PSI - Issue 68

Marwa Ben Bettaieb et al. / Procedia Structural Integrity 68 (2025) 297–302 M. Ben Bettaieb et al. / Structural Integrity Procedia 00 (2025) 000–000

298

2

1. Introduction Within the framework of the developments of the fourth generation Sodium cooled Fast nuclear Reactors (SFR), the non-replaceable components are supposed to operate in their hottest parts at about 550 °C for up to 60 years. Thus, the main damage mechanism to take into account in their design is long-term creep. The considered material for these components is AISI 316L (N) austenitic stainless steel. Depending on the type of component, different manufacturing routes are followed: while rolled plates are mostly used, large-scale thick components are made of hot forged thick plates with niobium addition to prevent grain growth at high temperature during manufacturing. Small variations in chemical composition or different manufacturing processes can substantially influence the creep behavior of the material. Many works have already been focused on the effect of microstructure on mechanical creep properties. (SOURMAIL, 2001) mentioned that the presence of niobium (Nb), titanium (Ti) or vanadium (V) improves the creep resistance by precipitation strengthening. In such a way, non-stoichiometric MX precipitates (M=Nb, Ti or V and X=N or C) can be observed in steels enriched in stabilizing elements. However, other studies shows that Z-phase (NbCrN) can be formed instead of MX phase at high temperature, during aging or creep, within the matrix or at grain and twin boundaries (LI et al., 2018; VODAREK, 2011). The stability of one or the other phase in the steel depends on the amount of nitrogen, carbon and niobium. (VODAREK, 2011) and (NASSOUR et al., 2001) proved that the addition of niobium to austenitic stainless steels reduces the minimum creep rate and the creep strain. Nitrogen (N) increases creep lifetime either by forming fine nitride precipitates or by solid solution strengthening (MATHEW et al., 2012; SOURMAIL, 2001). Molybdenum (Mo) plays also a role in improving creep properties by solid strengthening (SOURMAIL, 2001). In this article, the creep behaviors at 575 °C/ 310 MPa of two AISI 316L (N), one Nb-free and one Nb-rich alloys, are compared. Their creep damage mechanisms and microstructural evolutions are studied.

Nomenclature A

maximum elongation of tensile test (%) ultimate tensile strength (MPa)

UTS R p 0.2

0.2% proof stress (MPa)

2. Studied materials Resulting from two different manufacturing processes, two types of AISI 316L(N) austenitic stainless steel are studied : Nb-free rolled plate and Nb-rich hot-forged material. In the latter case, the niobium is added to prevent grain growth during forging at high temperature. After being manufactured, the materials are solution-annealed at temperature between 1050-1150 °C, followed by water quenching. Chemical composition of each material in weight per cent (wt.%) is presented in Table 1. ZEISS Sigma 300 Scanning Electron Microscopy (SEM) is used for microstructural analyses operating at 5 kV. First investigations are carried out at the-as received state of the two materials. A typical 316L (N) equiaxed microstructure with an average grain size of about 90 µm is observed for the Nb-free alloy. A larger average grain size of about 180 µm is measured for the Nb-rich alloy. Subgrain boundaries are further observed in the latter material. Thin foils of Nb-rich material at the as-received state are examined by JEOL 2100 Transmission Electron Microscopy (TEM).

Table 1. Chemical compositions of the studied materials (wt.%). Chemical composition Cr Ni Mo

Mn 1.7

N

C

Si

Cu

Nb

Rolled plate (Nb-free plate)

17.38 12.12 17.42 12.33

2.39 2.53

0.069 0.025 0.19 0.073 0.024 0.4

0.16 0.08

-

Hot-forged material (Nb-rich alloy)

1.73

0.119