PSI - Issue 68

A.F Perez et al. / Procedia Structural Integrity 68 (2025) 439–445

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A. F. Perez et al. / Structural Integrity Procedia 00 (2025) 000–000

Both methods require structural materials that can endure extreme conditions, such as complex thermomechanical loads and high-energy neutron irradiation, while maintaining low activation properties (Murry, 1999). EUROFER97, a Reduced Activation Ferritic/Martensitic (RAFM) steel, is a promising candidate for use in fusion reactors, particularly as a structural component for the breeding blankets. In Tokamak reactors, the objective is to confine plasma at extremely high temperatures – several million degrees Celsius – using strong magnetic fields to prevent direct contact with the reactor walls. The structural components are actively cooled to prevent overheating, typically using cooling fluids like water, gas, or liquid metal (Federici et al., 2019). This cooling system maintains the structural materials within a temperature range of 300 to 600°C, which is why P91 steel (possessing similar mechanical characteristics to EUROFER97) was studied at 600°C. To better capture the irradiation phenomena, it is essential to work with small samples, which allow for a more efficient and uniform distribution of radiation damage across the material. By reducing the irradiated volume, miniature compact tension (CT) samples help achieve more representative results. Calvet (2024) used miniature P91 samples – 8 mm wide and 4 mm thick – in his tests and obtained low fracture toughness values. Building on his findings, and for comparative purposes, this project aims to characterize the fracture toughness of P91 steel at 600°C using larger CT samples, specifically with dimensions of 50 mm in width and 25 mm in thickness. 2. Materials and Experimental procedures 2.1. Chemical composition, mechanical properties and geometry P91 is a steel alloy used mainly in industrial applications where high temperatures and pressures are common. It is particularly appreciated for its excellent mechanical properties and resistance to corrosion at high temperatures. P91 is a martensitic steel containing 9% chromium and 1% molybdenum, the chemical composition of which is presented in Table 1 (Zheng, Guo & Liu, 2022).

Table 1. Chemical composition of Grade P91 steel in weight %. Element Chemical composition (wt%) C Si Mn P

S

Cr

Mo

V

Ni

Nb

Estimated value

0.18

0.44

0.45

0.017

0.005

8.37

0.93

0.23

-

0.08

Standard value

0.08-0.12

0.20-0.40

0.30-0.50

0.02 max

0.005 max

8.00-9.50

0.85-1.05

0.18-0.25

0.40 max

-

The mechanical properties of grade P91 steel at room temperature are detailed by Fournier (2007) and Touboul (2012). For higher temperatures, a range of values is consistently provided by Ruiz-Moreno et al. (2020) and Zhang et al. (2022), whose data were used for the calculation of fracture parameters at 600°C, as shown in Table 2.

Table 2. Mechanical properties of the Grade P91 steel. Mechanical Properties

Young’s Modulus, [GPa] Poisson’s ratio, [-] Yield Strength, !" [MPa] Ultimate Tensile Strength, #$" [MPa]

Room Temperature, 25°C

600°C

205 0.3 550 650

130 0.3 280 320

The fracture toughness tests were conducted on CT samples, which are characterized by high triaxiality. This increased triaxiality lowers the strain a material can withstand before failure, thereby reducing its fracture resistance. Due to the significant influence of crack tip triaxiality on fracture toughness, CT specimens tend to provide more conservative estimates of material toughness compared to other fracture geometries (Calvet, 2024). The dimensions and characteristics of the tested CT samples are summarized in Table 3.