Fracture toughness assessment of in-service aged primary circuit elbows using mini-CT specimens taken from outer skin
Introduction
Type CF8M cast duplex stainless steels used in the primary circuit elbows of pressurized water reactors are known to undergo thermal aging embrittlement at their service temperature, at ∼300°C. This phenomenon affects their mechanical properties, mainly their fracture toughness (tearing resistance) (Bethmont et al., 1991, Massoud et al., 1991), which needs to be evaluated for mechanical analyses.
Fracture toughness estimation methods through predictive formulae using chemical composition and aging conditions are available (Slama et al., 1983), which provide safe values. However, in the case of the most embrittled elbows, more accurate evaluations are also needed. A new method was therefore considered: it consists of taking relatively thin slices of material from elbow surfaces and performing tearing resistance tests on very small fracture mechanics specimens (0.4T–CT with thickness reduced to 5 mm), called mini-CT.
Given the coarse macrostructure of the cast duplex stainless steels and the level of scatter usually encountered on tearing resistance curves using 1T–CT specimens, the use of mini-CT specimens is not obviously relevant. A validation program was thus undertaken before starting taking samples from in-service elbows.
This paper deals with the development and application of mini-CT specimens testing. It is divided into three parts: the first one presents the experimental procedures, the second shows the results of the validation program and the third is devoted to the application to an in-service elbow.
Section snippets
Experimental procedures
All tearing resistance tests were carried out according to the GFR procedure (French Fracture Group, 1990). Except for a few differences, this procedure is equivalent to the ASTM E813-89 standard.
Validation program
In order to check the relevance of tearing resistance tests using mini-CT specimens, a validation program was undertaken. Our major concern was the level of scatter of the J−Δa results, which we feared might be enhanced by reducing the affected volume of material: there are only one or two primary ferritic grains ahead of the crack front of mini-CT specimens, whereas they are five times as numerous for 1T–CT specimens (Jayet-Gendrot et al., 1994).
The validation program consists of testing
Application to an in-service elbow
Three samples of approximate dimensions 55×54×6 mm were taken from the outer skin of a hot leg 50° elbow in Gravelines 4 reactor. The choice of sampling areas was guided by industrial considerations (sufficient thickness, accessibility, absence of repair and of radiographic indication, presence of full lagging). This elbow had remained in service for 86 898 h at the temperature of 323°C. The ferrite contents measured on the samples range from 33 to 35%. The chemical composition measured on one
Conclusion
An experimental program was carried out to study the use of small 0.4T–CT specimens with reduced thickness to estimate the fracture toughness of embrittled cast materials. The results showed that such mini-CT specimens are relevant and guidelines for further practice are proposed. The main outcomes are the following:
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the J−Δa results obtained with mini-CT specimens are correctly centered around those obtained with regular 1T–CT specimens. Scatter is of the same order.
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the results corresponding to
Acknowledgements
We would like to thank M. Naumann for her work on specimen observations during her internship at EDF.
References (7)
- Bethmont, M., Meyzaud, Y., Soulat, P., 1991. Properties of Cast Austenitic Materials for Light Water Reactors. Proc....
- French Fracture Group, 1990. Test Recommendations for Measurement of the Ductile Tearing Resistance of Metallic...
- Jayet-Gendrot, S., Ould, P., Balladon, P., 1994. Effect of Fabrication and Test Parameters on the Fracture Toughness of...
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