Issue 35

X.C. Arnoult et alii, Frattura ed Integrità Strutturale, 35 (2016) 509-522; DOI: 10.3221/IGF-ESIS.35.57 510 As the current energy policy envisages an extended lifetime for the current Generation II nuclear power plants, large research effort must be devoted to understanding the ageing of power plant components, where delamination cracking is one of the possible ageing mechanisms. In this short review, the mechanism of delamination cracking is presented and open questions are identified in determining if the delamination cracks have negative or positive impact on the material toughness, which applies especially to the crack divider orientation. (a) L-T and T-L orientation: crack divider. (b) T-S and L-S orientation: crack arrester. (c) S-L and S-T orientation: crack splitting. Figure 1 : Terminology used to describe the various orientation of delamination cracks [13, 19]. I NFLUENCE OF HEAT TREATMENT , TEST TEMPERATURE AND IRRADIATION Tensile fracture surface ramfitt and Marder [22] tested a very low-carbon steel (VLC steel) under tensile loading at room temperature to understand the influence of the finishing temperature of heat treatment (960°C - 150°C) on the fracture surface of this steel. After fracture, delamination cracks were observed in all transverse specimens from plates where the finishing temperatures were at 480°C and below. In the case of longitudinal specimens, delamination cracks occurred on specimens made from plates having a finishing temperature at 370°C and below. Thus finishing temperatures at least 260°C below the A1 temperature (i.e. the eutectoid temperature above which austenitic phase is formed) promoted the development of delamination cracks in the fracture process of the VLC steel. It was also observed that the number of delamination cracks increased with decreasing finishing temperature of heat treatment and delamination cracks were always orientated parallel to the rolling plane. Baldi et Buzzichelli [23] investigated the influence of finishing temperature (cf. Table 1) and microstructure on seven different high-strength-low-alloy-steels (HSLA steels). They studied the tensile behavior of these steels as a function of finishing temperature and specimen orientation (0° and 90° relative to the rolling direction). The authors observed that on all studied HSLA steels, the fracture surface displayed longitudinal delamination cracks in both directions. Code Heat Temperature, °C Rolling schedule Finishing temperature, °C Final thickness, mm Cooling MC1 1150/2h 80% R.A. at < 750°C 700 12 Air MC2 1150/2h 80% R.A. at < 750°C 700 12 Air MC3 1150/2h 80% R.A. at < 800°C 750 12 Air MC4 1150/2h 80% R.A. at < 750°C 700 12 Oil quench F2A 1150/2h 80% R.A. at < 750°C 700 12 Air C1B 1150/2h 80% R.A. at < 750°C 700 12 Air M1 1150/2h 66% R.A. at < 900°C 750 12 Air Table 1 : Rolling and heat-treatment schedule [23]. Similarly, Yan et al [6] investigated the influence of tempering temperature on a HSLA steel. The 200°C, 400°C and 700°C tempered specimens exhibited a typical cup-and-cone tensile fracture (cf. Figure 2). There were no delamination cracks on the fracture surface. However, large and deep delamination cracks were observed on the fracture surfaces of tensile specimens tempered in the range between 500°C and 650°C (cf. Figure 9). The latter specimens did not have a typical cup-and-cone fracture shape, which indicates a low or very low ductility, and the fracture surfaces were divided in two B