Issue 42

A. Strafella et alii, Frattura ed Integrità Strutturale, 42 (2017) 352-365; DOI: 10.3221/IGF-ESIS.42.36 353 was developed and utilized in the PHÉNIX reactor where it exceeded the target burnup of 100 dpa. Addition of silicon to 15-15Ti further improved the swelling resistance. With silicon-modified 15-15Ti, the maximum fuel pin deformation was reduced by a factor of 2.4 as compared to the silicon-free 15-15Ti grade [3]. The addition of Ti also improves the high temperature mechanical properties due to the precipitation of carbide particles, either M 23 C 6 , or TiC in relation to the contents of Ni and Cr and to the matrix structure, grain size and dislocation density. It has been noticed for a long time that the high dislocation density found in cold-worked steels reduces the swelling. For this reason, Ti-modified austenitic stainless steels are often cold-worked to increase their mechanical properties. Dislocations trap vacancies and decrease their supersaturation. Nevertheless, a recovering of the dislocation network is observed at high temperature (T > 540 °C) and reactivates the swelling. Titanium has been added in the alloys to avoid this phenomenon. It induces the precipitation of nanosized titanium carbides which pin the dislocation network and stabilize it at high temperature. Indeed, the cold-working promotes the precipitation of carbides which are liable to precipitate on slip planes produced by pre-strain and creep strain. Therefore, the cold-worked steels have higher crack growth resistance at high stress intensity levels compared to solution-annealed stainless steel [4-7]. For its properties, 15-15Ti(Si) is one of the best candidates for high temperature components of nuclear reactor of IV generation Lead-cooled fast reactor (LFR) [2-3] which is one of the systems to be deployed in the future. The main problem in LFR reactor development is the compatibility of the structural materials (steels) with the coolant as well as the corrosion of structural components and fuel. When steel comes in contact with liquid metal, the loss of ductility in normally ductile steel could occur; this phenomenon is named Liquid Metal Embrittlement (LME) and takes place when steel is stressed in temperature under contact with liquid metal. Although there is a great interest on these steel properties, only few data on its characterization can be found in the literature. So, the aim of this work is the study of the 15-15Ti(Si) stainless steels creep properties both in air and in lead. For this purpose, the creep behaviour was investigated at 550° C, under a wide range of applied stresses, in hostile condition or in air , following by a morphological analysis of fracture surfaces. E XPERIMENTAL PROCEDURE Materials ests were carried out on creep specimens prepared from 15-15Ti(Si) steel, an austenitic steel provided by OCAS. The steel was manufactured in the form of plates (15mm X 750mm X 250mm), 20% cold-worked (CW): it was subjected to a homogenization annealing treatment in a furnace at 1230°C for 15h, hot-rolled at 1250°C for 1.5- 2h, annealed at 1080°C (followed by a water-quench treatment) and 20% CW. The steel was cold-worked because this process promotes the precipitation of carbides, which generally increases the mechanical properties of materials. Compared to solution-annealed stainless steel, the cold-worked steels have higher crack growth resistance at high stress intensity levels. The composition of 15-15Ti(Si) is given in Tab. 1. Table 1 : Chemical composition of 15-15Ti(Si) steel As anticipated in the previous paragraph, it must be underlined that the Ti presence in the steel is important for the purposes of this work because induces the precipitation of carbide particles, M 23 C 6 or TiC, then improves the high temperature mechanical properties. Moreover, Ti addition decreases the irradiation swelling; that is important for nuclear applications. The used lead is provided by Sigma-Aldrich in particles with a purity ≥ 99.9%, size ≤ 2 mm and melting point 327.4°C. Specimens Creep specimens were machined from the plate in the rolling direction to obtain 6 mm in diameter and 70 mm of gauge length, as shown in Fig. 1. T All elements C Mn Si P Ti Cr Ni B Mo % 0.090 1.502 0.791 0.041 0.404 14.392 15.607 0.007 1.509