Prediction of J-R curves and thermoelectric power evolution of cast austenitic stainless steels after very long-term aging (200,000 h) at temperatures below 350 °C

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Abstract

Cast austenitic stainless steels (CASS) are materials used to fabricate many important safety-related components in the primary circuits of light water reactors since the early 1970’s. The primary circuit, which transports heated water by nuclear reaction to steam generators, is subjected to in-service temperatures between 285 °C and 325 °C. Under these conditions, CASS undergo thermal aging which may significantly affect their mechanical properties and more especially their fracture toughness. From a metallurgical point of view, the changes in mechanical properties are attributable to several solid-state phase transformation processes including the spinodal decomposition of the ferritic phase and the precipitation of G phase. The kinetics of these phase transformations depends primarily on the time and temperature of aging, secondly on the chemical composition of the heat (chromium, molybdenum, silicon and nickel contents) and thirdly on the heat treatments performed during component manufacturing. The prediction of the long-term behavior of CASS is an important industrial issue for nuclear power plant operators. In the early 1980’s, Electricité De France (EDF) engaged an unparalleled laboratory aging program that is still in progress. Presently, some materials have been aged for more than 200,000 h at low temperatures. The results of this program allow the development of prediction models for Charpy-impact energies, JR curves and thermoelectric power values much more precise than those proposed so far, based on aging at 400 °C and rarely exceeding 30,000 h. These prediction models are described in this paper.

Introduction

Cast Austenitic Stainless Steels (CASS) have been selected to fabricate a certain number of Light Water Reactors (LWR) components such as pump casings, elbows, pipes, fittings, valve casings, thanks to their excellent ductility, high notch toughness, corrosion resistance, and good formability. The main feature of CASS (ASTM Specification A-351 for Grades CF3, CF8, CF3M, and CF8M) is their duplex structure consisting of ferrite (between 5 and up to 35% in volume) and austenite phases. The ferrite phase provides additional benefits as it increases tensile strength and also improves resistance to stress corrosion cracking.

However, it was established already in the early 1980’s that these materials undergo thermal aging embrittlement at the service temperature around 300 °C [1]. This thermal aging embrittlement results from the microstructural evolution of the ferrite phase and can affect mechanical properties: ductile tearing resistance decreases and tensile strength increases. Fracture toughness J0.2mm values, measured by tearing resistance tests, can be as low as 40 kJ/m2 at room temperature for very susceptible materials [2]. Thus, many experimental programs have been launched worldwide to investigate the reasons for this important evolution of the properties of interest (e.g. Trautwein [3], Chopra [4,5] or Miura [6]). This extensive work has helped establishing a fairly clear qualitative understandings of the on-going phenomena within these types of steels [[7], [8], [9]]. Within the temperature range 250–550 °C, the thermal embrittlement is due to the decomposition in ferrite only (the austenite phase does not evolve neither with time nor with temperature), that is to say the formation of Fe-rich (α) and Cr-rich zones (α′). This phenomenon is strongly temperature-dependent, and more precisely:

  • Between about 250 °C and 350 °C, this decomposition occurs by spinodal decomposition. This mechanism leads to the formation of a percolated “sponge-shaped” structure. This structure is reached by the appearance of composition fluctuations whose amplitude and wavelength (a few nanometers) will increase gradually [1,10];

  • Between 350 °C and about 500 °C, depending on the chemical composition, the spinodal decomposition process tends to speed up and is finally replaced by a nucleation and growth process at the highest temperatures [11,12];

  • Parallel to the decomposition, an intermetallic phase G (Ni16Ti6Si7, in which chromium, iron, molybdenum and manganese can be substituted for titanium and nickel) precipitates at α/α′ interphase [4,13];

  • At higher temperatures, other mechanisms of aging are involved. The decomposition of the ferrite is replaced by the precipitation in the ferrite or at the ferrite-austenite interfaces of intermetallic phases (σ, R, χ, …), carbides and nitrides [4,13].

To make it short, the spinodal decomposition of ferrite and G-phase precipitation are the main contributors to the degradation of the mechanical properties of the components fabricated out of CASS within the operating conditions of a pressurized water reactor.

The severity of the thermal embrittlement of this type of materials is controlled primarily by the amount and the chemical composition of the ferrite, and to some extent by the ferrite’s morphology or by the presence of carbides or nitrides at the phase boundaries. Differences in the thermal aging behavior have been observed in CASS materials produced by different foundries, suggesting that manufacturing parameters may also be important [14]. Thermal aging of CASS materials causes an increase in hardness and tensile strength, and a decrease in ductility, Charpy-impact energy and fracture toughness. The Charpy transition curves also shift to higher temperatures.

