Volume 50, Issue 1

The different stages of the polishing process of polycrystalline tungsten samples were investigated by scanning electron microscopy of both the sample surface and of cross-sections prepared with the help of a focused ion beam. It is shown that a distortion layer is present at the sample surface after mechanical fine grinding and even after polishing with diamond suspension, although the sample has a mirror-like finish. A sufficiently long chemo-mechanical polishing step using an alkaline colloidal silica suspension was able to remove this distortion layer. Although electropolishing produced an even smoother surface, the microstructure quality after chemo-mechanical polishing is comparable to that of an electropolished sample.

Experiments were conducted to determine the influence of the number of grains counted using the Jeffries planimetric procedure of ASTM E 112 with a single test circle compared to the number counted using rectangles as suggested by Saltykov. Both planimetric methods produced similar estimates of the ASTM grain size number when the counts per field placement were high. The Saltykov rectangle method is a viable test method and produced good data down to low count numbers. Bias was not observed at low counts, only data scatter. The intercept count method, using only a single test circle per field, yielded slightly higher estimates of the ASTM grain size number, but the difference was relatively insignificant.

On the occasion of the revision of a gas turbine power plant in Northern Germany, a surface crack was detected in one of the 44 stage 0 compressor stator blades during a routine non-destructive testing of the blades. By then the engine had completed more than 38,000 h of operation. The damaged blade was precision drop-forged and copy-milled of X20Cr13 and for some time was wearing a subsequently applied corrosion protection coating containing aluminium pigments. The blade's condition before the subsequent coating was applied is unknown. The metallurgical investigation found that the cause of failure was corrosion fatigue.