Effect of strength mismatch on fracture behavior of ferrite-austenite interface in Ni-base alloy dissimilar metal welds

Teemu Sarikka

Research output: ThesisDoctoral ThesisMonograph

Abstract

Dissimilar metal welds (DMW) are extensively used in energy production applications, such as nuclear power plants (NPP), to join the ferritic steel nozzles of reactor pressure vessels (RPV) to the austenitic stainless steel pipings using different types of safe-end designs. Traditional, V-groove safe-end designs made using filler metal Alloys 82/182 are being replaced by modern safe-end designs manufactured using narrow gap (NG) welding technique with filler metal Alloy 52/152. The higher Cr-containing Alloy 52/152 provides improved corrosion resistance, especially resistance to stress corrosion cracking (SCC), and NG technique provides technical and economical benefits. The difference in filler metal and NG technique require thorough understanding of the metallurgical and mechanical properties of the resulting weld as the changes may have an effect on the structural integrity of the DMW. The objective of this thesis was to determine the microstructural properties, mechanical properties, and mechanical mismatch across a ferritic-austenitic interface of a traditional V-groove DMW and a modern NG DMW. The mechanical mismatch was determined using tensile testing and microhardness and nanoindentation testing. The different mismatch states were compared to the fracture mechanical behavior of the ferritic-austenitic interface. Microstructural properties of the different DMWs were found to differ from each other in terms of the existence of a carbon-depleted zone (CDZ) and a narrow carbon pile-up near the fusion boundaries of the DMWs. The existence of the CDZ and carbon pile-up caused high mechanical mismatch at the fusion boundary region of the NG DMW and affected the fracture propagation behavior near the fusion boundary region of the NG DMW. The mechanical mismatch was observable with nanoindentation testing, but the tensile tests even with small-sized specimen did not reveal the high mechanical mismatch near the fusion boundary region.
Translated title of the contributionPaikallisen yli-/alilujuuden vaikutus nikkelipohjaisten ferriittis-austeniittisten eriparihitsausliitosten murtumiskäyttäytymiseen
Original languageEnglish
QualificationDoctor's degree
Awarding Institution
  • Aalto University
Supervisors/Advisors
  • Hänninen, Hannu, Supervising Professor
  • Nevasmaa, Pekka, Thesis Advisor
Publisher
Print ISBNs978-952-60-6984-5
Electronic ISBNs978-952-60-6983-8
Publication statusPublished - 2016
MoE publication typeG4 Doctoral dissertation (monograph)

Keywords

  • dissimilar metal weld
  • narrow gap welding
  • mechanical mismatch
  • fracture mechanics
  • fracture behavior

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