ML19323F281

Steam Generator 2 Feedwater Pipe Failure Analysis.

ML19323F281
Person / Time
Site:	Millstone
Issue date:	03/31/1980
From:	Koziol J, Ragl A, Schloss S ABB COMBUSTION ENGINEERING NUCLEAR FUEL (FORMERLY
To:
Shared Package
ML19323F277	List: ML19323F276 ML19323F281
References
IEB-79-13, TR-N-MCM-001, TR-N-MCM-1, NUDOCS 8005280724
Download: ML19323F281 (43)

Text

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March 14, 1980 I

I Millstone Unit #2 Steam Generator #2 Feedwater Pipe I Failure Analysis

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I Primary Boundary Materials i Nuclear Power Systems Combustion Engineering, Inc.

.I Windsor, Conn.

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4 Prepared by: Date: 3 ~ / V- If8 g a. aes1 7 8 Approved by:

S MM-chl Date: P2G#

Approved by: 4A , Date: 3' 3/ * [d

. J.

Kozip Q' l

Soorpo4Y

I ABSTRACT l

This report summarizes the findings of the metallurgical investigation into the Millstone Unit #2, Steam Generator #2 i

1 feedwater pipe cracks. Circumferential cracks were found in the counterbore of an elbow, approximately 7 feet away from the safe end of the steam generator feedwater nozzle. The remainder of the elbow did not contain cracks. All cracks I are the result of high cycle fatigue. A thin layer of cor-rosion products existed over the entire I.D. surface and on j all crack surfaces, but was not believed to be the major driving force for crack initiation. This investigation j could not assess the contribution of corrosion on the fatigue h crack growth rate.

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B Table of Contents Section Page Abstract 3 List of Figures 6 List of Tables . 9 I. Introduction 10 II. Analysis 10 General Examination 11 Metallography 11 Fractography 12 Chemical Analysis 13 Mechanical Testing 13 III. Discussion 14 IV. Conclusion 15 l

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I List of Figures Figure No. Title - Page 1 SCHEMATIC VIEW OF FEEDWATER PIPING SYSTEM- 16 STEAM GENERATOR #2 LOOP 2 CROSS SECTIONAL VIEW OF ELBOW AND PIPE 17 SECTION 3 LOCATION OF CIRCUMFERENTIAL CRACK IN ELBOW 18 COUNTERBORE PRIOR TO SAMPLE REMOVAL 4 GENERAL VIEW OF SAMPLE #3 19 5 ENLARGED VIEW OF SAMPLE #3 WITH CIRCUM- 19 FERENTIAL CRACK AND SURFACE CORROSION 6 GENERAL VIEW OF SAMPLE #9 20 7 ENLARGED VIEW OF SAMPLE #9 WITH c'.IRCUM- 20 FERENTIAL CRACK AND SURFACE CORROSION 8 ELBOW AS RECEIVED 21 9 ELBOW AS RECEIVED 21 10 SECTION OF ELBOW WITH LIGHTLY GROUND AREA 22 11 OXIDE LAYER ON INSIDE SURFACE OF ELBOW 22 12 MAJOR CRACK IN SAMPLE #3 23 13 MAJOR AND SECONDARY CRACKS 23 14 CRACK AT 2:30 O' CLOCK END OF SAMPLE 24 l -AS POLISHED-j 15 CRACK AT 2:30 O' CLOCK END OF SAMPLE 24 l -ETCHED-16 CRACK AT 3:30 O' CLOCK END OF SAMPLE 25

-AS POLISHED-l 17 CRACK AT 3:30 O' CLOCK END OF SAMPLE 25 *

-ETCHED-18 OUTER BEND RADIUS OF ELBOW -AS POLISHED- 26 19 OUTER BEND RADIUS OF ELBOW -AS POLISHED- 26 20 MICROSTRUCTURE OF ELBOW MATERIAL 27 21 ENLARGED VIEW OF MICROSTRUCTURE 27 R List of Figures (continued) l l Figure No. Title Page 22 INNER BEND RADIUS OF ELBOW -AS POLISHED- 28 23 INNER BEND RADIUS OF ELBOW -AS POLISHED- 28 24 MICROSTRUCTURE OF ELBOW MATERIAL 29 29 g 25 ENtARoED v1EW OF M1CROSrRuCTURt l 26 SEM PHOTO OF FRACTURE SURFACE 30 l -

