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Title: Case Study on a Cracking Failure in UNS N06693 Weld for Steam Generator Applications
Abstract UNS N06693 is a Ni-base alloy that provides metal dusting corrosion resistance in steam generator pipes with operating temperatures above 500°C. A crack failure occurred in a 6.5mm thick similar weld pipe joint, located at both fusion zone and heat affected zone, after about 10 years in service and 2 months after weld repair in adjacent weld, which warranted an investigation into possible root causes of failure. This study investigates the potential failure mechanisms that may arise during service (such as stress relaxation cracking, stress corrosion cracking, ductility dip cracking, and creep failure) for UNS N06693 in order to understand the observed cracking behavior. In this year, preliminary fractography, metallurgical characterization, thermodynamic and kinetic CALHAD simulations, and investigation into potential contributing factors (e.g., weld procedure specifications (WPS) and post weld heat treatment (PWHT)) to failure have been completed. The fracture surfaces indicate brittle, intergranular failure, such that no shear lips were observed, and radial lines (crack propagation) were primarily observed in weld fusion zone. Metallurgical characterization near the fracture surface is conducted to reveal the contributing factors to failure, such as intermetallic phases (e.g., Cr-rich α-phase) and distribution of carbide particles (e.g., intergranular chromium carbides), that may contribute to reduced cracking and sensitization resistance. Blocky, intergranular Cr-rich precipitates, either Cr-rich α-phase or Cr-rich M23C6., are observed behind secondary cracks. Based on the initial findings, contributing factors for failure considered are increase in tensile residual stresses due to nearby repair field weld and grain boundary embrittlement due to coarse, blocky Cr-rich phase that likely developed during initial PWHT and within the 10-year service window. In the following year, a more in-depth metallurgical characterization, discussion on contributing causes and possible mitigation strategies for improving microstructural stability and performance-based weldability (e.g., weld procedure and PWHT design), and conclusions with root cause analysis will be provided.  more » « less
Award ID(s):
2052819
PAR ID:
10609308
Author(s) / Creator(s):
; ; ;
Publisher / Repository:
American Society of Mechanical Engineers
Date Published:
ISBN:
978-0-7918-8850-6
Format(s):
Medium: X
Location:
Bellevue, Washington, USA
Sponsoring Org:
National Science Foundation
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