This paper presents an overview of stress corrosion cracking (SCC) in low-pressure (LP) turbine discs through 1993. Disc cracking experience in power plants and pertinent results of research programs are summarized. Analyses of field experiences and laboratory studies conducted in the United Kingdom, the United States, and other countries showed that stress corrosion cracking of LP turbine disc steels can occur in pure condensed steam or pure water, as well as in known cracking environments, such as hot hydroxide solutions.

It has been established that stress corrosion crack initiation in LP turbine disc steels exposed to high-purity water environments typical of those found in nuclear power plants depends upon seven factors: the presence of a liquid phase; the oxygen concentration of the liquid phase (or the electrode potential corresponding to the oxygen concentration); the yield strength of the steel used in discs; the disc temperature; tensile stress level; crevices and localized corrosion within crevices; and a combination of tensile stress and a flaw of size and shape sufficient to produce a Kl value greater than Klscc, i.e., greater than about 7.2 MPa√m (6.6 ksi√in.).

In general, susceptibility of discs to crack initiation increases with increasing oxygen concentration, steel yield strength, and disc temperature. Crack initiation is much more likely to occur on creviced surfaces, as are present at disc keyways, bores, and rim attachments, and the likelihood of cracking decreases when the clearance between two surfaces is greater than about 0.04 mm (0.001 in.). Crack initiation has been found to be independent of disc steel composition, inclusions, and pitting, although cracks may be initiated at pits formed on steel surfaces at inclusions.

While stress corrosion crack initiation depends upon the several factors listed above, the growth rate of stress corrosion cracks in LP turbine disc steels exposed to high-purity water and steam environments depends upon only three factors: the presence of a liquid phase; disc yield strength; and disc temperature. Importantly, stress corrosion crack growth rates in LP turbine disc steels are essentially independent of tensile stress level and stress intensity. Increases in both yield strength and temperature result in increased crack growth rate.

Subject
Pits, Crack growth, Intergranular stress corrosion cracking, Nuclear power, Turbines, Aqueous environments, Crack initiation, Steel, Oxygen, Steam, Yield strength, Stress corrosion cracking, Inclusions
Keywords:
stress corrosion cracking, low-pressure turbine disc steels, power plant experience, crack initiation, crack growth rate, presence of liquid, oxygen concentration, electrode potential, yield strength, temperature
© 1994 Association for Materials Protection and Performance (AMPP). All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means (electronic, mechanical, photocopying, recording, or otherwise) without the prior written permission of AMPP. Positions and opinions advanced in this work are those of the author(s) and not necessarily those of AMPP. Responsibility for the content of the work lies solely with the author(s).
1994
Association for Materials Protection and Performance (AMPP)
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