The effects of damage on high pressure gas turbine disc alloys

Abstract

The aim of this project is to assess the damage tolerance of High Pressure Turbine Disc (HPTD) components after the introduction of gouge damage and hence to optimise damage tolerance lifing approaches and minimise component retirements.

Historically, due to the relative hardness of Alloy 720Li, a multi-pass scratching technique has been used to simulate deep in-service scratch damage to HP turbine discs. A single-pass scratching method, which may better represent real-life damage formation, has been used in similar test programmes for IN718, which is comparatively easier to scratch. The reliability of the single-pass technique and estimations of its implications on component life have been assessed as follows:

i). The characterisation of a series of single-pass scratches at different depths. This includes metallurgical sectioning to assess microstructural changes/defects, analysis of the scratch profile conformity using non­ contact confocal and optical microscopes.

ii). Low-cycle fatigue (LCF) testing of representative Kt notch feature, shot peened specimens manufactured from disc forgings used for previous multi-pass LCF testing. The tests compare the fatigue properties between single-pass and multi-pass scratch damaged sites with the aid of a D.C.P.D. technique to monitor the initiation and growth of the fatigue cracks, and ultimately identify the main factors that contribute to fatigue life differences.

iii). Fractographic observations, SEM characterisation of the damaged material and microhardness map profiles were produced to assess the surface and sub-surface material states respectively.