Roller Pressing or Shot Peening of Fir-Tree Root of LPT Blades of 500 MW Steam Turbine

Author:  Sharma
Source:  Conf Proc: ICSP-8 Sept. 16-20, 2002 Garmisch-Partenkirchen, Germany
Doc ID:  2002064
Year of Publication:  2002
Abstract:  
Author Mahesh C. Sharma, Maulana Azad College of Technology, Bhopal, India Abstract An experimental investigation was carried out to explore the possibility of replacing pressure rolling of Fir-tree root of L.P. Blades by shot peening. For this purpose it was necessary to find out shot peening parameters which could achieve the required depth of compression and residual stress as pressure rolling does. Depth is especially crucial when very poor water condition come into effect. Further more surface roughness is a concern for bearing lands. It was necessary to achieve residual stress not lower than -300 MPa down to a depth of 0.25 mm and roughness value Rz must not be larger than 22 um after shot peening. In the present investigation surface roughness and residual stress data obtained with different peening parameters were reported. It was observed that pressure peening with 10 mm convergent divergent nozzle, 1 to 1.2 mm cast steel shots, at 0.32A peening intensity, 3 kg/cm2 tank pressure 12" stand off, 125 to 150% coverage gave residual stress value -418 MPa at 0.25 mm depth in 12% chrome steel test pieces. The surface roughness too was with in required range (Ra = 2.80 um). Residual stress on an actual blade profile cut section was also verified. Thus roll pressing can be replaced by shot peening.