Handbook of Turbomachinery Second Edition Revised - Ventech!

With the advent of larger turbines in the 1920s, a 5% Ni steel became
commonly used. Since the 1930s, most LP turbine blades have been
manufactured from a 12% Cr stainless steel; typically Type AISI 403, 410, or
410-Cb has been chosen depending on the strength required. Types 403 and
410 have better corrosion resistance than Type 422, an important
characteristic for use in the wet stages of the LP turbine.
There are numerous specifically customized versions of these generic
materials, for example, Carpenter H-46 and Jethete M152. Jethete M152 has
higher hardness and is thus more resistant to liquid droplet erosion in the LP
than Types 403 and 410. So far it has only been used in LP turbines, but
could be used in the HP and IP if needed. European designations for 12% Cr
blading alloys include X20CrMoV121 and X20Cr13.
More recently the precipitation-hardened stainless steel designated 17-
4 PH (AISI 630) was developed by one manufacturer for the last blades of
the LP turbine in the largest 3,600 rpm machines. It has a nominal
composition that is 17% Cr, 4% Mn, and 4% P. The hardening temperature
can control a wide range of mechanical properties. Alloy 17-4 PH is
somewhat difficult to weld and requires postweld heat treatment. Other
precipitation hardened steels that have been investigated for blade
construction in Europe include 15-5 PH (European designation: X5CrNi-
MoCu145) and 13-8 PH (designation: X3CrNiMoAl1382).
Titanium alloys, chiefly Ti-6Al-4V (6% aluminum and 4% vanadium),
have been used for turbine blades since at least the early 1960s. The use of
titanium in the last few rows of the LP offers a number of advantages over
other materials:
1. Titanium has about half the density of 12 Cr steels, which allows
for longer last-stage blades without an increase in centrifugal stresses in the
blade and thus an increase in annular area and improved turbine efficiency.
The capability of LP turbines to produce power is limited by the long last
row of blading and the strength of the rotor to support the blades. The
practical limitation for blades constructed of 12% Cr martensitic steel was
reached with 840-mm (33.5-in.) blades operating in 3,600 rpm machines and
1,200-mm (48-in.) blades operating in 3,000 rpm machines. In contrast,
titanium offers an opportunity to go to 1,000-mm (40-in.) and 1,350-mm
(54-in.) blades for 3,600 rpm and 3,000 rpm machines, respectively. This
represents a marked increase in power and makes possible a new generation
of LP steam turbines [47].
2. Titanium has particularly favorable mechanical properties in
applications involving high stresses at low temperatures. Because titanium
has half the density and about half the elastic modulus of steel, the natural
vibration frequencies and mode shapes of titanium blades are very similar to
Copyright © 2003 Marcel Dekker, Inc.