Effect of the honing drum upon the inducement of compressive residual stresses

Abstract Rotor blades for large-output steam turbines are subjected to a wet honing treatment when they have attained their finished state. The blades are processed in a closed drum which de-burrs and gives a smooth finish to all sharp edges, generally improving their surface state. Following the honing process with its slow mild effects, a compression shot-peening treatment is also conducted on the last stage blades, which are highly stressed in service. Fatigue-strength tests (rotation bending and pulsating push-pull) carried out on test-pieces manufactured from an appropriate steel have shown, by inference, that the drum-honing treatment improves the quality of the blades by between 15 and 20% in air and by between 30 and 50% in an aqueous medium (water + 1 g/l sodium chloride), and that such an improvement can be compared to that obtained from shot-peening. Residual stress measurements by X-ray diffractometry carried out on rotor blades have revealed a remarkably uniform and widespread compression of the surfaces processed in the honing drum, to achieve compressive stress values of between approximately 300 and 400 MPa, depending upon the material tested (12% chrome steel and Hastelloy X). Although the penetration of compression with the drum method is inferior to that for shot peening (disappearance of the compression effect at between 0.1 and 0.2 mm depth as opposed to at more than 0.5 mm depth for shot-peening), the honing treatment is capable of eliminating all risk of cavity formation, or at least of retarding the effects of stress corrosion, which latter is the initial phase in the majority of crack occurrences and fatigue fractures encountered on materials subjected to such high stress as turbine blades.