Handbook of Turbomachinery Second Edition Revised - Ventech!

those made of steel. Note, however, that the elastic modulus is dependent on
the particular titanium composition.
3. Titanium has greater corrosion resistance and as a result may have
better performance in dry/wet transition-phase regions of the LP.
4. Titanium also has excellent resistance to impact and water droplet
erosion damage and, in many applications, can be used without erosion
shields.
The drawbacks to titanium include:
1. Higher cost than steel, even though titanium’s lower density
means that more blades can be manufactured for a given mass of
material, which somewhat offsets the higher cost per pound of the
material.
2. More difficult to machine.
3. More difficult to weld. Titanium requires a high state of
cleanliness and an inert welding atmosphere.
4. Poor resistance to sliding wear, which can allow fretting corrosion
in some conditions, although fretting has not been found to be as
much of a problem as was once anticipated.
5. Lower internal material damping than stainless steel.
6. Finally, a disadvantage of titanium for blades is that recent high
cycle fatigue studies have shown an endurance limit in air and in
steam smaller than for 12% Cr stainless steels. However, shot
peening has been successfully used to restore the fatigue life lost
after machining, production, or repair processes.
Duplex stainless steels are those stainless steels that contain very high levels
of chromium and about equal amounts of ferrite and austenite. They have
been evaluated for use for LP blading, primarily in Europe. There are a
variety of duplex stainless steels with ferrite contents ranging from about
45–75%. The composition of an alloy produced in the United States
(Ferralium 255) is the same as the composition of a European alloy
(X3CrMnNiMoN2264). The duplex stainless steels have excellent corrosion
fatigue properties. Their primary drawback for blading may be somewhat
lower yield strength than Type 403/410 (in general, although the specific
properties for the Ferralium alloy indicate that good yield and tensile
strength can be achieved) and they do show some long time service
embrittlement at temperatures above 300 8C (570 8F).
Copyright © 2003 Marcel Dekker, Inc.