Service failure of hot-stage turbine blades:
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I.
INTRODUCTION
P O R O S I T Y in hot-stage turbine blades has for a long time been suspected to be a leading cause of blade failure. Unfortunately, the situation was confused by conflicting nondestructive inspection data. Fluorescent penetrant inspection (FPI) of coated blades before being entered into service showed no porosity in acceptable blades. On the other hand, inspection using a radioactive gas penetrant, KET,* commonly showed porosity to be *KET is a proprietary process of Qual-X Inc. The blades are examined, using a radioactive penetrant (Kr85), which is adsorbed in pores. Hence, porosity can be seen on an autoradiograph. This process reveals surface connected pores reproducibly.
(3) Explain, if possible, why the defects were not found by FPI.
II. THE RELATIONSHIP BETWEEN DEFECTS IN NEW AND USED BLADES
A. Components Used in the Study 1. Samples of new and used blades for three current military engines were obtained. The type A data were for equiaxed structure blades made from IN-100* coated *IN-IO0 is a trademark of Huntington Alloys, Huntington, WV.
present. The porosity could be confirmed by metallographic inspection. In this study, a database of KET indication and other data for new and used hot-stage turbine blades was set up. The data were then analyzed statistically, and sample blades were examined metallographically. The study had three objectives. (1) Show statistically if the porosity indications on new blades had any relationship to crack formation in service. (The new blades had all been passed by FPI.) A crack is assumed to be blade failure. (2) If porosity in new blades does relate to crack development in service, examine the mechanism by which this takes place. Hence, show why the porosity found in the new blades had the observed effects on service life. WILLIAM OLDFIELD, President, and FREDA M. OLDFIELD, Vice President, are with MRCS Corporation, St. Augustine, FL 32084. Manuscript submitted July 16, 1992. METALLURGICAL TRANSACTIONS A
with PWA73 nickel aluminide coating. All had been inspected and passed by FPI. Both sets of blades were from the same time period, between May 1988 and mid1990. (The used blades were obtained 1 year after the new blades had been introduced to service). The KET inspection of the new blades showed that of the 6940 inspected (sampled from a batch of 20,000), 196 blades had defects. Several types of defects were found. They were identified as porosity, inclusion, gas hole, cold shut, crack, coating crack, and gouge. The 250 used blades inspected by KET were culled at random, from a group of approximately 5000 which had been taken out of service. A total of 202 blades of 250 inspected had defects. The defect population was slightly different from the new blades, as would be expected. It included crack, porosity, inclusion, coating erosion, foreign object damage (FOD)/starburst cracks, coating cracks, and gouges. The results are summarized in Table I. VOLUME 24A, OCTOBER 1993--2313
Table I.
Defect Type Porosity Crack Inclusion Coating crack Unknown Coat
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