Prakt. Met. Sonderband 46 (2014) 321
conditions, METALLOGRAPHIC INVESTIGATION OF A FATIGUE EVENT
ON A HYDROPOWER PENSTOCK MADE FROM HIGH
ly sprayed STRENGTH STEEL
2. Wear 257
H. Cerjak, N. Enzinger, G. Stéfan
Cracking in Institute of Materials Science and Welding, Graz University of Technology, Graz, Austria
NF8 (2002)
omponents. ABSTRACT
This investigation was performed on test pieces removed from a pipe of an Hydropower
and testing. Penstock made from high strength steel S 690 QT where leakages in the vicinity of a
longitudinal weld were observed after only 5 years of service .
The fracture surface was investigated by application of Macroscopic Light Metallography
a] at 600°C. (LOM) and SEM. The macro appearance of the fracture surface represents fatigue as
cause of the failure in spite of the fact, that micro striations at a first glance could not be
detected. At the crack start position on the inner surface of the pipe, beside of heavy local
e. Practical corrosion pits attack, intergranular grain separations could be found. This appearance is
typically for the existence of hydrogen induced cold cracking phenomena in the heat
affected zone of the weld. Metallographic investigations on cross sections confirmed that
ng and Life observation.
; By these investigation results it could be explained why fatigue failure appeared after an
nics (2008) : ,
unexpected short service period. Fatigue failures in such components usually start in
areas of welding caused surface notches which represent locally high stressed regions. In
this special case it could be observed by metallographic investigations, that the fatigue
crack started from an already existing cold crack by crack propagation mechanism without
suffering the fatigue crack initiation phase.
1. INTRODUCTION
| In a Penstock of a Pumping Storage Hydropower Plant, 3700 mm diameter, wall thickness
15 mm, Material S 690 QT after only 5 years of service two leaks appeared in the vicinity
of longitudinal welds.
k rim The scope of the performed investigation was to find the reason of the appearance of the
leaks observed after the short time of service.
2. MATERIAL INVESTIGATED
Fig. 1 represents a cross section of a test piece removed from the damaged area. The
weld was performed by submerged arc welding (SAW) process. A sample showing a crack
depth of about 7 mm after forced fracture under liquid nitrogen (Fig. 2) was used for this
investigation The original fracture surfaces were covered with heavy rust layers. The crack
started in the notch of the weld in the heat affected zone (HAZ). The crack ended in the
unaffected base material. At the crack tip branching of the crack could be observed.
CF fracture
