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Handbook of Turbomachinery Second Edition Revised - Ventech!

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Similarly, LP turbine blades are subjected to liquid droplet erosion,<br />

especially in the last stages <strong>of</strong> the turbine. For both solid particle and liquid<br />

droplet erosion, there is a general trend that increased hardness <strong>of</strong> the target<br />

material (blade) within a given class <strong>of</strong> materials leads to improved<br />

resistance to erosion. Outside these general guidelines, however, the<br />

development <strong>of</strong> quantitative predictions <strong>of</strong> erosion resistance from basic<br />

material properties is still evolving.<br />

Materials <strong>of</strong> Construction for HP and IP Blades<br />

The most common material for HP and IP rotating and stationary blades<br />

and nozzles is 12 Cr martensitic stainless steel. Three generic martensitic<br />

stainless steels are widely used for turbine blading, most commonly Type<br />

AISI 422 for HP blading and Types AISI 403 and AISI 410 in LP blading.<br />

There are numerous specific application materials where turbine manufacturers<br />

have customized the generic grade by the addition or deletion <strong>of</strong><br />

specific alloying elements, or by modification <strong>of</strong> the production or heattreating<br />

process. The final properties <strong>of</strong> these steels are strongly influenced<br />

by tempering temperature.<br />

Austenitic stainless steels (AISI series 300) are used in some hightemperature<br />

applications [42]. An example is Bohler’s Turbotherm 17 13 W<br />

(a 16 Cr–13.5 Ni variety). Austenitic stainless steels have excellent<br />

mechanical properties at elevated temperatures and are typically readily<br />

weldable. There is a thermal expansion coefficient difference between<br />

martensitic and austenitic stainless steels so that care is required when<br />

designing attachment clearances for fitting austenitic blades into martensitic<br />

disks. Also there is a potential for stress corrosion cracking when 300 series<br />

stainless steels are used in wet-steam conditions.<br />

The rings and webs <strong>of</strong> HP and IP nozzle diaphragms are commonly<br />

manufactured from stainless steels, although if the working steam<br />

temperature does not exceed 350 8C (660 8F), then welded diaphragms can<br />

be made from carbon steels.<br />

A number <strong>of</strong> superalloys have been investigated for use in blading,<br />

particularly for combustion turbines, primarily because <strong>of</strong> their hightemperature<br />

tensile and creep strength. However, these materials have<br />

typically not been used in steam turbines due to their higher manufacturing<br />

costs.<br />

Materials <strong>of</strong> Construction for LP Blades<br />

Early LP blade materials included cartridge brass (72 Cu, 28 Zn), nickel<br />

brass (50 Cu, 10 Ni, 40 Zn), and Monel (typically 66 Ni, 31 Cu, 1.3 Fe).<br />

Copyright © 2003 Marcel Dekker, Inc.

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