Buckets and Nozzles

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4.4.1
Buckets and Nozzles
Stephen J. Balsone
GE Gas Turbines LLC
P.O. Box 648; GTTC 174D
Greenville, SC 29602
Phone: 864 254-5294
Email: stephen.balsone@ps.ge.com
411 411
4.4.1-1 Introduction
Gas turbine engines, both aircraft and industrial power generation,
represent one of the most aggressive applications for structural materials. With
ever growing demands for increasing performance and effi ciencies, all classes of
materials are being pushed to higher temperature capabilities. These materials
must also satisfy stringent durability and reliability criteria. As materials are
developed to meet these demanding requirements, the processing of these
materials often becomes very complicated and expensive. As a result, the cost
of materials and processes has become a much larger consideration in the design
and application of high performance materials. Both the aircraft engine and
power generation industries are highly cost competitive, and market advantage
today relies on reducing cost as well as increasing performance and effi ciency.
The fi ring temperatures of all gas turbines, both industrial power
generation and aircraft engines, have increased over the past ~30 years. More
recently, the rate of temperature increase has slowed for aircraft engines but not
for industrial gas turbines (IGT). As a result, the materials temperature capability
requirements for these two classes of gas turbines are converging. For many
years, the high performance requirements of military and commercial aircraft
engines fueled the development of advanced materials and processes. Many of
these high temperature materials are now being used in industrial gas turbines as
output, effi ciency, and reliability requirements continue to grow. Directionally
solidifi ed and single-crystal nickel-base superalloys have been developed for
investment casting of hot gas path components and have been scaled up to the
part sizes required for IGT components but not without signifi cant challenges
in producibility, defect allowances, and repair. The application of nickel-
base superalloys in industrial gas turbines has required particular emphasis on
technology development for the production of buckets and nozzles in large IGT
sizes. Processing scale-up from aircraft engine-sized parts to large IGT-sized
parts has presented unique materials development and processing challenges.
There has been much synergy in the development of these materials for both
aircraft engines and industrial power generation turbines, and this synergy is
likely to continue to grow as we strive to push materials capability to the limit
while providing robust designs for reliable, long-life service.
4.4.1-2 Background
Higher operating temperatures are historically the primary means of
improving aircraft engine thrust or industrial power generation gas turbine output.
Higher operating temperatures require higher temperature capability materials
and associated technologies such as improved oxidation and environmental
coatings. For industrial power generation gas turbines, the fi ring temperature (as
defi ned by the gas temperature that enters the fi rst rotating stage of buckets or
blades) has a profound effect on the performance of the turbine.
Since the early 1970’s there has been a continuous increase in the
output and effi ciency of large industrial gas turbines (IGT) for electrical
power generation. This increase is due in large part to the introduction of high
temperature structural materials. The use of these advanced materials has resulted
in an increase in gas turbine fi ring temperature from 982ºC (1800ºF) to greater
than 1427ºC (2600ºF) over the past 30 years. For every 10ºC (50ºF) increase in
the fi ring temperature, the gas turbine combined-cycle effi ciency improves by
approximately 1%. A 1% improvement in effi ciency means millions of dollars
in savings to an electrical power producer looking to deliver electricity at the
lowest cost to its customers.
Nickel-base (Ni-base) superalloys are the alloys of choice for high
temperature, high strength structural applications, and they have become
the standard for IGT hot gas path components such as buckets, nozzles, and
shrouds. Many of these investment cast Ni-base superalloys were derived
from aircraft engine alloys developed for use in both commercial and military
aircraft gas turbines. In addition to investment cast Ni-base superalloys, other