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More Common in the Powerplant The popularity of using 91 Cr-Mo Alloy materials has been rising
significantly in the past few years. Due to its good mechanical
properties at high temperatures, powerplant designers now routinely
specify P91/T91 for Reheaters and Superheaters. Although great
for boiler performance, the weld-ability of the material requires
extreme care to avoid hydrogen cracking and reheat cracking problems
in the shop and in the field.
Experience Counts
Chanute has been welding T91 Tubes and P91 Pipes since 1996 for
package boiler superheaters, and HRSG reheaters and superheaters.
To date Chanute has not had a P91 weld failure reported in the
field. This is due to Chanute's development of high quality
fabrication processes in the shop that ensure successful P91 weld
joints.
Chanute has time-tested welding processes for selecting and controlling
preheat temperatures, interpass temperatures, postheat temperatures
and durations, postheat cooling parameters, and the ever-so-critical
PWHT methods. In addition, Chanute is very particular about selection
and control of the chemical composition of its welding consumables.
The trace element chemistry for the amount of Ni, S, Ni, P, 5Sb,
Sn, and As in the filler metal contributes significantly to a high
quality end product.
When it comes to producing reliable P91 and T91 heat exchanger
components, weld process control counts, attentive Quality Control
counts, and Chanute's track record and experience counts.
Reheat Cracking
Reheat cracking is the most common type of failure that can occur
in P91 and T91
weld joints. This type of cracking is almost always found in the
coarse grained regions of the heat affected zone (HAZ) beneath
the weld and in the coarse grained regions within the weld metal.
Stress concentrations, impurities, hydrogen, poor
weld process, and/or poor PWHT in combination or as stand-a-lone
propagators can lead to various degrees of reheat cracking. The
cracking itself occurs in the post weld heat treatment cycle between
1000°-1100°F where the grain structure of 91 Cr-Mo alloy weld joints
characteristically weakens.
Chanute pro-actively addresses these factors through its quality
controlled weld techniques to produce the maximum refinement of
the coarse grain HAZ. Then, utilizing its custom heat treating
equipment, Chanute performs a thorough PWHT
process while properly supporting the components to induce the
least stress possible.
NDE Inspection Portfolio
Chanute is capable of performing in-house NDE procedures to insure
the integrity of its weldments. Chanute's inspectors are certified
to SNT-TC-1A Level II for visual testing (VT), magnetic particle
testing (MT), dye penetrant testing (PT), and radiographic examination
(RT). In addition, Chanute has a Certified Weld Inspector on staff,
and all pressure parts are fabricated to the ASME code requirements
or stricter.
Finning and Bending of T91 Tubes
In addition to its experience fabricating and welding T91
tubes and P91 header pipes, Chanute has been spiral finning and
bending T91 tubes for many years. Chanute's spiral fin welding
process
meets the ASME Code requirements as outlined in ASME Code Case
2215, and Chanute has literally tens of thousands of T91 fintubes
in powerplant operations throughout North America.
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