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7/8/03

10:02 PM

Page 1637

The Role of the Slope Out Region on Fatigue Crack Initiation in Electron Beam Welded Waspaloy A.K. ABDUL JAWWAD, M. STRANGWOOD, and C.L. DAVIS Electron beam (EB) welding can be used to produce compressor drum assemblies for gas turbine aero-engines. The compressor disc components are fatigue limited and the weld has been identified as a fatigue susceptible region. Electron beam welds in forged and double-heat-treated WASPALOY have been characterized in terms of microstructure, hardness, and fatigue initiation for the base metal, full penetration, and partial penetration (slope-out) weld metal. It has been found that the grain size increases from the base metal through the full penetration weld to the partial penetration weld metal, being largest at the end of full penetration (EOP). Fatigue initiation was found to occur preferentially at the EOP and was associated with the presence of large sulfides at, or close to, the weld surface. The sulfides arise from free surface segregation of sulfur, from the fused metal and surface contamination (arising from a preweld sulfuric acid etching stage), into the slope-out region during weld power down. The sulfides provide fatigue initiation sites and also modify the local composition, changing the type and number density of grain-boundary carbides and g’ precipitates, in the slope-out region near EOP, resulting in lower hardness regions. Removal of the sulfuric acid etching stage resulted in a more uniform microstructure.

I. INTRODUCTION

GAS turbine aero-engines contain a number of compressor stages prior to the combustion chamber in order to raise the gas pressure and improve combustion efficiency. These stages consist of aerofoil section blades held by a rotating compressor disc, which are catastrophic failure items. The compressor discs are fatigue limited and can experience operating temperatures up to 700 °C;[1] hence, they are made from nickel-based alloys processed to optimize hightemperature fatigue performance. Discs are forged polycrystalline materials and reducing the matrix grain size increases fatigue life in the low-cycle regime.[2] Heat treatment, after shaping, controls the strength level through the development of specific g’ precipitate populations. For example, to achieve less-planar slip and increase fatigue resistance requires the presence of large nonshearable particles with a high stacking fault energy (SFE). As the deformation temperature increases, the SFE increases and cross-slip out of the easy glide plane occurs, increasing the strength of the g’ hardening precipitates so that dislocation looping occurs rather than cutting. This is manifested as cyclic hardening during elevated temperature fatigue.[3] The large precipitates reduce planar slip but do not harden the matrix effectively and so multistage heat treatments are employed to develop a range of g’ precipitate sizes and so increase the overall strength levels. Heat treatments over a range of temperatures and composition modification also allow a greater volume fr