Abstract
Large hot-rolled rings made of Alloy 718 were used as semi-finished product for turbomachinery components. Samples taken from one such ring exhibited non-uniform mechanical properties, partly below specified limits. Too high a volume fraction of delta phase in the microstructure was identified as the metallurgical root cause of failure. The case study presented in this contribution shows that such deviation from specified material properties does indeed constitute component failure, since the subject raw forging could not be used as is for fabrication of rotating equipment for gas turbine engines. To salvage the subject part and avoid having to scrap it at high cost, successful heat treatment tests were performed in the laboratory with the aim of eliminating delta phase to an extent, sufficient for property recovery. Heat treatment parameters were derived and recommended to the client. They were successfully applied before final machining and the subject hot-rolled ring was salvaged.
Kurzfassung
Große warmgewalzte Ringe aus Alloy 718 dienten als Halbzeug für den Einsatz in Turbomaschinenkomponenten. Proben von einem dieser Ringe wiesen uneinheitliche mechanische Eigenschaften auf. Dabei wurden die spezifizierten Grenzwerte teilweise unterschritten. Als metallurgische Fehlergrundursache wurde ein zu hoher Volumenanteil der Deltaphase im Gefüge ermittelt. Die in diesem Beitrag vorgestellte Fallstudie zeigt, dass man bei einer solchen Abweichung von den spezifizierten Werkstoffeigenschaften durchaus von Bauteilversagen sprechen kann, da das entsprechende Rohschmiedeteil im Istzustand nicht zur Herstellung von rotierenden Maschinenelementen für Gasturbinen verwendet werden konnte. Um das entsprechende Teil einer Verwendung zuführen zu können und eine kostenintensive Verschrottung zu vermeiden, wurden im Labor erfolgreiche Versuche mit Wärmebehandlungen durchgeführt. Diese zielten darauf ab, die Delta-Phase in einem für eine Verwendung des Produkts ausreichenden Umfang zu eliminieren. Aus den Versuchen wurden Parameter der Wärmebehandlung abgeleitet und dem Kunden als Empfehlung weitergegeben. Die Parameter wurden mit Erfolg vor der Endbearbeitung angewendet, so dass der entsprechende warmgewalzte Ring gerettet werden konnte.
About the authors
Madeleine Giller
Apprenticeship as a materials testing technician at the welding institute SLV Berlin-Brandenburg. After that, she studied materials science at the Berlin Institute of Technology. She graduated with a masters degree in materials science in 2012. Since November 2012 employment as engineer at the Metallography Laboratory of the Siemens Gas Turbine Works in Berlin. She has been appointed laboratory manager within the Berlin Testing Center of the Large Gas Turbine Engineering Group in 2014. Her main fields of expertise comprise failure analysis and microstructural investigations using the field emission scanning electron microscope.
Boromir Fischer
completed a training as State Certified Technical Assistant for Metallography and Physical Material Analysis at Lette-Verein in Berlin. He has worked in the metallographic laboratory of the Siemens Gas Turbine Plant (Siemens-Gasturbinenwerk) Berlin for more than a decade. The key areas of his work are failure analysis and microstructural examinations using the field emission scanning electron microscope.
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