27 SEM PHOTO OF FRACTURE SURFACE SHOWING THUMBNAIL INITIATION SITE 30 l g 28 Stu PHOTO OF THUuBNAIL 31 l 29 SEM PHOTO OF FRACTURE SURFACE BELOW 31 THUMBNAIL j' 30 SEM PHOTO OF FRACTURE SURFACE PRIOR TO 32 l DE-SCALING 31 SEM PHOTO OF FRACTURE SURFACE AFTER 32 l DE-SCALING I 32 SEM PHOTO OF FATIGUE STRIATIONS 33 I

l 33 SEM PHOTO OF FATIGUE STRIATIONS

-ENLARGED VIEW-33 34 EDAX OF OXIDIZED SURFACE 34 35 STRESS-STRAIN CURVE AT 70*F 37 l

g 36 STRESS-STRAIN CURVE AT 250*F 38 l 37 STRESS-STRAIN CURVE AT 450*F 39 38 IMPACT ENERGY VS. TEMPERATURE CURVE 41 t

39 LATERAL EXPANSION VS. TEMPERATURE CURVE 42 40 SHEAR VS. TEMPERATURE CURVE 43 1

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1 List of Tables

!l' i Table No. Title Page

1 1 1 CHEMICAL ANALYSIS 35

-ELBOW MATERIAL-TENSION PROPERTIES 36

-ELBOW MATERIAL-IMPACT PROPERTIES 40 f

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-ELBOW MATERIAL-i I

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l I. Introduction The Millstone Unit #2 generating station, operated by the Northeast Nuclear Energy Company (NNEC) since December 1975, consists of a two loop Nuclear Steam Supply System (NSSS). The general config-uration of the feedwater pipe system for steam generator #2 con-sists of a straight horizontal six foot - eight inch run of eighteen inch O.D. schedule 60 seamless pipe (ASTM A 106 Gr. B - after warm bending the pipe into an elbow its designation is ASTM A234 WPB) from the feedwater nozzle through the shield wall; the pipe makes a 90* downward bend and runs for four feet - six inches to where it makes another 90' right bend and continues for nine feet; here it turns upward from a 90* bend. Figure 1 is a schematic view of the g system. W As a result of radiographic examinations performed in August 1979, NNEC reported that circumferential cracks were discovered in the l counterbore transition taper on both sides of the steam generator feedwater nozzle safe end to pipe weld and in the adjacent pipe to elbow weld in both feedwater loops (Weld #3 in Figure 1).

The samples received by Combustion Engineering (CE) for analysis consisted of two counterbore ring segments (Sample. #3 from the 2:30 to 3:30 o' clock position and Sample #9 from the 8:30 to 9:30 o' clock position) and an entire elbow. The position and location of these samples and elbow are shown in Figure 2. The circumfer- ._

ential cracks in the elbow counterbore region, as revealed by liquid penetrant examination, were determined to be 4"-6" long and spaced 120 apart. Figure 3 (upper photo) shows the crack in this region tnd the pipe to elbow weld prior to and after sample re-moval (lower photo).

II. Analysis The analysis was performed on the two ring segment samples (#3 and

1. 9) and the entire elbow sample. The metallurgical investigation -

performed was to determine where cracks existed, the cracking mechanism, the characterization of the material microstructure, material properties, and the influence of corrosion.

I General Examination:

o Both counterbore ring segment samples (#3 and 79) and the inside surface of the elbow were visually examined for presence of cracks and to determine general surface appearance. A circumferential crack, perpendicular to the I.D. surface, was found near the machined edge (machining performed by Millstone to remove the ring) of Sample #3 shown in Figure 4. This crack was located at the root of the counterbore. Closer examination of the crack showed small corrosion pits along with original machining marks.

This is represented in Figure 5. The same features were discovered in Sample #9 (see Figures G & 7). In this sample, the crack was apparent only halfway along the length, apparently the remainder was removed in machining of the edge.

To permit an examination of the inside surface of the elbow, shown in Figures 8 & 9 as received at CE, it was necessary to section the elbow axially. Several locations along the inside and outside bend radii of the elbow were then lightly ground to remove the oxide layer. This revealed only shallow pits in the underlying base material. .1dditional light grinding removed these pits (Figure 10)

There were no cracks discovered in any of these areas. A very thin I layer of an adherent oxide (Magnetite) was noted on the entire in-side surface of the elbow, as shown in Figure 11.

Metallography For metallographic characterization of the counterbore ring segment and elbow material, Sample #3 and representative areas of the elbow were analyzed. Due to the closeness of the machined edge to the  !

crack, Sample #9 was selected for binocular examination of the crack 1

profile, but not for metallography.

