sion or some other wear occurring in
addition to corrosion,” he says.
For that second case, “we would create … a different type of coating that
would be able to support the erosive flow
through it and protect against damaging
the surface of the valve,” he explains.
Hardcoating (Figure 2), which
deposits a fairly thin layer of material
and is often done with a thermal spraying technique, is distinguishable from
hardfacing, which uses a form of welding for a thicker deposit, though considerable overlap in applications exists.
The choice of which to use “usually has
to do with the geometry of the part followed up by the volume of pieces that
needs to be done,” says Ozzie Bell, manager, business development, Deloro Stellite. Manual methods used include MIG
or TIG welding or oxyacetylene. If volumes are high and the part geometry is
simple, he continues, “then there will be
an automated process where we would
have something like a plasma transferred arc piece of equipment being
used, or an HVOF [high velocity oxygen
fuel] system.” (Figure 1)
One of the oldest hardfacing materials used in valves is Stellite 6 (the name
Stellite was trademarked in 1911),
which is based on cobalt, chrome and
tungsten carbide. However, Tribaloy
alloys are increasing in popularity; the
alloy consists of cobalt, chromium and
COURTES Y OF DELORO STELLITE
Figure 2. Hardcoating being applied to a valve
seat.
molybdenum. Bell explains that: “They
tend to stand up better under high abrasion and high corrosion applications.”
They’re typically “used on the trim, on
stem, seats, some bushings and possibly
some of the flapper valves,” he says.
Another hardcoating gaining favor is
Hardide Coatings’ tungsten carbide-based material, which consists of tungsten carbide nanoparticles dispersed in
a tungsten metal matrix. This material
is applied using chemical vapor deposition (CVD), which can coat both external and internal parts. The coating has
been approved for use on a new line of
coated metal ball and seats.
Meanwhile, Sub-One Technology uses
a different deposition technique; the
company employs a process applied by
using “hollow cathode plasma immersion ion processing,” which can deposit
carbon-based coatings, metal carbides
and oxides—for example silicon, tantalum and chromium carbides—and dia-
