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Blade life comparison is provided in the form of the stress required for rupture as a function of a parameter that relates time and temperature (the Larson-Miller Parameter). The Larson-Miller parameter is a function of blade metal temperature and the time the blade is exposed to those tempera-tures. Figure 11-4 shows the comparison of some of the alloys used in blade and nozzle application. This parameter is one of several important design parameters that must be satisfied to ensure proper performance of the alloyin a blade application， especially for long service life. Creeplife， high-andlow-cyclefatigue， thermal fatigue， tensile strengthand ductility， impactstrength， hot corrosion and oxidation resistance， producibility， coatability and physical properties must also be considered.
Turbine Wheel .lloys
.lloy71. Nickel-Based . Thisnickel-based，
.lloyprecipitation-hardened alloy is the newest being developed for the next generation of Frame type gas turbine machines. This alloy has been used for wheels in aircraft turbines for more than 20 years. Alloy 718 contains a high concentra-tions of alloying elements and is therefore difficult to produce in the very large ingot sizes needed for the large Frame type turbine wheel and spacer forgings. This effort requires closecooperation between the manufacturer， and its superalloy melters and large forging suppliers to conduct the solidification and forging flow studies that are necessary to bring into production a new wheel material for large wheels. This development effort has resulted in the production of the largest ingots ever made and forged into high-quality qualification turbine wheel and spacer forgings.

.lloy706 Nickel-Based . Thisnickel-based，

.lloyprecipitation-hardened alloy is being used in the large frame type units by GE such asthe frame 7FA，9FA，6FA， and 9EC turbine wheel and spacer alloy， and it offers a very significant increase in stress rupture and tensile yield strength compared to the other wheel alloys. Figures 11-6 and 11-7 show the stress rupture and tensile yield strength of the various alloys. This alloy issimilar to Alloy718， but contains somewhat lower concentrations of alloyingelements， and is therefore easier to produce in the very large ingot sizes needed for the large frame type gas turbines.

Cro-Moo-V .lloy. Turbine wheels and spacers of most GE single shaft heavy-duty gas turbines are made of 1% Cr -1.25% Mo-0.25% V steel. This alloy is used in the quenched and tempered condition to enhance bore toughness. Stress rupture strength of the dovetail region (periphery) is con-trolled by providing extra stock at the periphery to produce a slower cooling rate during quenching.
The stress rupture properties of this alloy are shown in Figure 11-6.
Stress
200

14.0
100

6.0
60

4.0
2.0
20

0
0
Temp

°C
100,000 Hr.
Life

800 900 1000 1100 1200
Figure 11-6. Turbine .heel Alloys stress rupture comparison

450 500 550 600
140
120
0.2%
YIELD

100STRENGTH 80
60

TEMP °F
12 Cr .lloys. This family of alloys has a combination of properties that makes it especially valuable for turbine wheels. These properties includegood ductility at high-strength levels， uniform properties throughout thick sections and favorable strength at temperatures up to about 900 oF (482 oC)
M-152 alloy is a 2-3% nickel-containing member of the 12 Cr family ofalloys. Initially， it was used as an upgrade in gas turbines as a replacement for A286. It features outstanding fracturetoughness， in addition to the properties common to other 12 Cr alloys. M-152 alloy is intermediate inrupturestrength， between Cr-Mo-V and A286 alloy (Figure 11-6)， and has higher tensile strength than either one.These features， together with itsfavorable coefficient of expansion and good fracture toughness， make the alloy attractive for use in gas turbine applications.
　
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