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Metallurgical evaluation and failure analysis of turbo expander Tie-bolt

Attarian, M ; Sharif University of Technology | 2023

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  1. Type of Document: Article
  2. DOI: 10.1016/j.engfailanal.2023.107392
  3. Publisher: Elsevier Ltd , 2023
  4. Abstract:
  5. Failure in a turbo-expander Tie-Bolt (stud) made of superalloy A-286 was investigated. The evaluation of the chemical composition and tensile properties confirmed compliance with the relevant standard. However, the coarsening of γ′ due to over aging condition, the non-uniform grain size and banded coarse MC carbide particles, the segregation of phosphorus to grain boundaries and the growth of carbide particles at the grain boundaries led to a decrease in fatigue strength of the alloy. Fractography studies showed that fracture occurred due to primary axial and secondary bending loads. Multiple ratchets and tire tracks on the fracture surface were consistent with low-cycle fatigue (LCF) failure mechanism. Fatigue striations spacing and micro hardness profile of the threads were used to calculate the stress amplitude and estimate the fatigue strength of Tie-bolt. Neuber's rule was used to estimate the local stress and strain in the thread root. The calculation results showed that the failure occurred due to the application of cyclic loads at a stress level lower than fatigue strength of the stud. A large amount of sulfur detected at the multiple cracks initiation areas by EDS analysis, indicated the presence of H2S in the environment, which has caused cathodic hydrogen charge and reduced the steel ductility and accelerated fatigue crack nucleation and growth. © 2023
  6. Keywords:
  7. Failure ; Hydrogen embrittlement ; Low cycle fatigue ; Tie-bolt ; Tire tracks
  8. Source: Engineering Failure Analysis ; Volume 151 , 2023 ; 13506307 (ISSN)
  9. URL: https://www.sciencedirect.com/science/article/abs/pii/S1350630723003461