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Engineering to Control Cracking on Low Pressure (LP) Turbine Discs

Stress corrosion cracking can occur on low pressure (LP) turbine disks which operate at or near saturated steam conditions.

Though a majority of LP steam turbine disc cracks have been found in keyways, cracking may also occur in the disc bore, and on the sides of the disc. Reducing tensile stresses at the surface of the disc will provide protection against the initiation of stress corrosion cracking.

O’Donnell evaluated the feasability of various processes for introducing residual compressive stresses at the surface of the turbine disc. Large compressive surface stresses, obtained by plastic prestraining of the material, super-imposed on the operating stresses reduces the resulting surface stresses below the stress corrosion threshold. Several methods for introducing high residual compressive surface stresses in the disc were analyzed. These included overspeeding, bore pressurization, and thermal quenching.

Elastic analyses were performed to determine the operating stresess due to shrinkfit, blade load and angular rotation. An elastic-plastic analysis was also performed to establish the residual stresses resulting from the processes used to develop compressive surface stresses. The results were superimposed and the combined stresses evaluated to determine the most beneficial process.

All three processes generate compressive residual stresses in the bore. Pressurization and overspeeding both can introduce large compressive stresses in the bore, hence they are useful for disc designs where stress corrosion cracking is limited to the bore region. Quenching generates a uniform surface layer of compressive stressses over the entire disc surface. Thus, it can reduce the high operating tensile stresses in stress concentration areas and produce a more favorable operating stress field over the entire disc, including the rim region.

O’Donnell performed finite element studies on discs subjected to spray quenching, and developed methods for fabricating crack-resistant discs.

 

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