phenomenological law to predict damage occurrence. Based on critical value of creep damage Dc, an attempt is made to show how a crack initiates under mode I and mode II creep loading conditions. ANALYSIS OF AGEING, BEHAVIOUR AND DAMAGE Microstructural ageing Similar studies of weld metal existed in the literature but conclusion was clear that precipitation process varied with the composition, even for minor elements [3,4]. Hence TEM samples were observed of material aged for 2000 hours and 10000 hours at 600°C. With the present composition and welding parameters [1], ageing process takes place in the ferrite which tends to disappear to the benefit of chromium carbides and Vphase formation as shown in figure 1. Laves phases have also been identified, as well as secondary austenite. Chromium carbides of small size precipitate as well in the few J J boundaries deprived of ferrite. Similar results for the characterisation of a weld metal can be found in detail in [5]. As far as the mechanical behaviour analysis is concerned, ageing and creep are intimately combined, even after a 2000 hours 600°C heat treatment as discussed later. Constitutive equations The first set of coefficients identified and reported in [1] failed in reproducing well the behaviour of notched specimens. A new identification was performed and the following coefficients were obtained with H in % and V in MPa: primary creep: H V C t n C 1 1 2 n1= 3.63 ; C1=0.1999 10 -9 (MPa-n1s-C2) ; C2=0.3144 secondary creep: H V C n n= 11.47 ; C=0.413 10-29 (MPa-ns-1) Tertiary creep stage was not modelled. Comparison between experiment and model for the notched specimens are to be based on a same strain basis. The new values for the parameters given here make no change to the results obtained on figure 3 of (1) relative to the distribution of maximum principal stress in the notched section of the axisymmetrical specimens. Stress is maximum at the centre of the 4-mm notch radius specimens when it tends to be closer to the notch tip for the 1-mm radius specimens. Damage could not be observed in the material at the end of the secondary creep stage. Some tests were interrupted after the onset of the tertiary stage in order to study creep damage micro-mechanisms. Creep damage. Qualitative analysis Creep cavitation in the weld metal is intergranular, as in the base metal [6]. Figure 2 presents a SEM photograph showing the formation of a real alignment of cavities of sub-microscopic size along a J J grain boundary. Electron Back Scatter Diffraction (EBSD) analysis was used to characterise those boundaries from a crystallographic point of view. The light micrograph of figure 3a) shows the cross section of a crept notched specimen. The microstructural features have been revealed with oxalic acid, showing indiscriminately in dark grey colour ferrite, precipitates, grain boundaries and cavities. It has to be noticed that damage tends to be located at the centre of the notched part of the 4-mm notch radius specimens whereas it can be seen closer to the notch tip for the 1 mm notch radius ones. Orientations of the austenite grains of this area were obtained using the EBSD automatic mapping technique (figure 3b). Each colour corresponds to a specific orientation. It is clear from this experiment that cavities, sometimes large enough here to be called micro-cracks, are located along the J J grain boundaries. Furthermore, measurements of misorientation angles between grains on each side of the boundary [reported table 1] prove that damage is located at high angle boundaries.
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