Page 64 - Handbook of Materials Failure Analysis

58 CHAPTER 3 Boiler tube failures: Some case studies

Table 3.3 Microstructural Characteristics and Mean Hardness Values
Mean
Location of Hardness
Sample Examination Microstructural Characteristics Value (HV)
Platen Adjacent to Elongated grains of ferrite and 136
superheater failure coalescence of alloy carbide at ferrite
tube grain boundaries (Figure 3.9)
Diametrically Dispersed alloy carbide in ferrite matrix 128
opposite to and coalescence of alloy carbide at ferrite
failure grain boundaries along with isolated
creep cavities (Figure 3.10)
Away from Ferrite, alloy carbide mostly at grain 127
failure boundaries along with isolated creep
cavities at the grain boundaries
(Figure 3.11)

FIGURE 3.9
Micrograph showing elongated grains of ferrite and coalescence of alloy carbides.

2.1.1.5 Scale thickness measurement and estimation of tube
metal temperature
Inside diameter scale-thickness measurement was carried out on three specimens
from failed platen superheater tube. Tube metal temperature of the tube was esti-
1
mated based on maximum measured oxide scale thickness on ID surface for 2 / 4
Cr. 1 Mo steel. The observed maximum scale thickness and estimated tube metal
temperatures are given in Table 3.4.