Session: MF-20-01 Material Quality and Failure Analysis-1

Paper Number: 123473

123473 - Case Study on Metallurgical Cracking Failure in Uns N06693 Weld for Steam Generator Applications 

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, including both matching filler fusion zone and heat affected zone, after a few months in service, which warranted an investigation into possible root causes of failure. In addition to potential weldability issues generated prior to service, such as ductility dip cracking, weldability issues from service are considered in this case study that may contribute to crack propagation, such as stress relaxation cracking, stress corrosion cracking, thermal fatigue, and creep corrosion. The fracture surfaces indicate brittle failure, such that no shear lips were observed, and radial lines were primarily observed in weld fusion zone. Metallurgical characterization behind the fracture surface is conducted to understand contributing effects to failure, including phase and microstructure development of embrittling phases, e.g., intergranular gamma prime, NbC and M₂₃C₆., that may contribute to reduced elevated temperature ductility and corrosion resistance. Additionally, the weld procedure specifications (WPS) and post weld heat treatment (PWHT) conditions are investigated, and optimized welding and PWHT procedure recommendations are provided for improved microstructural stability and weldability performance. 

Keywords: nickel base alloys, stress corrosion cracking, stress relaxation cracking, ductility dip cracking, PWHT

Presenting Author: Timothy Pickle Metallurgical and Materials Engineering at Colorado School of Mines

Presenting Author Biography: Timothy Pickle is a PhD candidate in Metallurgical and Materials Engineering department at Colorado School of Mines (CSM) in the Center for Welding, Joining, and Coatings Research (CWJCR). His research focus is on reheat cracking or stress relaxation cracking (SRC) of austenitic stainless steel and additively manufactured Ni-super alloy welds. He is expected to graduate in 2024. He earned a B.S. in Materials Joining and Welding Engineering at LeTourneau University in 2018 with an undergraduate research focus on multi-pass welding effects on super duplex stainless steel microstructure. He graduated in 2021 with a M.S. in Metallurgical and Materials Engineering at CSM with a thesis title on “Effect of PWHT and Filler Metal on Stress Relaxation Cracking Susceptibility in 347H Stainless Steel Welds for Elevated Temperature Service.”

Authors:

Timothy Pickle Metallurgical and Materials Engineering at Colorado School of Mines
Kok-Theng Kho Shell Global Solutions Inc.-Shell MDS Sdn Bhd
Jorge Penso Shell Global Solutions (US) Inc.
Zhenzhen Yu Colorado School of Mines; National Renewable Energy Laboratory