Abstract
A first-stage turbine vane was received in the laboratory directly from fabrication, prior to its use in engine service. The part had not yet been covered with its customary coating system that protects these parts against hot corrosion. A first visual inspection revealed multiple cracks on the airfoil’s hot gas path side, fairly centered in the part. After cutting the part open, it soon became apparent that the cracking was even more severe inside, suggesting crack initiation from that cooled side. Fractography allowed to determine liquation cracking as the metallurgical failure mechanism. Since the part was received immediately after pre-heat before plasma coating, that process step was concluded to have caused the cracking.
Kurzfassung
Eine Leitschaufel der ersten Stufe ging direkt ab Fabrikation im Labor ein, noch bevor sie beim Betrieb der Turbine zum Einsatz kam. Das Bauteil war noch nicht mit der üblichen Beschichtung ausgestattet, die solche Teile vor Hochtemperaturkorrosion schützt. Im Rahmen einer ersten Sichtprüfung wurden am Schaufelblatt auf der Seite des Heißgaskanals in etwa mittiger Position Risse gefunden. Nachdem das Bauteil aufgeschnitten wurde, stellte sich schnell heraus, dass es im Innern zu einer noch gravierenderen Rissbildung kam, was für eine von dieser gekühlten Seite ausgehenden Rissinitiierung spricht. In fraktographischen Untersuchungen konnte das Auftreten von Wiederaufschmelzungsrissen als metallurgischer Schadensmechanismus ermittelt werden. Da das Teil kurz nach der vor dem Plasmabeschichten stattfindenden Vorwärmung zur Untersuchung einging, wurde davon ausgegangen, dass dieser Verfahrensschritt zur Rissbildung führte.
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
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