Prakt. Met. Sonderband 46 (2014) 323
3.2 MACROSCOPIC INVESTIGATION BY STEREO MICROSCOPE (LMS)
By the investigation using the stereo microscope especially pronounced three different
areas of topographic appearance of the fracture surface can be distinguished: Fig. 3. An
area located at the crack start position, on the inner surface of the pipe, extension about 1-
2mm is covered by the presence of heavy corrosion pits.
ntation, using
EO 1450 VP
MikroskopvergréBerung: 19,9x; Bildbreite: 7.00 mm
Bildname: 01012a0143.ipa
Fig. 3: LMS Investigation of deoxidized fracture surface
Following the direction to the outside of the plate a zone with smooth surface appears
which was defined as “deformed surface”. A distinct area was observed in direction
towards the crack front which is characterized by parallel striations of distance about 30-
70um. This area was defined as “macro striation area”,
These three distinct areas of topographic appearance could principally be observed at all
samples investigated.
2d (left) and
3.3 SCANNING ELECTRON MICROSCOPY INVESTIGATION (SEM)
The macro appearance of the fracture surface clearly represents fatigue as cause of the
failure. In contrary, at the first glance, no micro-striations could be observed at almost the
d with heavy entire thickness of the fracture surfaces.
surface from By application of higher magnifications in the SEM investigation, see Fig. 4 left, in addition
e fracture can to the corrosion pits intergranular separation of the coarse grain HAZ microstructure, can
be observed. In addition to the martensitic/bainitic microstructure in the grains, small holes
fully removed can be observed. This observation is typical for the occurrence of the phenomena of so
> surface. All called hydrogen induced cold cracking. This is a typical phenomenon of welding caused
naintained by failure appearance which often is observed when welding high strength steels by using not
optimal welding parameters (preheating, soaking, hydrogen control of environment and
welding fluxes). [2]
re
