Reducing Tensile Stresses at Disk Surface Provides Protection
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 disk cracks have been found in keyways, cracking may also occur in the disk bore, and on the sides of the disk. Reducing tensile stresses at the surface of the disk will provide protection against the initiation of stress corrosion cracking.
O’Donnell Consulting evaluated the feasability of various processes for introducing residual compressive stresses at the surface of the turbine disk. Large compressive surface stresses, obtained by plastic pre-straining 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 over-speeding, bore pressurization, and thermal quenching.
Elastic analyses were performed to determine the operating stresses due to shrink-fit, 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 over-speeding both can introduce large compressive stresses in the bore, hence they are useful for disk designs where stress corrosion cracking is limited to the bore region. Quenching generates a uniform surface layer of compressive stresses over the entire disk 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 disk, including the rim region.
We performed finite element studies on disks subjected to spray quenching, and developed methods for fabricating crack-resistant disks.
We Perform Stress Corrosion Cracking Mitigation for Clients in Various Industries.