Page 393 - Failure Analysis Case Studies II

Fig. 5. Typical cracks found at external support welds.

1‘ L -x
Fig 6 Typical cracks found at external support welds

3.2. Metallurgical examination
Three of the more easily accessible cracks were chosen for microstructural replication, polished
to a 1 pm diamond finish and etched with 2% nital.
Examination of the replicated micrographs in the laboratory showed that in each case the
cracks followed the martensitic region of the heat affected zone. Figures 8 and 9 are “panorama”
micrographs across the HAZ from parent metal to weld metal in order to illustrate more clearly the
location of the cracks in relation to the microstructure.
A significant feature is that all three of the cracks were filled with an oxide phase, which is shown
particularly clearly in Fig. 10. This is indicative that the cracks were exposed to an elevated
temperature oxidizing environment after initiation [2, 31.

3.3. Discussion
All three of the cracks examined above are typical of heat-affected-zone cracking which is also
referred to as “hydrogen-induced cracking”, “weld cracking”, “delayed cracking” or “underbead
cracking”.