PSI - Issue 68

Julie Papin et al. / Procedia Structural Integrity 68 (2025) 727–733

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Julie Papin et al. / Structural Integrity Procedia 00 (2025) 000–000

1. Introduction The operating time of Pressurized Water Reactor nuclear power plants is limited by the safety of non-replaceable components, like its reactor pressure vessel, made of low-alloy, quenched and tempered bainitic steel (A508 Gr.3). Accurate assessment of the fracture toughness of this steel is essential to guarantee the integrity of the pressure vessel especially in accidental scenarios considered in their design. This fracture toughness depends on many factors including the testing temperature (because of the ductile-to-brittle transition, DBT) and the initial microstructure. Indeed, the high thickness of the component induces large variations of local heat treatment conditions during manufacturing, particularly of the quenching rate, from the skin to the mid-thickness, and it leads to a broad range of microstructures, from proeutectoid ferrite mixed with bainite to martensite, that has an impact on the mechanical properties. The current study focuses on the effects of heat treatment parameters on the fracture toughness and on microstructural features affecting the brittle crack initiation, in the as-manufactured material. The DBT of low-alloy steels has been extensively studied for the last fifty years, through fracture toughness and impact toughness tests. Probabilistic approaches – such as the local approach to fracture (Beremin, 1983; Wallin et al., 1984) – have been developed to predict the effects on fracture toughness of the specimen size and temperature, as well as the large scatter of experimental results in the DBT. These approaches led to the standardized Master Curve method at the end of the 90’s (Wallin, 1998). Microstructural analyses identified brittle particles as potential cleavage initiators. However, the identification of microstructural features at the initiation site has been sparsely reported and no systematic link between brittle particles (such as inclusion or precipitates) and the triggering of fracture has been established. Some recent studies addressed cleavage initiation (location and particles type), and/ or the effect of heat treatments on fracture properties (Gibson et al., 1989; Chekhonin et al., 2023; Delattre et al., 2024, 2022a, 2022b). In the work reported by Delattre et al., several heat treatment conditions were applied to a unique SA508 Gr. 3 steel to produce a large panel of microstructures encompassing the ones that can be observed on industrial components. The characterization of these microstructures, as well as the ranges of the resulting tensile and Charpy impact toughness properties have been reported in the previous editions of ECF23 and Fontevraud 10 conferences (Delattre et al., 2022a, 2022b). In the present work, fracture toughness tests were performed on some of these microstructures, followed by thorough fractographic analyses of the initiation sites of specimens broken in the brittle domain. The purpose is to evaluate how the populations of brittle phases responsible for cleavage fracture initiation might evolve with heat treatment parameters, and whether there is a link between the crack initiation mechanism and the value of the fracture toughness.

Nomenclature EDX

point energy dispersive spectrometry ductile-to-brittle transition DBTT ductile-to-brittle transition temperature T 0 DBT estimated reference temperature Charpy index temperature at 41J energy T41J T 0 *

fracture toughness reference temperature (according to ASTM E1921 standard)

TP tempering parameter K Jc (1T) median stress intensity factor

2. Materials and testing conditions 2.1. Material and heat treatments

The starting material was a macroscopically homogeneous part of an industrial A508 Gr.3 ring shell manufactured by forging a hollow ingot. Chemical composition is given in Delattre et al. (2022a). Coupons were austenitized at 870°C for 2h, then cooled down at prescribed quenching rate, and tempered. The tempering parameter, TP, was defined by equation 1 with T the tempering temperature in °C and t the tempering duration in