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Several sections we.a cut from Sample #3 and metallographically pre-l pared to reveal cracks, their profile and depth. One major crack was found through the entire length of the sample. A representative I sectional view of this crack is shown in Figure 12. It reveals l

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I several circular pits along the crack front as well as some branching following these pits. The depth of this crack was measured to be 28 mils. In several other metallographically prepared areas of.this sample, small secondary cracks, parallel to the major crack, were found. A representative sectional view is shown in Figure 13. These smaller cracks were not circumferentially continuous and ranged in depth from .005" to .015".

Near each end of Sample #3, two sections were selected for micro-structural characterization. Figures 14 a 15 show a section near the 2:30 o' clock position in both as polished and etched conditions. The microstructure revealed ferrite-pearlite banding typical for A-234 WpB steel. Similar observations were made in a section from the 3:30 o' clock position (Figures 1E a 17). The corrosive attack of the microstructure was not selective of the ferrite grains, but rather general to the surrounding structure. This is clearly evident in Figure 17. Branching, where it was noted, occurred from a corrosion pit.

Representative sections from the elbow material were also metallo-graphically prepared. Only light pitting on the inside surfaces, -

with a maximum depth of about .004". was observed with no significant differences between the inner and outer bend radii of the elbow.

Figures 18 & 19 show as polished sections from the outer bend radius.

Figures 20 & 21 show the etched microstructure of these sections.

It reveals a ferrite-pearlite banded structure typical for A-234 WPB steel. The sections from the inner bend radius revealed similar features. These are shown in Figures 22 through 25.

Fractography A one-inch long section of the crack from Sample #3 was opened in order to study the topography of the crack surface. Scanning Elec-tron Microscopic (SEM) examination of the opened c:'ack surf ace re-vealed a relatively flat smooth fracture surface. This is shown in Figure 26. Crack initiation sites (thumbnail at the top of Figure 27) were observed. Also evident in this figure are typical features of I

I crack branching. These are represented by material separation on the crack surface. An enlarged view of one initiation site and an area below it is shown in Figures 28 & 29 respectively. This area, upon closer scrutiny, shows evidence of fatigue striations.

To determine if this evidence of fatigue striation was masked by I the oxide layer, the crack surface was descaled. Figures 30 & 31 show the crack surf ace prior to and af ter chemical descaling.

Evident from Figure 31 is the presence of high cycle fatigue striations. An enlarged view of the fracture surface clearly shows these fatigue striations (Figures 32 & 33).

Chemical Analysis A chemical analysis was performed of the oxide layer covering the inside surface of the elbow, and also o f .the . elbow' material itself .

The composition of the oxide layer was qualitatively determined using an Fnergy Dispersive X-ray Spectrometric Analysis (EDAX).

The results (shown in Figure 34) reveal the presence of iron, copper, nickel, and zinc. The presence of these elements is normal for Millstone steam generator feedwater lines, the absence of chlorin.;,which often contributes to corrosion mechanisms, should be noted.

The results of a quantitative analysis of the elbow material are shown in Table I. A comparison made with the ASTM /ASME specification for this material shows that it meets all chemical requirements. A mill test analysis is also included in Table I.

I Mechanical Testing

a. Tension Tests (Longitudinal)

The tension tests were conducted in accordance with ASTM Method E-8

" Tension Tests of Metallic Materials", and Method E-21, "Short-Time

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Elevated Temperature Tension Tests of Materials". The tests were performed at 70*F, 250*F, and 450*F. Included in the data reported (Table 2) are Yield Strength, Tensile Strength, Reduction of Area, and Elongation. A typical Engineering Stress vs. Strain diagram for I

I each test temperature is shown in Figures 35 through 37. A com- i parison of the test results with the applicable AS5!E Specifications for tension properties shows that the material fails to meet the minimum yield strength requirements of 35ksi. The results of a mill test are included in Table 2.

b. Impact Tests (Charpy V-notch . Tyne A-Axial)

The Impact tests were conducted in accordance with ASTH 1 f.iethod E-23,

" Notched Bar Impact Testing of lietallic 51aterials". The tests were performed over a range of temperatures from -40*F to +450*F, with g tests concentrated in the transition region. The material exhibited 5 a 50 ft-lb index temperature of 38'F and a 35-mils Lateral Expansion index temperature of 30*F. The upper shelf energy of the material was deter mined to be 136 f t-lbs (absorbed energy) .

