained weld test was performed in a close approxiraa-tion of the geometry of the ,st ainless steci control rod drive housing tube, the Inconel stub tube, and tlse A-516; Grade 70-vessel bottom head. A force of 56,500 pounds was applied normal to the control rod housing tube with a 100 ton jack at the beginning of welding with a reduction to 32,000 pounde during the remainder of the welding cperation. No cracking was observed in the resulting weld-ment. A photomicrograph taken of a section of Inconel tubing in the area of maximum stress showed no deleterious effects.

7. A corrosion study was made using an Inconel weldment from Heat No.

6400 in an cicctrolyte containing 200 ppm c1 for 28 days. No cracking was noted.

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J. P. O'Reilly -4. . July 10, 1968

8. A corrosion test similar to that described in-item 7 above is now in progress in the CB&I laboratories at a-180 F. The test v111-run for ~<100 days.

9. CB&I and their consultant concluded that the inclusions in the material in question did not affect its mechanical properties or its veldability.

D. Observations Made by the Inspectors

1. The inclusions.in question extended from two to fou: metal grains in length. In many cases they appeared to be intergranular. The inclusions appeared to be empty _over a considerabb portion of their volume. The carbon may have been removed during the sectioning and polishing process.

2. The polished micrographic specimens contained small inclusions which had a pink color which were explained to be titanium carbo-

, nitrides. Most of these very small inclusions, pencil dots at a magnification of x1000, were intergranular.

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3. Mr. Lofy was contacted by the writer. Mr. Lofy stated that he had discussed the matter with Mr. J. ' J. Chyle. The writer also con-tacted Mr. Chyle after returning from CB&I. Mr. Chyle had the following comments on the matter and the licensee's investigation into the problem:

a. He observed that the licensee't investigation was' proper and responsible,

b. He observed the carbon does not dissolve and migrate in nicael as it does in iron elloys.

c. He agreed that the physical properties of the Incon21, including its acidability did not appear to be impaired.

d. He speculated that the carbon particles may have orig'inated.

from carbon electrodes in an electrical melting process,

c. " stated that the real question centers around the effect of ac inclusions on the long term performance of the material' 1 le service environment.

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J. P. O'Reilly July 10, 1968 i

E. Region-III position

1. We agree that the inclusions apparently have not impaired-the physical properties of the material in question.

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2. We believe that CB61 has performed a respousibic investigation of i the properties of the material. We also commend CB&I and NSP on I their efforts to inform Compliance of the problem.

3. In the light of experience with stainless steels, we have reserva-tions with regard to the long term performance of the material.in the service envir: ament. This concern could be reduced by the initiation of an in-service material surveillance program. The long term performance of Inconel 600 is unknown at this time.

4 It is perhaps unlikely that the carbonaceous inclusions will result' in a localized region of higher EMF, resulting in a phenomenon similar to intergranular corrosion in stainless steel, but we

, cannot rule out a new mechanism in Inconel or mechanism which takes advantage of the inhomogeneity of the parent material, no matter how slight.

5. We recommad that the matter be considered by our specialists and consultants.

a. -If they agree that the physical properties of the mat'erial are unimpaired by the presence of the inclusions, we should request DRL to require that the applicant carry out a meaningful long term in-service material surveillance ,

program which includes the system material in question.

b. If they are aware of a possible failure mechanism which the presence of the inclusions might cause in the parent material, other action msy be necessary.

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.arold D. Thornburg Senior Reactar Inspector f

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