Nuclear Engineering Division

Corrosion and Mechanics of Materials

Light Water Reactors

 

Fatigue Testing of Carbon Steels and Low–Alloy Steels

Carbon and low-alloy steels and austenitic stainless steels are used extensively in LWR steam supply systems as piping and pressure vessel materials. The environmentally related increase of fatigue crack growth rates in pressure vessel and piping steels in high-temperature water is well known. LWR coolant environments can also significantly affect the fatigue lives of these steels. Under certain conditions of loading and environment, the fatigue lives of these steels can be a factor of 70 lower in coolant environments than in air. Fatigue tests are being conducted in air and simulated LWR environments to obtain data under conditions where information is lacking in the existing fatigue data base. Additional studies have been undertaken to determine crack initiation and crack growth characteristics, and to better understand the actual mechanism of degradation. The results indicate that the decrease in fatigue life of pressure vessel and piping steels is caused primarily by the effect of the environment during the early stages of fatigue damage, i.e., growth of cracks that are <200 mm deep (Stage I crack growth). The mechanisms for the enhancement of the growth rates are slip dissolution/oxidation in carbon and low-alloy steels and hydrogen-induced cracking in austenitic stainless steels.

For carbon and low-alloy steels, the magnitude of the environmental effects on fatigue life increases as the level of dissolved oxygen (DO) in water is increased. In contrast, the fatigue lives of austenitic SSs are decreased significantly in low-DOwater (<0.01 ppm DO). Also, the effect of environment is not influenced by the composition or heat treatment condition of the steel; the effect increases with decreasing strain rate and increasing temperature.

Fracture surface morphology in high-dissolved oxygen environment

Fracture surface morphology in high-dissolved oxygen environment. Click on image to view larger image.

The effect of heat treatment can be quantified in terms of electrochemical potentiodynamic reactivation (EPR). The degree of sensitization of stainless steels is assessed by a potentiodynamic sweep over a range of potentials from passive to active and measures the amount of charge associated with the corrosion of chromium-depleted regions surrounding chromium carbide-precipitated particles. An EPR value less than 1.0 designates an unsensitized microstructure, while greater than 4 corresponds to a fully sensitized microstructure.

In high-DO water, the fatigue lives of austenitic SSs are influenced by the composition and heat treatment of the steel. As shown in the figure, fatigue life is lower for sensitized Type 304 steel in BWR water; the decrease in life appears to increase as the degree of sensitization is increased. Furthermore, the fracture mode is different in water. In air, irrespective of the degree of sensitization, the fracture mode for crack initiation and crack propagation is transgranular, most likely along crystallographic planes, leaving behind relatively smooth facets. In the high-DO water, the initial crack appeared intergranular for sensitized material, implying a weakening of the grain boundaries. By contrast, in low-DO environments, cracks initiate and propagate in a transgranular mode irrespective of the degree of sensitization.

The effect of material heat treatment on the fatigue life of Type 304 stainless steel in air, BWR, and PWR environmentsThe effect of material heat treatment on the fatigue life of Type 304 stainless steel in air, BWR, and PWR environments. Click on image to view larger image.

Corrosion and Mechanics of Materials: Light Water Reactors
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Last Modified: Thu, April 21, 2016 4:56 AM

 

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