Abstract
Short fillet welds used to fasten a large retainer ring to so-called dog bone seals failed in the turbine exhaust casing of a non-OEM heavy-duty gas turbine engine used for power generation. The subject fillet welds fractured due to high cycle fatigue loading. Neither weld imperfections nor any other material defects were found that could have contributed to the failure. It was concluded that an unfavorable design, specifying very short fillet welds for fastening the dog bone seal segments to the retainer ring, was the root cause of failure. In a purely static loading situation, this design would probably not have failed. However, in a dynamic loading scenario as is the case in any gas turbine engine exhaust, such a design is simply not sturdy enough.
Kurzfassung
An kurzen Kehlnähten zur Verbindung eines großen Sicherungsrings mit sogenannten Knorpelringen im Turbinenaustrittsgehäuse einer nicht mit Originalteilen ausgerüsteten Großgasturbine zur Stromerzeugung kam es zum Schadensfall. Durch HCF(high cycle fatigue)-Beanspruchung kam es zum Riss der besagten Kehlnähte. Es wurden weder Fehlstellen in den Schweißnähten noch sonstige Materialfehler entdeckt, die zum Schadensfall hätten beitragen können. Daraus wurde geschlossen, dass die Hauptursache für den Riss in einer ungünstigen konstruktiven Gestaltung lag, d. h. in sehr kurzen Kehlnähten zur Verbindung der Knorpelringsegmente mit dem Sicherungsring. Bei einer rein statischen Beanspruchung wäre es wahrscheinlich nicht zum Schadensfall gekommen. Gegenüber einer dynamischen Beanspruchung jedoch, wie sie bei diesem Turbinenaustrittsgehäuse vorliegt, ist eine solche konstruktive Gestaltung einfach nicht robust genug.
About the authors
Tobias Gädicke was born in 1986 and joined the Siemens Energy Sector in 2005 as apprentice in the Berlin Gas Turbine Works. As materials testing technician, his main field of expertise is metallographic target preparation of laser-drilled holes in turbine blading. Also, he is expert in the field of portable metallography by means of the replica technique with both putty and replica foils which he regularly applies to microstructural analyses of large gas turbine casings made of ductile iron and steel castings. He is a lecturer in the DGM seminar on portable metallography. Metallurgical failure analysis of gas turbine engine hot gas path components are another field of expertise of his.
Susanne Riesenbeck joined the Gas Turbine Plant of Siemens' Energy Sector in 1984 and first underwent vocational training there. As a materials testing technician, one of the first fields of her professional endevours was steam turbine service, namely remaining lifetime assessment by means of the replica technique. She is a metallographer and one of the principal failure analysts in the materials testing laboratory. Her main fields of expertise comprise microstructural analyses of steel and iron castings.
References / Literatur
[1] Neidel, A.; Gädicke, T.; Riesenbeck, S.: Internal Report BLN MT/2018/0189, Berlin, March 1, 2018Search in Google Scholar
[2] Neidel, A.; Gädicke, T.; Riesenbeck, S.: Internal Report Addendum, BLN MT/2018/0189, Berlin, March 1, 2018Search in Google Scholar
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