Page 70 - Handbook of Materials Failure Analysis

64 CHAPTER 3 Boiler tube failures: Some case studies

Table 3.6 Microstructural Characteristics and Mean Hardness Values
Mean
Tube Hardness
Identification Microstructural Features (Hv)
Adjacent to failure (i) Discontinuous cracking (Figure 3.17) 121.5
zone (ii) Discontinuous grain boundary cracking initiated
from ID surface and presence of decarburization
zone (Figure 3.18)
Slightly away from Ferrite, complete breakdown of pearlite in association -
the failure zone with intergranular grain boundary cracking and
decarburized zone (Figure 3.19)
Diametrically Ferrite and partial breakdown of pearlite (Figure 3.20) 126
opposite to failure
Away from the failed Ferrite and pearlite (Figure 3.21) 124.5
zone

FIGURE 3.17
Micrograph showing discontinuous cracking.

The microstructural examination revealed ferrite, complete breakdown of pearlite in
association with discontinuous intergranular cracking, and decarburized zone. This
was probably due to hydrogen embrittlement. The carbon in the material reacted with
the hydrogen to form methane (CH 4 ). Methane was a large molecule, which exerted
pressure and caused discontinuous cracking along the grain boundaries [1,13,14].
Removal of the carbon from the sample caused the decarburization zone. The hard-
ness measurement also showed no abnormal variation of hardness adjacent to the
failure zone.