The Effects of Aging at 343°C on the Microstructure and Mechanical Properties of Type 308 Stainless Steel Weldments (NUREG/CR-6628, ORNL/TM- 13767)

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Publication Information

Manuscript Completed: May 1999
Date Published: November 2000

Prepared by:
D. J. Alexander, K. B. Alexander,
M. K. Miller, R. K. Nanstad

Y. A. Davidov, Visiting Scientist
Institute for Metal Science
Bulgaria

Oak Ridge National Laboratory
Managed by Lockheed Martin Energy Research Corp.

Oak Ridge National Laboratory
Oak Ridge, TN 37831-6151

C. J. Fairbanks, NRC Project Manager

Prepared for:
Division of Engineering Technology
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

NRC Job Code W6953

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Abstract

Some piping systems in light-water reactors contain welds made with type 308 stainless steel filler metal that are routinely subjected to operating temperatures above 300°C. The effect of long-term aging of stainless steel welds at 343°C (650°F) has been studied by the Heavy-Section Steel Irradiation Program to determine the extent of degradation of the mechanical properties of the weld metal. Three multipass shielded metal-arc welds were prepared from type 304L base plate with type 308 filler metal. The chemistry of the filler wire was adjusted to obtain different ferrite levels (4, 8, or 12%). Portions of these welds were aged for 3,000, 10,000, 20,000 or 50,000 h at 343°C. Charpy V-notch and tensile specimens were taken from both the tops and the bottoms of these welds. The tensile results were similar for all the specimens and showed little effect of aging on the yield or ultimate tensile strength or the ductility. In contrast, the Charpy impact toughness, as characterized by either the ductile-to-brittle transition temperature or the upper-shelf energy, was significantly degraded by aging. The extent of the degradation increased with increasing ferrite content and increasing aging time. Embrittlement continued to increase up to 50,000 h of aging with no indication of a saturation. At lower test temperatures the fracture path appeared to preferentially follow the ferrite phase, which fractured by a brittle cleavage-like mode. Sections taken perpendicular to the fracture plane revealed microcracks in the ferrite and strain-induced martensite in the austenite phase near the fracture surface. Aging for 50,000 h at 343°C also resulted in a significant decrease in the J-Integral fracture toughness at 290°C of the 8 and 12% ferrite welds. The microstructures of the welds were examined by optical metallography, scanning and transmission electron microscopy, and atom probe field-ion microscopy. Examination of material aged for 20,000 h showed that the ferrite phase undergoes spinodal decomposition, creating iron-rich and chromium-rich regions. Additionally, relatively large G-phase particles were observed heterogeneously associated with dislocations, and fine G-phase particles were distributed homogeneously through the ferrite phase. No changes were observed in the austenite phase. The hardening of the ferrite phase due to the spinodal decomposition is believed to be the primary factor responsible for the loss of toughness observed after aging.