324 Prakt. Met. Sonderband 46 (2014) 
4. Mf 
Cross 
Microsc 
in the 
situatio 
produc 
can be 
the LM 
Fig. 4: SEM investigation of crack start area (left) and deformed area (right) in Fig. 3 
Cold cracking, in this case also called “Type 4 cracking“ or “Toe Cracking”, appears 
typically in the coarse grain HAZ at the surface region. This crack occurs directly after the 
welding process, under certain conditions some hours after finishing of the welding. 
Fig. 4 right show the fracture surface situation in the so called “deformed area” The grains 
seem to represent a smooth surface. The presence of small cracks is remarkable. When 
looking in these open cracks one can observe the presence of micro striations, which are 
caused by the crack growth mechanism of the fatigue loading during the service of the 
penstock. The presence of these cracks can be explained by recognition of the 
observation shown in Fig. 1. In the cross section “branching of fatigue cracks” can be “ 
observed. Obviously the main fatigue crack was squeezed together during. emptying of Fig 
the penstock and the surface was deformed plastically. 
Very in 
area. F 
plastic 
plastic : 
crack c 
of micr 
conditic 
In this « 
can be 
The ex 
presen 
an influ 
5. CC 
In the p 
inducec 
EHTSZOOKY Mage S00X @ isnot beginni 
LATA Da At oe crack. 
Fig. 5: SEM investigation in crack front area of Fig. 3 thicker 
| 
This is also the reason that the micro striations (which caused the crack propagation Sots 
during the service of the penstock) in our investigation could be observed only in a very from be 
narrow area (smaller than 100um) at the crack front border area of fatigue crack to the 
forced liquid Nitrogen brittle fracture, as well as in the branching cracks, see Fig. 5.