Practical Failure Analysis

, Volume 1, Issue 2, pp 71–82 | Cite as

Investigation of fatigue-induced socket-welded joint failures for small-bore piping used in power plants

  • Daniel N. Hopkins
  • Daniel J. Benac
Peer Reviewed Articles


Nuclear power plants typically experience two or three high-cycle fatigue failures of stainless steel socketwelded connections in small bore piping during each plant-year of operation. This paper discusses fatigue-induced failure in socket-welded joints and the strategy Texas Utilities Electric Company (TU Electric) has implemented in response to these failures. High-cycle fatigue is invisible to proven commercial nondestructive evaluation (NDE) methods during crack initiation and the initial phases of crack growth. Under a constant applied stress, cracks grow at accelerating rates, which means cracks extend from a detectable size to a through-wall crack in a relatively short time. When fatigue cracks grow large enough to be visible to NDE, it is likely that the component is near the end of its useful life.

TU Electric has determined that an inspection program designed to detect a crack prior to the component leaking would involve frequent inspections at a given location and that the cost of the inspection program would far exceed the benefits of avoiding a leak. Instead, TU Electric locates these cracks by visually monitoring for leaks. Field experience with fatigue-induced cracks in socket-welded joints has confirmed that visual monitoring does detect cracks in a timely manner, that these cracks do not result in catastrophic failures, and that the plant can be safely shut down in spite of a leaking socket-welded joint in a small bore pipe.

Historical data from TU Electric and Southwest Research Institute are presented regarding the frequency of failures, failure locations, and the potential causes. The topics addressed include 1) metallurgical and fractographic features of fatigue cracks at the weld toe and weld root; 2) factors that are associated with fatigue, such as mechanical vibration, internal pulsation, joint design, and welding workmanship; and 3) implications of a leaking crack on plant safety. TU Electric has implemented the use of modified welding techniques for the fabrication of socket-welded joints that are expected to improve their ability to tolerate fatigue.


failure analysis fatigue piping stainless steel welding 


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  1. 1.
    EPRI Report TR-107455, Electric Power Research Institute.Google Scholar
  2. 2.
    D.J. Benac and G.G. Chell: SwRI Report No. 06-5229-166, Southwest Research Institute, San Antonio, TX, Sep 1993.Google Scholar
  3. 3.
    CPSES Welding Manual, Welding Procedure Specifications CP-301 for stainless steel and CP-201 for carbon steel, Comanche Peak Steam Electric Station.Google Scholar
  4. 4.
    CPSES Welding Manual, Procedure WLD-106, Section 6.9.1, Comanche Peak Steam Electric Station.Google Scholar
  5. 5.
    ASME Boiler And Pressure Vessel Code, Section III, Division 1, Subsections NB-/NC-/ND-, Article 4427, American Society of Mechanical Engineers, New York, NY.Google Scholar
  6. 6.
    ASME Boiler and Pressure Vessel Code, Section IX, par. QW 451.4, American Society of Mechanical Engineers, New York, NY.Google Scholar
  7. 7.
    CPSES Welding Manual, SWQT CS-6, SS-5, Comanche Peak Steam Electric Station.Google Scholar
  8. 8.
    P. Hirschberg, P. Riccardella, M. Sullivan, J. Scheletz, and R. Carter: “Vibration Testing of Socket Welds, Phase II”ASME Pressure Vessel and Piping Conference 2000, Vol. 407, American Society of Mechanical Engineers, New York, NY, 2000.Google Scholar

Copyright information

© ASM International - The Materials Information Society 2001

Authors and Affiliations

  • Daniel N. Hopkins
    • 1
  • Daniel J. Benac
    • 2
  1. 1.Texas Utilities ElectricGlen Rose
  2. 2.Southwest Research InstituteSan Antonio

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