Page 21 - Handbook of Materials Failure Analysis

3 Case Studies 13

Cr-containing steel, probably corresponding to the 13CrMo44 or 10CrMo910 grades
also normally used for such kind of application. The other tube, within which cracks
have been formed, was identified as a low-carbon-low-alloy ferritic steel, as in the
previous case. A clear interface is noticed between the two welded tubes; the joint
seems free of flaws and discontinuities and failure is located outside the bonding line.
Concerning hardness distribution on both sides of the welded joint, the following
findings could be reported:

• In the case of Cr-containing carbon steel grade, the area near the seam
exhibited an average hardness up to 200 HV10, while at a distance of 3.5 mm
from the bonding line the hardness decreased to 170 HV10 toward the parent
metal hardness ( 150 HV10).
• In the case of ferritic carbon steel, no appreciable differences were accounted,
that is, the hardness varied between 110 and 120 HV10 at all the examined areas,
from weld zone to parent metal.

The area close to the joint boundary of Cr-containing carbon steel manifested the
characteristic upper-bainite microstructure (Figure 1.9). The adjacent area of inter-
mediate hardness exhibited the presence of tempered ferrite with a fine distribution
of spheroidized cementite, while the parent metal exhibited a microstructure of fer-
rite with partially spheroidized cementite.
The adjacent area of ferritic low carbon steel grade consisted of equiaxed ferrite
grains, of uniform distribution, an observation indicating that the material was
recrystallized while the carbide precipitates found at the grain boundaries probably
attributed to spheroidized cementite. In the same area, individual creep voids of
small size were scarcely observed.
The parent material exhibited round and transgranular creep cavities with an
average diameter of 30-50 μm(Figure 1.10). A close view on this area revealed that

Acc.V Spot Magn Det WD 10 µm Acc.V Spot Magn Det WD 5 µm
20.0 kV 3.0 2000x SE 5.6 20.0 kV 3.0 4000x SE 5.6
(a) (b)
FIGURE 1.9
SEM micrographs showing details of the bainitic structure of the Cr-containing steel grade. (b)
Detail of (a).