Session: OAC-06-04 Continued Safe Operation of Existing Assets - 4

Paper Number: 105755

105755 - A Study of Stress Relaxation Cracking Mechanism in a 347h Steel Pipe-Shoe Weldment After Five-Year Service 

347H stainless steel has been widely used for piping, furnace tubes, pressure vessels and other pressure boundary components in oil refining plants because of its excellent high-temperature strength and high oxidation and sulfidation resistance. However, cracking failures in 347H steel weldments could occasionally occur and significantly shorten lifetimes of those welded heater components. In this work, a comprehensive metallurgical analysis was conducted on a cracked 347H steel pipe-shoe weldment after only about 5-year service at a petroleum refinery. Premature cracking mechanisms of this pipe-shoe weld were studied in detail by correlating multiple factors, including service conditions, weld configuration, chemical compositions, and microstructure degradations. Microstructural characteristics of different regions in the weld, including fusion zone, heat affected zone, and base metal, were examined using multi-scale microscopic techniques. The results show the cracks initiated at the fusion boundary in the weld toe and propagated towards the heat affected zone and base metal in an intergranular manner. The cracks grew via connecting grain boundary cavities formed during service. Insufficient precipitation strengthening from M₂₃C₆, and MX assisted nucleation and growth of cavities along grain boundaries was observed. Ferrite formation along grain boundary induced a high localized strain along grain boundaries, which also favored crack propagation. Due the thermal gradient during service, the microstructure degradation (cavities) along the pipe wall thickness direction was not uniform, the region near the outer diameter has more cavities than that of the inner diameter wall. The high stresses in the weld toe region caused by the deviation from component’s original design and rotational forces on the weld resulting from the weld configuration, were the main external driving force responsible for the observed early failure. A combination of stress relaxation cracking, thermal fatigue, and sensitization was likely the main mechanism of this premature cracking issue.

Presenting Author: Yiyu Wang Oak Ridge National Laboratory

Presenting Author Biography: Dr. Yiyu (Jason) Wang is a R&D Associate Staff in the Materials Science and Technology Division at Oak Ridge National Laboratory. He earned his Ph.D. degree from the University of Alberta in 2017. His current research focuses on welding metallurgy and testing of high temperature materials. He is a co-recipient of the 2017 W. H. Hobart Memorial Award and the 2018 Warren F. Savage Memorial Award from the American Welding Society.

Authors:

Yiyu Wang Oak Ridge National Laboratory
Yi Yang University of Tennessee
Yanfei Gao University of Tennessee
Jorge Penso Shell Global Solutions (US) Inc
Zhili Feng Oak Ridge National Laboratory