In such instance, a predictive methodology is necessary for analyzing the structural integrity and developing monitoring strategies. Therefore, an assessment of the degradation of mechanical properties caused by thermal aging at all operating conditions is required to evaluate the performance of CASS components during prolonged exposure to service temperatures and environments. Recently, the Argonne National Laboratory (ANL) has published a detailed report on how to assess at best the thermal embrittlement of CASS using data available in the literature [4,5]. Kawaguchi [15] proposed a prediction method of tensile properties and fracture toughness of thermally-aged cast duplex stainless steel piping, and Tucker [16] worked on the assessment of thermal embrittlement in duplex stainless steels UNS 2003 and 2205 for nuclear power applications. Work has also been undertaken by Electricité De France (EDF), whether in understanding aging [10,[17], [18], [19], [20]], predicting it [21] or developing in-situ monitoring techniques, such as ThermoElectric Power (TEP) portable devices [22]. A large amount of data has now been collected by EDF within the framework of a laboratory aging program, an on-site monitoring of components and material investigations on removed components. The resulting database has several unique characteristics that distinguish it from others. First, the number of products aged and characterized is very large: several tens. The chemical composition and the ferrite content have been determined using the adequate techniques for each single heat. Secondly, the aging times at moderate temperatures (i.e. below 350 °C) exceed for some products 200,000 h making the database unique.

The main focus of this paper is the detailed description of empirical predictive models for Charpy-impact energies, JR curves and thermoelectric power evolution. A brief description of the experimental program is provided as well as the methods used for the material characterization. The validity of the proposed models was checked by comparison to the data available in the literature, and especially those mentioned in Ref. [4].

For EDF Nuclear Power Plants (NPP), the in-service assessment of component integrity is made according to Appendix 5.5 of the RSE-M Code [23] that makes use of partial safety factors on loads and toughness properties. Thus, it was necessary to develop toughness prediction formula able to predict not only the median but also specific fractiles (typically 0.16 and 0.05). In order to limit the length of the paper, we have chosen to present only the formulae relative to the median values.

Section snippets

Experimental aging program

As mentioned earlier, this program has started in the early 1980’s and has been described in great detail by Bonnet and al [17]. This program consists in measuring the evolution versus aging of several properties (hardness, Charpy-impact energy, toughness, tensile properties, thermoelectric power …) of acceptance blocks coming from the same heats as components and from representative products. These materials have been chosen in order to cover the entire range of CASS components installed on

Methodology

First, it is recalled that:

  • The formation of α′ phase by spinodal decomposition of the ferrite phase and G-phase precipitation are known to be the primary mechanisms for CASS thermal aging below 400 °C;

  • Other microstructural evolutions such as nucleation and growth of α′ phase or some carbides precipitations at ferrite/austenite phase boundaries may occur at the highest temperatures;

  • For the aging duration and temperature range of interest, thermal aging is considered to have no effects on the

Validation of the prediction models

The validation of the prediction models consists mainly on the comparison between the trends given by the formulas obtained by fitting the data of sets 1 and 2, described in the former chapter, with the experimental data of set 3. Comparisons with open literature results are also presented.

Conclusions

Over the last 30 years, an extensive aging program of cast austenitic stainless steels has been conducted by EDF. It made it possible to characterize several dozens of products of various chemical compositions with ferrite contents from 5 to 35%. These products were isothermally aged between 285 °C and 400 °C. For several of them, laboratory furnace holdings of 200,000 h were realized. The heats with very long aging times made it possible to reach saturation of the Charpy-impact energy, which

Research data for this article

Data not available/The data that has been used is confidential.

CRediT authorship contribution statement

Sébastien Saillet: Conceptualization, Methodology, Software, Formal analysis, Data curation, Writing - original draft, Writing - review & editing, Visualization, Project administration. Patrick Le Delliou: Conceptualization, Methodology, Validation, Formal analysis, Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors thank their predecessors at EDF R&D laboratories, Suzanne Bonnet, Josseline Bourgoin, Jean-Paul Massoud, Marielle Akamatsu and others, for having engaged the very long agings that allowed the achievement of this work. The authors are grateful to Abderrahim Al Mazouzi for fruitful discussions, and the authors also gratefully acknowledge Omesh Chopra for providing valuable information and helpful discussions.

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