POWER January 2021 - 17

FOCUS ON O&M
Supplement 8 (A Well-Engineered Approach
for Establishing the Minimum
Allowable Post-Weld Heat Treatment
for Power Generation Applications of
Grade 91 Steel [3002005350]).
■ In-service Failures. Many in-service
failures of Grade 91 steel components
have been documented (such
as Service Experience with Grade
91 Components [1018151] and Service
Experience with Creep Strength
Enhanced Ferritic Steels in Power
Plants in the Asia-Pacific Region
[3002005089]). Explanations include
poor design, poor operation, bad construction
practices, metallurgical risk
factors, and other causes. Regardless
of the root cause for each documented
failure, the number of documented
failures highlights the need for validation
of weld repair options that can be
used for the range of Grade 91 steel
components in service.
■ Inappropriate Repair Techniques. The
technical details for " best practice "
regarding welding of low-alloy bainitic
steels should not be applied directly
to welding of tempered martensitic
steel. Thus, repair techniques established
for " conventional " low-alloy
steels are not directly relevant for repairs
on Grade 91 steels.
EPRI Research on Well-Engineered
Weld Repair Approaches
These concerns led EPRI to conduct a
comprehensive multi-year, state-of-theknowledge
research program to evaluate
the integrity of well-engineered options
for repair of Grade 91 steel components.
Alternative weld repair approaches that
do not mandate the use of PWHT provide
end-users with added flexibility, reduced
outage schedules, reduced cost,
reduced risk of over-tempering, reduced
risk of destroying the existing component,
and reduced risk of ancillary damage
due to localized PWHT and other
potential downstream impacts.
EPRI's research program resulted,
first, in development of Welding Method
6 (WM6), which allowed tube weld repairs
of Grade 91 steel without PWHT.
WM6 was approved by the NBIC in
2015. After only five years of code acceptance,
multiple utilities have successfully
applied WM6 to Grade 91 steel
components-including Tennessee Valley
Authority (TVA), American Electric
Power (AEP), Prairie State Generating
Co., Florida Power and Light, and Xcel
Energy-and application of WM6 has
been documented in thousands of reJanuary
2021 | POWER
pairs. WM6 was recently modified to
permit the repair of attachment-to-tube
configurations or to allow for local weld
buildup in the event that the tube wall
has been reduced by erosion or corrosion.
These enhancements will appear in
budget years that allow potential replacements
to be better accommodated
(valued in the $100,000s), ability to bring
critical power assets back to the grid rapidly
(valued in the millions of dollars with
regard to both reduced outage times
WM6 was recently modified to permit the
repair of attachment-to-tube configurations
or to allow for local weld buildup in the
event that the tube wall has been reduced
by erosion or corrosion. These enhancements
will appear in the 2021 edition.
the 2021 edition.
Supplement 8 was first introduced to
the NBIC Part 3 in the 2017 edition to
permit repair options for thick-section,
balance-of-plant components, including
large-bore girth welds, small-bore penetrations,
attachments, and other configurations
that are not addressed in WM6.
As an engineered repair option, Supplement
8 requires that, " The organization
performing the engineering evaluation
shall have demonstrated experience
with Grade 91 CSEF steels. " To date,
hundreds of repairs consistent with the
language in Supplement 8 have been executed
and documented throughout the
power generation industry.
More recently, WM7 was developed
to permit dissimilar-metal weld repair
between Grade 91 steel boiler tubes
and austenitic steels or low-alloy ferritic
steels without PWHT (Figure 2). This development
represents more than four
years of sustained testing and parallel
efforts within the NBIC.
The method, similar to the original
language for WM6 printed in 2015, will
be limited to butt welds in tubing NPS 5
(Nominal Pipe Size, in.) DN 125 (Diameter
Nominal, mm) or less in diameter and 1/2
in. (13 mm) or less in wall thickness and
to non-pressure part welds for which the
applicable rules of the original code of
construction did not require notch toughness
testing. WM7 will eventually permit
a more comprehensive set of rules within
Supplement 8 to allow for dissimilar-metal
weld repair for balance-of-plant applications
in NBIC's 2023 edition.
From a monetary perspective, the
cost savings for utilities applying these
alternative weld repair approaches include
avoidance of PWHT (valued in the
$10,000s to $100,000s cumulatively),
delayed
component
replacement
www.powermag.com
to
and reduced purchase of power to meet
current supply especially within high-demand
months), and through elimination
of potential component damage induced
by mal-PWHT (valued in the millions of
dollars). Overall, the direct cost savings
demonstrated by innovative PWHT are
easily in the $100,000s, while the indirect
cost savings, which include both
deferred and minimized impact, are well
within the millions of dollars.
Need for Fitness for Service
Assessments
Another current materials and repair issue
in the energy industry involves " Fitness
for Service. " Pressure-retaining
systems and other " high-energy " components
in power plants may be at-risk
to fracture. When these events occur,
they can present serious safety hazards,
including personnel injuries and fatalities,
can lead to significant economic
consequences, and should be assessed
in terms of the risk to leak or break.
The failure of components generally
gives rise to a number of generic concerns
and questions. For example:
■ Why do cracks develop in components
designed to conservative construction
codes or standards?
2. Example of a simulated repair weld in a
dissimilar-metal weld between Grade 91 and
Grade 22 steels where the original weld deposit
was made with an American Welding
Society (AWS) type -B3 consumable. Note
that the repair was made with AWS type -B8
filler material. Courtesy: EPRI
17
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POWER January 2021

Table of Contents for the Digital Edition of POWER January 2021
