Handbook of Turbomachinery Second Edition Revised - Ventech!

lading material be weldable, particularly last-stage LP blading, as many
designs require that cover bands, tiewires, and erosion shields be attached by
thermal joining. Weldability is also important for blade repairs. It is
important to note that up to about 1990 welding of blades and rotors was
considered ‘‘impossible’’ by many major manufacturers [33].
Creep Strength and Creep-fatigue Resistance. Creep resistance is important
in the first two or three rows of the HP and IP turbines to resist elongation
and accumulation of strain at the higher operating temperatures,
particularly at stress concentrations such as at the blade-to-rotor
attachments.
Tensile strength is required to withstand steady centrifugal and steambending
loads. Corrosion resistance is important to maintain blade life in
the turbine environment. Ductility is required for three reasons:
1. To allow localized plastic flow to relieve stress peaks and
concentrations, which can occur in the local regions of complex
geometries, e.g., at the blade root, and interblade connections such
as lashing wires and shrouds.
2. To allow for rivet formation from blade tenons, which is required
in many designs to attach shrouds to blades.
3. To provide for plastic deformation that can accommodate stress
from a rub or impact of a foreign body and thus limit damage to a
localized region instead of a brittle fracture followed by multiple
consequential failures.
High impact strength will also help resist failure from sudden contact by
foreign bodies such as a fragment from a failed blade.
Fatigue strength is important to prevent failures from the vibratory
stresses imposed by steam flow and system resonances. In LP blades, even
more important than simple fatigue strength is the resistance of the material
to cyclic loads in aggressive or corrosive environments in order to avoid
corrosion fatigue.
Notch Sensitivity. Notch sensitivity describes the effect of stress
concentration on fatigue strength. Since blades have a variety of stress
concentrations, low notch sensitivity is a desirable characteristic. There are,
however, tradeoffs. Higher tensile strengths tend toward improved fatigue
strength, but unfortunately, increasing tensile strength also leads to
increased notch sensitivity. The practical implication is that the designer
must balance the benefits of selecting a stronger material against the
reduction of fatigue strength [34].
Copyright © 2003 Marcel Dekker, Inc.