Metallurgical analysis of creep failure in M16 fastening bolts made of N80A
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M. Giller
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.
Abstract
Fractured fasteners were received from a large gas turbine engine. The subject bolts are M16 fasteners with rolled threads. They are made of N80A, a wrought precipitationhardenable nickel-base superalloy. This alloy features one of the highest creep strengths among this class of alloys. The fasteners are used to join two large casings in the turbine exhaust duct. The flange of this connection has more than one hundred bolts on the outer diameter, and the same number on the inner diameter. Only six out of those many bolts exhibited creep fractures. All failed bolts sat adjacent to each other on the outer diameter flange in the 12 o’clock position. Five of those fractured bolts were received at the authors’ laboratory. It was concluded that excessive service temperatures and/or higher than intended stresses caused the premature creep failures. An unfavorable microstructure with elephant grains may have contributed to the failures. More details about the assembly situation cannot be provided here.
Kurzfassung
Aus einer großen Gasturbine wurden gebrochene Verbindungselemente eingesandt, bei denen es sich um M16-Gewindebolzen handelt. Sie bestehen aus N80A, einer schmiedbaren, ausscheidungshärtbaren Nickelbasis-Superlegierung. Diese Legierung weist eine der höchsten Zeitstandfestigkeiten ihrer Klasse auf. Die Verbindungselemente werden eingesetzt, um zwei große Gehäuse im Abgaskanal der Turbine miteinander zu verbinden. Der Flansch dieser Verbindung hat mehr als hundert Bolzen am Außendurchmesser und ebenso viele am Innendurchmesser. Von diesen vielen Bolzen wiesen nur sechs ein Kriechversagen auf. Alle defekten Bolzen befanden sich nebeneinander auf dem Flansch des Außendurchmessers in der 12-Uhr-Position. Fünf dieser defekten Bolzen wurden an das Labor der Autoren geschickt. Es wurde festgestellt, dass überhöhte Betriebstemperaturen und/oder höhere Spannungen als vorgesehen zum vorzeitigen Kriechversagen geführt hatten. Eine ungünstige Mikrostruktur mit „Elefantenkörnern“ könnte zum Versagen beigetragen haben. Weitere Details zur Einbausituation können hier nicht gegeben werden.
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.
References / Literatur
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