Ball Peening Machines And Some Aspects Of Ball Peening Of Steam Turbine Blades

Author:  Korzynski, M.
Source:  Conf Proc: ICSP-4, (p. 19-26)
Doc ID:  1990008
Year of Publication:  1990
Abstract:  
The paper undertakes a problem of bearing-ball peening of large steam turbine blades and presents results of tests concerning surface layer condition and fatigue strength of 23H12MNF steel, that has been surface treated by polishing or bearing-ball peening. The tests have shown decrease in surface roughness, considerable increase in surface layer microhardness and compressive stresses arising in the surface layer after ball-peening. Modification of the surface layer condition by bearing-ball peening has caused increase in fatigue limit. The paper presents also schematic figure of machine for ball-peening of large steam turbine blades made in Rzeszow Technical University for steam turbine factory ZAMECH (Poland). This machine enabled to perform ball peening of blades up to 940 mm in length and 150 mm in width. There are four vacuum nozzles of special design, which are generating negative pressure and throw the balls at the surface to be peened. Machine control system make possible adjustment of individual working periods for every one vacuum nozzle and to get different degrees of strain hardening at different sections of the blade, according to requirements. Control of the process is performed periodically, using test blade provided with Almen's test strips. The peened blades have fatigue limit about 30% better than belt ground blades. Therefore ball peening process allows to eliminate previous belt grinding operation. Descriptors: Bearing ball peening, Large steam turbine blades, Surface layer condition, Compressive stresses, Microhardness, Fatigue limit, Bearing-ball peening machine