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ADVANCED MATERIALS & PROCESSES •
JANUARY 2014
22
been identified in several gas-fired, cycling units over the
last decade and has resulted in confusion and misinforma-
tion regarding the best inspection, repair, and fabrication
techniques to prevent it.
EPRI’s research efforts, in collaboration with The Ohio
State University and Carpenter Technology Corp., are tar-
geting these two key issues in the power generation indus-
try through a wide range of practical and fundamental
disciplines to find viable, long-term solutions.
Need for Co-free hardfacing materials
The development of Co-free hardfacing materials is
highly challenging in several aspects. In general, there is a
lack of uniform, consistent wear performance data for
hardfacing materials candidates. Such information may
exist in proprietary databases with material manufacturers
or OEMs, however, little is publicly available. This is evi-
dent given the difficulties in assessing wear-resistant mate-
rials across a wide range of potential wear mechanisms
(abrasion, erosion, and galling), and across the range of op-
erational temperatures. To address some of these chal-
lenges, nuclear OEM research facilities may use one-off
tests to assess potential materials. These tests simulate, on
a small scale, actual valve operating conditions. Aside from
several decades of acceptable service experience in both
power generation industries, relatively little comparison
data exists between mainstay hardfacing materials such as
Alloy 6 and other material options.
The evaluation of candidate hardfacing materials also
presents issues in producing quantitative results to existing
wear standards as well as extrapolating lab results to real-
world material and operational behavior. Two ASTM stan-
dards recognize and validate this point: ASTM G98
“Standard Test Method for Galling Resistance of Materials”
and ASTMG133 “Standard Test Method for Linearly Recip-
rocating Ball-on-Flat Sliding Wear” aim to provide qualita-
tive comparisons
[2, 3]
. However, regarding ASTM G133, the
reported volume loss of the flat specimen (conducted in ac-
cordance with the standard) deviates from measurements
taken by a laser microscope by as much as 150%
[4]
.
Advanced characterization methods such as laser micro-
scopy demonstrate the ability to improve test method accu-
racy, and also provide a quantitative means to evaluate
candidate materials. The confocal laser microscope used by
EPRI features a resolution as low as 5 nm in the laser axis,
yielding extremely precise results
[5]
. An example of a wear
scar on a flat and ball specimen, analyzed using laser mi-
croscopy, is shown in Fig. 2.
Understanding wear mechanisms
A detailed, predictive understanding of all potential
wear mechanisms incorporating fundamental materials
properties as well as mechanical behavior,
microstructure, and part geometries does
not exist—especially for the highly varied
structures found in some alloy/composite
systems. The initial lattice structure and
transformation behavior of a given mate-
rial under wear conditions may play a crit-
ical role in assessing performance, but few
experimental studies are available, and
conducting such an assessment is chal-
lenging. Through the combination of syn-
chrotron diffraction, neutron diffraction,
scanning electron microscopy (SEM),
electron backscatter diffraction (EBSD),
transmission electron microscopy (TEM),
and an exhaustive physical examination of
a large variety of alloy specimens tested to
ASTM standards, a coherent description
of the physical mechanisms of a variety of
Fig. 1 —
Delamination of hardfacing material from a ferritic valve disc operating in a cycling, gas-fired power plant at 1050°F after
roughly 35,000 hours.
Fig. 2 —
Representative laser image and color height map for the flat and pin specimens
tested to ASTM G133.
Wear scar
Flat
Pin
Wear
Before test
After test