244 Prakt. Met. Sonderband 46 (2014)
such as hot cracks or lack of fusion were observed in the welded joint. After creep loading particles
numerous separations of grain boundaries without cavity formation could be seen. The heat trea
microcracks occur, often as wedge cracks at the grain triple points, which suggests the
grain boundary sliding as the creep mechanism, typically at high loads. All cracks are
concentrated along the grain boundaries. The crack paths are, depending on fracture
location, interdendritic in the weld metal and intercrystalline in the base material. The heat
affected zone (HAZ) area was free of cracks.
Fig. 3: °
W1, ¢) st
From the:
= stabilizati
} N } . formation
Fig. 2: virgin a) BM, b) WM; after creep test c) fracture in WM (state A3/W3), d) fracture in At the gre
BM (state A4/W4) (Fig.4 c).
3.2.2 TEM INVESTIGATIONS Saree
; ; ; Es states rat
The influence of short-term creep loading at 700 °C on the microstructure of welded joints The diffe
of Alloy 617 in as-welded condition, after PWHT at 980 °C and after creep loading was boundarie
investigated by TEM in the area of the base material and the weld metal, see Tab. 2. precipitati
TEM images of all examined states are shown in Fig. 3. In the as-welded condition only strength ¢
few M23Cs carbides are found both in the BM and in the WM within the grain and isolated grain bol
at the grain boundaries (Fig. 3 a and b). The dislocation density in the weld metal is towards t
10 times higher than in the base material. Y phase was not identified in any state. 980 °C of
After creep at 700 °C, the density of M23Cs carbides within the grain increases. At the grain stable ant
boundaries the number of particles increase in BM but decrease in WM. vy ‘phase occurs can abso
within the grain in both states. Fig. 3 ¢ and d show the results for states after PWHT at grain bou
980 °C. Compared with the non-heat treated initial state more M23Cs carbides are found in the non-h
both areas (BM and WM) within the grain and at the grain boundaries. means th:
After short-term creep loading there is still a difference between two conditions. In heat If areas o
treated state, the size of My3Cs carbides within the grain increases, the y ‘phase is, that in bo
however, smaller (Fig. 4 a and b). This could be observed for both the base metal and the low amou
weld metal. The changes during the creep loading are not significant especially concerning GB-gliding
M23Cs carbides in the PWHT-state. The density of Mx3Ce particles in heat treated state and maki
does not increase, however, is increasing in the non-heat treated condition, since only few formation