ML20198A248

Metallurgical Failure Analysis of Check Valve Guide Pin

ML20198A248
Person / Time
Site:	Turkey Point
Issue date:	04/04/1986
From:	Leonard L CALSPAN CORP.
To:	Cortland P NRC
Shared Package
ML20198A241	List: ML20198A236 ML20198A248
References
CON-NRC-05-83-216, CON-NRC-5-83-216 F-5896-015, F-5896-15, NUDOCS 8605200392
Download: ML20198A248 (11)

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A NRC Contract 05-83-216 FRC Project 5896-015 NRC Task No. TA-M-215 bA Y

METALLURGICAL FAILURE ANALYSIS OF A CHECK VALVE GUIDE PIN FRC Report No. F-5896-015 Prepared for:

Engineering and Generic Communications Branch Office of Inspection and Enforcement Author: L. Leonard

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U.S. Nuclear Regulatory Commission FRC Task Leader: G. J. Toman

- Washington, DC 20555 NRC Project Manager: P. Cortland April 4, 1996 Prepared by: Reviewed by: Approved by:

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- FRANKLIN RESEARCH CENTER DIVISION OF ARVIN/ CAL 5 PAN 8605200392 860501 aoen a eaci steurs.r.ama.pa mos PDR ADOCK 05000250 S PDR I

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1. INTRODUCTION At the request of the U.S. Nuclear Regulatory Commission's Office of Inspection and Enforcement, Franklin Research Center (FRC) conducted a metallurgical failure analysis of a check valve guide pin from Florida Power &

Light's Turkey Point Station. The check valve disc (of which the guide pin is an integral part) and the piece of the guide pin which had broken free from the disc wera provided to FRC. The failure analysis consisted of a fracto-graphic study using a scanning electron microscope (SEM), as well as metallo-graphy and hardness testing to evaluate the role of the guide pin's material in the failure process.

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2. ANALYSES AND DISCUSSION Macrographs of the valve disc and guide pin are presented in Figures 1 through 3. In Figure 2, it is clear that the pin had interacted with the disc and its seat after the fracture, resulting in deformation of the pin as well as its fracture face. The fracture face on the pin, as shown in Figure 3B, exhibited a series of steps or ratchet marks around the periphery of the fracture. Such steps were caused by the intersection of cracks that origi-nated at different locations. Multiple crack initiation of this type is indicative of high stresses (relative to the material's strength) and/or high stress concentrations, particularly under cyclic (i.e. , fatigue) loading.

Both the stress concentrations and cyclic loading can be readily identi-fied. Stress concentrations were plainly evident at the pin-to-disc transi-tion in the form of a very large section size change, a coarsely machined surface, and an apparently small radius of curvature at the transition.

Vibration-induced cyclic loading in service can be postulated to have been imposed on the small pin at the transition point by the massive disc body.

To verify that the mode of failure was indeed fatigue, the stub end of the fracture was cut from the valve disc and examined in the SEM. It was found that most of the fracture had been rubbed over from interaction with the mating fracture face during the failure process. This' rubbing was particularly heavy, as would be expected, around the outer periphery or earlier stage of the fracture. However, very distinct striations, characteristic of fatigue, were found at various locations, particularly near the center of the pin.

Examples of these striations are shown in Figure 4. The presence of fatigue fracture rather than ductile or brittle fracture near the center of the pin rules out the possibility that an overload somehow had occurred, and establishes that cyclic bending of the pin had led to its failure.

The inherent resistance of the pin material to fatigue cracking was evaluated by metallography and hardness testing. Both longitudinal and transverse cross sections were prepared. As shown in Figure 5, the elongated inclusions in the longitudinal section reflect the fact that the disc body and its pin had been machined from bar stock or frcs a forging. The microstructure a

F-5896-015 shown in Figure 6 is typical of a normalized or annealed steel with about 0.25% carbon. Consistent with the microstructure, the hardness was R

• B This hardness, which also indicates that the pin material was a plain carbon rather than an alloy steel, corresponds with a low strength, i.e.,

approximately 77,000 psi ultimate strength.

For most steels, the fracture limit of smooth samples is about one-half of the tensile strength. Thus, for the 3/8-inch-diameter pin, in the absence of a stress concentrator, the fatigue limit in reversed bending loading at the failure location would be on the order of magnitude of 4000 pounds. When this value is further reduced by a stress concentration factor and by the fact that the strength of a steel is reduced at elevated temperatures (the oxidized surfaces of the valve and fracture indicate the valve had experienced such service), it is readily apparent that the combination of the design, surface finish, and material of the disc pin offered limited resistance to cyclic loading.

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3. CONCLUSIONS Based upon the analyses discussed above, the following conclusions can be presented:

1. The guide pin had fractured from the disc body in a fatigue mode.

2. Cracking originated at many locations around the pin at the pin-to-disc body transition owing to a combination of:

a. service-induced applied bending stresses ~ ,

b. large stress concentrations resulting from a pronounced section size transition and a coarse surface finish, and

c. a low strength material.

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B. Side View 1X Figure 1. Macrographs showing the valve disc. Arrow !. indicates the seat, and Arrow B points to the base of the fractured guide pin.

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I B. 1.5X Figure 2. Macrographs showing two views of the guide pin, with the fractured end at the riSht. -The gouges and dents along the side of the pin and the bendit.g resulted from the pin having been wedged between the disc and the seat after the pin had fractured from the disc body. (The hardness indentations were on the pin when it was j received at FRC.)

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1000X l, Figure 4. SD4 micrograph showing fatigue striations on the disc-side f racture ,

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