Content Provided By Miller Welds
As a result of nationwide power emergencies and an
abundance of natural gas, 300 to 400 new power plants are projected to be
built by the end of this decade. Maintenance and repair of the U.S.'s
aging power plants proceeds at a heady pace, too, as utility companies try
to keep their units producing at a high capacity.
Whether building a new facility or upgrading an old one,
utility companies and designers want to reduce fabrication and materials
costs. In addition, utility companies want systems that can withstand
higher temperatures and operating pressures to increase operating
efficiency and output. To meet these needs, designers are specifying
higher-alloyed steels, while fabricators, installers, and contractors must
use equipment and techniques that help them work quickly and
efficiently.
Advanced Chromium-Molybdenum
While power plants built in the 1970s used 21/4Cr-1Mo
steel (P22), today they increasingly use a modified 9Cr-1Mo-V (P91).
"In the past three years we've seen an exponential rise
in the specification of P91 for main steam lines, headers, tubing, and
heat recovery steam generators [HRSGs] in the United States," says W.F.
Newell Jr., P.E., vice president of Euroweld Ltd., Mooresville, N.C., a
supplier of specialty welding consumables and equipment. Newell chairs a
new subcommittee to write an American Welding Society (AWS) D10 standard
for welding advanced chromium-molybdenum steels, including those in the 9
to 12Cr range.
P91 is significantly stronger than P22 at elevated
temperatures. P91 typically allows a wall thickness reduction by at least
a 2-to-1 ratio. If the wall is thinner, hanger loads are lighter. Also, a
wall that is 50 percent thinner takes less time to weld and uses less
filler metal.
"From a designer standpoint, it's a wonderful material,"
says Newell. "From a fabrication standpoint, P91 presents some challenges.
Preheat and postweld heat treatment are mandatory and absolutely critical
to the service life of the alloy. It is incumbent on the fabricators and
installers to learn how to work with it."
Bring on the Heat -- But Control It
Depending on the P91 welding application, codes that
apply to power plants and refineries may include ASME B31.1 Power Piping,
ASME B31.3 Chemical Plant and Petroleum Refinery Piping, ASME Section I
Power Boiler Parts, ASME Section VIII Unfired Pressure Vessels, and ASME
Section III -- Division I Nuclear Components.
Welding P91 generally requires preheating the joint,
maintaining interpass temperatures, hydrogen bakes, and postweld heat
treatment (PWHT). Preheating, typically to 400 to 500 degrees F, drives
off moisture and thereby reduces hydrogen. Hydrogen embrittlement can lead
to cold cracking of the finished weld. For this same reason, hydrogen
bakes are recommended for P91 if a weld cools to ambient temperature
before PWHT.
Preheating also reduces the thermal gradient between the
base material and weld puddle, and it improves weldability by reducing the
heat necessary to make the weld while reducing hot-cracking
tendencies.
Welding thicker materials and high-alloy steels usually
requires maintaining minimum preheat and maximum interpass temperatures.
If a material cools below the minimum temperature between passes,
restraint issues may arise or hydrogen may contaminate the weld. If the
material becomes too hot, which can happen during multiple-pass welding,
some steels can lose their corrosion resistance. With P91, the matter is
more practical: higher than 600 degrees, the weld puddle usually becomes
too fluid and difficult to control.
PWHT requires controlling temperature in four phases to
relieve the stress caused by welding. The following example describes the
four phases with P91 and follows
ASME B31.1. However, always refer to the appropriate code for procedural
details and never work from memory.
1. Control temperature.
The weldment usually may be brought to this temperature, generally 600
degrees, without any time constraints.
2. Control-temperature
rise. Once the control temperature is reached, the code requires a
controlled temperature rise of no more than 600 degrees per hour divided
by one-half the maximum thickness, but in no case more than 600 degrees
per hour.
3. Soak or hold
temperature. This is the stress-relieving temperature. For heavy-wall P91
weldments, that temperature is typically 1,400 ±25 degrees. Other alloys
may be between 1,100 to 1,400 degrees. Soak time depends on the thickness
of the material. One hour per inch is common for most alloys, but Newell
recommends that heavy-wall (greater than 3¼4 in.) P91 components should
have a minimum soak time of two hours.
4. Control cool. After
the soak time, the material must be cooled at a controlled rate to prevent
stressing the part, which could lead to cracking. Typically, the
control-cooling rate is the same as the control-temperature rise -- cool
from 1,400 to 600 degrees at a rate of no more than 400 degrees per hour.
Below 600 degrees, the weldment can air-cool to ambient
temperature.
Stress relieving restores ductility. Without ductility
at ambient temperature, power piping and other chromium-molybdenum
components may not withstand hydrostatic testing and other aspects of
fabrication, transportation, start-up, and shutdown phases.
According to Newell, "Prior to stress relieving, it is
not unusual to see an as-welded P91 weldment with a Rockwell C hardness in
the mid 40s to low 50s and a Charpy V-notch toughness of just 3
foot-pounds. After PWHT, P91 is significantly more ductile. Its Rockwell C
hardness drops to the low 20s and its Charpy V-notch toughness can
increase to 40 to 70 foot-pounds, depending on the welding process."
Newell says he cannot emphasize enough that P91 provides
no forgiveness when fabricators and installers fail to stress-relieve the
material properly. It will fail under circumstances that P22 would
not.
Also, like most heavy-wall, large-diameter piping, a P91
pipe's inside diameter (ID) and outside diameter (OD) can exhibit a
substantial temperature gradient (50 to 100 degrees) if the procedure and
technique are not carefully evaluated and qualified, so installers need to
bring ID and OD temperatures as close together as possible.
New Induction Technology
Modern induction heating systems can heat parts up to
1,500 degrees. Newer induction heating systems are different from their
early counterparts. Induction heating systems of the 1970s used power
sources the size of a small car that required enormous amounts of primary
power. The old systems also had copper tubes to carry the induction energy
and for cooling water flow.
Today's more advanced induction heating systems use
compact, lightweight, and energy-efficient inverter technology. Modern
induction heating power sources weigh from 165 to 500 lbs., and some can
fit on a small, wheeled cart along with a temperature controller and
recorder and water cooler (see Figure 1).
Figure 1
Figure
2
(Figure 2) In induction heating, electric current flows
through the coil, generating a magnetic field in the part. The rapidly
changing magnetic field creates eddy currents in the part, causing heat to
be generated. This is similar to the heat generated in a transformer. The
coils do not heat up, so welding can be done while the coil is still attached. This gives precise pre-heat and post-heat control.