A
schematic diagram of a welded elbow pipe illustrated in Figure 3(a) is failed pipe
that located at first stage superheater tubes in the boiler. It had been
moderately cold-bent during installation. Non-destructive testing result shows
the longitudinal
cracking was located on the welding heat affected zone, along the weld beads (Figure 3b).
The failure occurred after only 3 months of operation.
The steam drum lacked adequate devices for separation of steam and water, and
load swings were frequent, possibly causing carryover of boiler water.
Microstructural analysis revealed plastically deformed grains from the cold
bending. The cracks were highly branched and ran between the grains
(intergranular) as they passed through the tube wall. The corrodent contains a slight sodium
chloride from boiler-water carryover. The tube was then removed and sends to
your consultancy firm for further failure investigation.
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| Figure 3: Schematic diagram of (a) welded elbow pipe & (b) the cracking heat affected zone along the weld bead
(i)
Give hypothesis the root cause of this
pipe failure.
(ii) Justify your hypothesis.
(iii) If metallographic
analysis performed on the failed area, determine the characteristics of the
microstructure that will be observed under optical microscope.
(iv) Recommend a few possible solutions to
prevent the occurrence of this defect in the future?
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Answer
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(i) Give hypothesis the root cause of this pipe failure.
The root cause of failure is hypothesized as Stress Corrosion Cracking
(ii) Justify your hypothesis.
Generally welding contains internal stress.
“Moderately cold-bent during installation” results residual/ internal stress”
“The steam drum lacked adequate devices for separation of steam and water, and load swings were frequent, possibly causing carryover of boiler water” results hot water enter the tube & forces the acceleration of oxidation reaction
“Micro structural analysis revealed plastically deformed grains from the cold bending” shows an elongated grain proves the presents of residual/internal stress
“The cracks were highly branched” shows the presents of stress ran between the grains (intergranular)” shows the presents of corrosion
“The corrodent was consist of small content of sodium chloride from boiler-water carryover” shows chloride ion from NaOH acts as active corrosion agent
(iii) If metallographic analysis performed on the failed area, determine the characteristics of the microstructure that will be observed under optical microscope.
(iv) Recommend a few possible solutions to prevent the occurrence of this defect in the future?
Stress corrosion
cracking (SCC) requires three conditions to occur simultaneously,
(a) a
susceptible alloy,
(b) tensile
stresses, and
(c) a
specific corrosive environment.
For instance, the necessary tensile stress can
be either externally applied or existing in the material due to forming or
welding. If one condition is eliminated, SCC will not occur.
Prevention can
be achieved by:
- Reducing the overall stress level and
designing out stress concentrations
- Selection of a suitable material not
susceptible to the environment
- Design to minimise thermal and residual
stresses
- Developing compressive stresses in the
surface the material
- Use of a suitable protective coating
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