Prakt. Met. Sonderband 46 (2014) 245 
‘eep loading particles were present there before the creep test. The density of Y phase increases in 
> seen. The heat treated condition more than in the initial state. 
uggests the 
| cracks are 
on fracture 
al. The heat 
Fig. 3: TEM bright-field images of Alloy 617 for different conditions: a) state A1, b) state 
W1, c) state A2, d) state W2, e) state A3, f) state W3, g) state A4, h) state W4, see Tab.2. 
From these observations it can be concluded that the PWHT at 980 °C effected a thermal 
stabilization of the My3Cs within the grain, and causes simultaneously an increased 
formation of ¥ phase due to the reduced precipitation behaviour of M»3C. 
) fracture in At the grain boundaries, the M,3Cs carbides are larger by a factor of 2 than within the grain 
(Fig.4 c). Although the carbides after PWHT are already twice as large as in the untreated 
state, they grow continuously, while they rather decrease in the initial state (solution 
annealed for BM and as-welded for WM). The number of M23Cs carbides increases in both 
Co states ratherby comparable size. During the creep MC carbides are forming (Fig.4 d). 
velded joints The differences in the changes of the microstructure within the grain and at the grain 
loading was boundaries affect the strength and failure behaviour of the material. The increasing 
ab. 2. precipitation of y phase in combination with the formation of fine M3Cs promote the 
ondition only strength of the grain, while the coarsening of the particles decreases the strength at the 
and isolated grain boundary. For time-dependent loading the failure position increasingly shifted 
eld metal is lowards the grain boundary, which results in a reduction of creep ductility. A PWHT at 
te. 980 °C of the initial state causes precipitation of M3Cs. Since these are relatively thermally 
At the grain stable and only few new fine M23Cs form due to the already occurred precipitates, the grain 
hase occurs can absorb more deformation in this state. In addition, coarsening of the particles at the 
er PWHT at grain boundaries is smaller, so that the grain boundary strength is increased compared to 
are found in the non-heat treated state. Thus, the ductility of the material is increased in this state, this 
means that the (residual) stress can be better reduced by plastic deformation. 
ons. In heat If areas of the base material and the weld metal in both states are compared, it is found 
Y ‘phase is, that in both areas that have failed (WM without PWHT and BM after PWHT) at the GB only 
etal and the low amount of precipitates exist. Thus, there are not enough obstacles which can limit the 
y concerning GB-gliding. Due to the high number of intragranural precipitates, the grains are stronger 
reated state and make strain relaxation along the grain boundaries more difficult. favouring crack 
nce only few formation.