The index temperatures were determined by using a full Charpy impact I

curve developed from the minimum data points of all Charpy 'V' notch tests performed. The upper shelf energy is reported E. the lowest impact energy absorbed at that test temperature where two or more g specimens exhibit a 100% shear fracture appearance. The results of a the Impact tests are presented in Table 3 and shown graphically in Figures 38 to 40. Table 3 also includes results of a mill test on three Charpies at 20*F.

III Discussion I

Two counterbore ring segment samples and the adjoining elbow from the 1!illstone #2 steam generator :=2 feedwater pipe system were analyzed.

Circumferential cracks were discovered in both counterbore ring seg-ment samples with a maximum depth of 28 mils. Smaller, circumfer-entially discontinuous cracks with a maximum depth of 15 mils were also discovered parallel to the major crack. There were no cracks discovered in the elbow material. Only a thin adherent oxide layer ,

was found on all inside and crack surfaces. The pitting on the inside curf aces was very d1 allow and randomly distributed.

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I During the metallographic examination, it was noted that circular pits had formed at various intervals along the crack profile. When branching was noted, it was preceded by a circular pit. Neither the I crack branches nor the pits selectively followed the ferrite grains; but were rather indiscriminate to the surrounding micrcstructure.

Fractographic examination revealed thumbnail initiation sites and fatigue striations indicative of high cycle fatigue. Although cor-rosion was noted on all surfaces, the contribution to the crack growth rate could not be assessed.

lI The chemical analysis of the material indicated that it is in com-pliance with appropriate ASME BPV code requirements. The tension values, however, fall short of meeting the code requirement for yield strength.

IV Conclusion In summarizing the findings of this investigation, the following conclusions can be supported:

I All cracks are the result of high cycle fatigue. They were located at the root of the counterbore in the hori-I

• zontal side of the elbow, oriented circumferential1y, and propagated perpendicular to the inside I.D. surface.

The contribution of the feedwater environment on the crack growth rate could not be assessed in this in-I #

vestigation. There was no evidence of ther-mal " craze cracking" or stress corrosion cracking.

The pitting corrosion mechanism which formed the cir-cular pits along the crack profile blunted the crack tip at various intervals. Subsequent renewed crack propagation was accomplished by crack branching.

Removing the geometric discontinuities in the feedwater lines may delay future cracking, but will not prevent

• its re-occurrence, unless the stresses are reduced below the fatigue endurance limit.

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TABLE 1 I

I Chemical Analysis

-Elbow Material-

. Element Weight Percent ASTM /ASME Spec. Mill Test I

A334 WPB Report I C 0.199 0.30 max. 0.26 V <0.005 AL 0.086 P 0.016 0.048 max. 0.006 I S 0.009 0.058 max. 0.018 0.14 Si 0.248 0.10 min.

Ti <0.005 j Cu 0.027 Mn 0.89 0.29 - 1.0G 0.81

! Ni 0.006 Cr 0.021 Co <0.005 I Pb Mo

<0.005 0.012 W <0.005 Ta 0.018 l Nb <0.005 Sn <0.005 j Mg <0.005 Zr 0.007 Fe 98.28 i

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TABLE 2 Tension Propertier

-Elbow Material-Yield Tensile Fracture Reduction Elongation Specimen

• Test Strength Strength Strength of Area (1 in. gage)

No. Temp ( F) (ksi) (ksi) (ksi) (%) (%)

MFP 1 70 30.5 62.9 38.7 70.0 38 3 70 32.2 63.0 3G.8 71.4 NA

" 10 70 30.2 59.4 39.1 72.0 NA MFP 4 250 29.1 SG.7 36.5 74.7 3G

" 5 250 27.9 5G.6 35.0 73.0 38 6 250 28.5 56.4 37.4 74.7 36 MFP 8 450 28.2 57.2 39.3 69.6 34 9 450 28.2 57.3 38.5 70.8 37 NA= Sample broke outside of gage

• 1" round, 1" gage Mill Test Report RT 38.9 74.5 NR NR 39(2 in. gage)

NH=Not reported M M M M M M M M M M M M M M M M M M M

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I TABLE 3 ,

Impact Properties

-Elbow Material-Impact Lateral I,

Specimen Test Energy Expansion Shear No. Temp ( F) (ft-lbs) (mils) (%)

MFP 1 -40 4 1 0

" 4 0 11 -40 7 "9 0 11 8 0 g "8 0 27 24 20 g j

"2 40 55 46 30 I

"4. 40 96 77 40

" 14 80 111 78 70

" 16 80 159 70 100

" 12 120 136 87 100 "3 120 162 84 100 "7 160 154 86 100 I

"6 160 213* 68 100 "5 450 188 77 100 g 13 450 188 60 100 g

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