Page 130 - Steam Turbines Design, Applications, and Rerating

Turbine Blade Design Overview 111

6.1 Blade Materials
Among the different materials typically used for blading are 403
stainless steel, 422 stainless steel, A-286, and Haynes Stellite Alloy
Number 31 and titanium alloy. The 403 stainless steel is essentially
the industry’s standard blade material and, on impulse steam tur-
bines, it is probably found on over 90 percent of all the stages. It is
used because of its high yield strength, endurance limit, ductility,
toughness, erosion and corrosion resistance, and damping. It is used
within a Brinell hardness range of 207 to 248 to maximize its damp-
ing and corrosion resistance. The 422 stainless steel material is
applied only on high temperature stages (between 700 and 900°F or
371 and 482°C), where its higher yield, endurance, creep and rupture
strengths are needed. The A-286 material is a nickel-based super alloy
that is generally used in hot gas expanders with stage temperatures
between 900 and 1150°F (482 and 621°C). The Haynes Stellite Alloy
Number 31 is a cobalt-based super alloy and is used on jet expanders
when precision cast blades are needed. The Haynes Stellite Number
31 is used at stage temperatures between 900 and 1200°F (482 and
649°C). Another blade material is titanium. Its high strength, low den-
sity, and good erosion resistance make it a good candidate for high-
speed or long-last stage blading. For a typical materials overview, refer
to Table 4.1.

6.2 Blade Root Attachments
The standard root attachment used on the majority of blade configura-
tions is commonly referred to as the dovetail root. The drawn blading
produced by Dresser-Rand uses a single tooth dovetail type root (Fig.
6.4) while the milled bladings have three variations of the dovetail root
design. These variations are the two tooth backside (2TBS), two tooth
milled (2TM), and two tooth center milled (2TCM) illustrated in Fig.
6.5. The 2TBS root is used for shorter, lower speed applications while
the 2TM and 2TCM root is normally used for longer blades and higher
speed applications. In addition to the dovetail type root designs manu-
facturers such as Dresser-Rand and others also use two additional spe-
cial root attachments: the axial entry (fir tree) root and the finger type
root. The axial entry root, Fig. 6.6 is primarily used where the root cen-
trifugal stresses are high while the finger type root is used on large,
last stage blades.
Some of the special design features of milled blading should be men-
tioned. The root teeth of milled blades are machined on a radius to
match the groove diameter and to ensure full tooth contact along its
pitch length. The concave (front) and convex (back) sides of the root are