POWER December 2014 - 18

3. Removing a sample. Using the
electric-discharge sampling equipment (left), a
4-mm thick " boat sample " was cut out from
the pipe (center), leaving a round-edge cavity
at the sample location (right). Courtesy: ETD
Consulting
weld. Microstructural examination, hardness
testing, and micro-hardness testing
were carried out on these three samples.
Representative photographic images of
the microstructures of each EDSE sample
were taken at three locations:
■ Close to the component surface (approximately
0-1 mm depth)
■ Middle of thickness of EDSE sample (approximately
1-3 mm depth)
samples from 1 millimeter (mm) to 20 mm
thick from the surface of a component,
which means that it is possible to obtain
samples in a nondestructive, or minimally
invasive, manner (Figure 3).
Using EDSE, weld repair is not necessary,
provided that removal of a few millimeters
from the surface does not compromise the
design minimum wall thickness. The design
code wall thickness requirements in
this case allowed the removal of 4-mm
EDSE samples from the P91 component.
Three samples were removed within about
75 mm of the weld/heat-affected zone. In
this way, the sampled material was representative
of base material that had been
tempered during the at-site PWHT of the
■ Close to cut-surface of EDSE sample
(approximately 3-4 mm depth)
The microstructures of the three P91
samples consisted of a mixture of ferrite
and martensite, with relatively coarse carbide
particles in some of the ferritic areas.
There was no significant change in the
microstructure between the component
surface and 4-mm depth.
In ETD's experience, the effect of decarburization
at the surface of P91 piping
components can extend to 2-mm depth in
extreme cases. If the surface of the component
is decarburized, then the structure at
the surface would contain a high proportion
of ferrite and the amount of ferrite would
decrease on moving away from the surface.
It was thus considered that the mixed ferrite/martensite
structure would be representative
of the component as a whole.
Abnormally slow cooling (that is, furnace
cooling rather than air cooling) from the
austenitizing temperature is one possible
way in which significant amounts of ferrite
could be formed during manufacture. Another
possibility arises during PWHT, if the
weldment is unintentionally heated above
the Ac1 temperature. If this situation occurs,
partial re-austenitization of the structure
takes place and, if followed by slow
cooling to the correct PWHT temperature,
then the austenite phase transforms to ferrite/carbide,
resulting in a mixed ferrite/
carbide and martensite structure.
It thus became essential to either replace
the P91 component or remove it
from the installation and carry out full
reheat treatment (that is, normalizing and
tempering) in order to obtain the correct
P91 microstructure and normal level of
creep rupture strength.
It may be feasible to test miniature test
specimens extracted from the EDSE samples
for their rupture strength. If this is done,
then the data can be used to justify operation
of the component for a significant period of
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POWER December 2014

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