Torsional Overload Fracture of Twist-off Bolts During Assembly
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A. Neidel
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.Thomas Ullrich born in 1974, trained as a materials tester in physics at Energiewerke Nord AG in Greifswald, where his work included nuclear service. He then gained further experience in power plant service at TÜV Rheinland Berlin-Brandenburg, particularly in the context of non-destructive testing and component metallographic residual life assessments. He joined the Berlin gas turbine plant in 2001. In addition to component metallography, he is primarily involved in the target preparation of bores and welded joints in the metallography laboratory.
Abstract
Supposedly simple cases of failure are most often best suited to communicate the principles of component failure analysis in the field of materials engineering to a wide readership, especially to those peers in the specialist community who are just beginning to familiarize themselves with the subject. The present case of failure relates to components that failed as early as during the assembly, and more specifically, during the final assembly stage of combustion chamber components for heavy-duty gas turbine engines. Hence, they lost their functionality (in fact, the common definition of component failure). At tightening torques of the nuts opposite of the tapered heads as low as below 25 Nm, so-called twistoff bolts which, when welded into combustion chamber sheets, take on the function of stud bolts, sheared off. By way of exception, a materialographic failure analysis could show that the primary cause of the failure was not the component’s design, but the disregard of the drawing specifications during final assembly. However, on a secondary level, design deficiencies had to be mentioned, as untempered welded joints in martensitic chromium steels invariably act as metallurgical notches. If the respective part is subjected to dynamic loads, as is the case in virtually all turbo machinery, they are thus to be avoided.
Kurzfassung
Meist eignen sich die vermeintlich einfachen Schadensfälle am besten, um die Prinzipien der werkstofftechnischen Bauteilschadensanalyse einer breiten Leserschaft zu vermitteln, vor allem jenen Kolleginnen und Kollegen der Fachgemeinde, die sich in die Thematik erst einzuarbeiten beginnen. Der vorliegende Schadensfall betrifft Bauteile, die bereits in der Fertigung, konkret in der Endmontage von Brennkammerkomponenten für Großgasturbinen, versagten, also ihre Funktionsfähigkeit verloren, was ja die gängige Definition für einen Bauteilschaden ist. Sogenannte Würgeschrauben, die durch Einschweißen in Brennkammerbleche die Funktion von Stehbolzen erhalten, scherten bereits bei geringen Anzugsdrehmomenten der den konischen Würgeschraubenköpfen gegenüberliegenden Muttern von unter 25 Nm ab. Eine werkstofftechnische Schadensanalyse konnte zeigen, dass die primäre Schadensursache ausnahmsweise nicht die konstruktive Bauteil auslegung war, sondern die Nichtbefolgung der Zeichnungsvorgaben in der Endmontage. Sekundär war die Konstruktion dennoch zu bemängeln, weil nichtangelassene Schweißverbindungen martensitischer Chromstähle in jedem Fall eine metallurgische Kerbe darstellen und bei dynamischer Beanspruchung, die im Turbomaschinenbau praktisch immer vorliegt, zu vermeiden sind.
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.
Thomas Ullrich born in 1974, trained as a materials tester in physics at Energiewerke Nord AG in Greifswald, where his work included nuclear service. He then gained further experience in power plant service at TÜV Rheinland Berlin-Brandenburg, particularly in the context of non-destructive testing and component metallographic residual life assessments. He joined the Berlin gas turbine plant in 2001. In addition to component metallography, he is primarily involved in the target preparation of bores and welded joints in the metallography laboratory.
References / Literatur
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- Effect of Different Heat Treatments with and without Deep Cryogenic Treatment on the Morphology of Mg17Al12 Intermetallic Phase and Hardness of AZ91 Magnesium Alloy
- Failure analysis
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Articles in the same Issue
- Contents
- Editorial
- Editorial
- Quantitative Structural Analysis of Superalloy Engine Disks
- Effect of Different Heat Treatments with and without Deep Cryogenic Treatment on the Morphology of Mg17Al12 Intermetallic Phase and Hardness of AZ91 Magnesium Alloy
- Failure analysis
- Torsional Overload Fracture of Twist-off Bolts During Assembly
- Picture of the Month
- Picture of the Month
- News
- Green hydrogen from New Zealand
- Meeting Diary
- Meeting diary
