Effects of Thermal Aging on Fracture Toughnessand Charpy-Impact Strength of Stainless Steel Pipe Welds (NUREG/CR-6428, Revision 1, ANL/EVS-17/3)

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Manuscript Completed: February 2017
Date Published: August 2018

Prepared by:
Omesh K. Chopra

Argonne Associate
Argonne National Laboratory
Argonne, IL 60439

Appajosula S. Rao, NRC Project Manager

Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

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Abstract

The effect of thermal aging on the degradation of fracture toughness and Charpy-impact properties of austenitic stainless steel (SS) welds has been characterized at reactor temperatures. The solidification behavior and the distribution and morphology of the ferrite phase in SS welds are described. Thermal aging of the welds results in moderate decreases in Charpy-impact strength and fracture toughness. The upper-shelf Charpy-impact energy of aged welds decreases by 50–80 J/cm². The decrease in fracture toughness J-R curve, or J_Ic is relatively small. Thermal aging has minimal effect and the welding process has a significant effect on the tensile strength. Fracture properties of SS welds are controlled by the distribution and morphology of second-phase particles. Failure occurs by the formation and growth of microvoids near hard inclusions. Differences in fracture resistance of the welds arise from differences in the density and size of inclusions. However, the existing data are inadequate to accurately establish the effect of the welding process on fracture properties of SS welds. Consequently, the approach used for evaluating thermal and neutron embrittlement of austenitic SS welds relies on establishing a lower-bound fracture toughness J-R curve for unaged and aged, and non-irradiated and irradiated, SS welds. The existing fracture toughness J-R curve data for SS welds have been reviewed and evaluated to define lower-bound J-R curve for austenitic SS welds in the unaged and aged conditions. Thermal aging decreases the fracture toughness by about 20%. The potential combined effects of thermal and neutron embrittlement of austenitic SS welds are also described. Lower-bound curves are presented that define the change in coefficient C and exponent n of the power-law J-R curve and the J_Ic value for SS welds as a function of neutron dose. The potential effects of reactor coolant environment on the fracture toughness of austenitic SS welds are also discussed.