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In this contribution, a case study is presented describing the failure of a combustion chamber assembly in a non-OEM (Original Equipment Manufacturer) gas turbine engine used for power generation. It showed how even advanced fabrication methods, such as Electron Beam (EB) welding, could trigger fatigue fracture, even if there are no material defects, no weld imperfections, no fabrication flaws, and even if everything is within specified limits. As is so often the case in component failures, the fact that failures occur anyway, despite the absence of out-of-spec material properties, and even though there were no fabrication flaws, is attributable to the design; which is often not sturdy enough to withstand unexpected dynamic loading.

Impingement cooling plates welded to the inner shrouds of first-stage turbine vanes were found cracked after refurbishment, before installation in a heavy-duty gas turbine engine. The metallurgical root cause of the failure was determined to be embrittlement and surface melting from thermal over-exposure. This was not brought about by engine service, but was a result of the complex repair process, part of which are several heat treatment operations. The latter are tailor-made for the base alloy of the turbine vanes. For the lower-grade impingement cooling plates, however, the peak temperature from these furnace runs was too high. They should have been removed from the vanes prior to the high-temperature heat treatment operations, but were left attached to the vanes for cost reasons.

In this contribution, some aspects of fractography are discussed, arguably the failure analysts' most potent analytical tool in metallurgical failure analyses. The characterisation of fracture surfaces is indispensable when it comes to getting to the bottom of things in regards to component failures. When machine parts or entire assemblies fail by fracture of individual members, the fracture surface contains the data that tells the failure analyst about the history of the failed component, and hopefully will also shed some light on the exact failure mode that eventually lead to fracture. That makes fractography, the art of reading fracture surfaces, so valuable. This applies to all material classes, i. e. metals, ceramics, and plastics alike. This paper, however, focuses on failures of metallic components only. When there is no fracture surface in a failed part, one is produced by forcing open cracks or other imperfections that might be suitable for this purpose. Once this is achieved, the failure analyst will open his or her toolset of macro- and microfractographic analysis equipment. It is not exaggerated to say that the scanning electron microscope (SEM) revolutionised the art of microfractography from the early 1960 s on. In this paper, a number of examples for this are given.

X-ray computed tomography (XCT) is a powerful volumetric non-destructive testing tool. In this contribution, its application for large pieces of equipment is demonstrated, after the main components of an XCT scanner are described. While the technology has been well known in the scientific community for many years, its industrial-scale use is relatively new, especially as far as very large units are concerned. In a number of case studies, the benefits to the failure analyst of readily available XCT machines will be shown. The advantages in employing non-destructive XCT for metallurgical root cause failure analysis, prior to cutting up a failed part for sectioning and opening of cracks or other imperfections, will be revealed. It will be indicated how XCT can be conveniently applied in failure cases, for the sake of both the failure analyst and the fractographer. XCT can be used as a first testing method to get an understanding of the exact location of imperfections, fields of porosity, crack systems and the like, before macro- and microstructural anomalies are opened by the fractographer. To state it more clearly, the fractographer who has a high-powered XCT, with excellent spatial resolution at his or her disposal, will never again run the risk of cutting away a crack tip that is so valuable in determining the fracture mode.

A first stage turbine vane of a heavy-duty gas turbine engine, exhibiting an outward bulge of approximately 0.6 mm and a visible crack in the top coating, was delivered by the client for a detailed destructive failure investigation. The subject component was an experimental part used in a test field engine. The suction side wall of the casting was cut off to reveal a gaping crack on the inside of the aft cavity that was also visible by borescopic inspection. The cooling insert of the aft cavity was clean and did not have any blocked cooling holes. The cooling pattern on the inside wall of the aft cavity was unremarkable. No evidence whatever of impact damage was found on the suction side surface of the subject vane in the cracked region. The hypothesis as to the most probable root cause of the cracking that is still standing is a combination of low wall thickness, non-optimal process parameters for bond coat pre-heat, and potentially high thermal/mechanical loading during engine operation. It appears this is not a wide-spread phenomenon but rather an isolated occurrence.

Cracking and leakage, not repairable by welding, was found in premix piping of heavy-duty gas turbine engine for power plant applications. The subject laboratory specimen, a pipe section cut out on site and received at the investigating laboratory, failed by transgranular stress corrosion cracking (SCC). The stress corrosion cracking has probably been induced by chlorides (halide-induced stress corrosion cracking). Chlorine peaks were found in EDS spectra acquired on the inner pipe wall surfaces. However, it cannot completely be ruled out that those were artifacts caused by handling, cleaning, and cutting. The cracking is located adjacent to welds.

A number of M8 fuel oil pre-mix (FOPM) burner nozzles in a heavy-duty gas turbine engine were found loosened after some 32,000 operating hours. They were removed and tended to crack in the shaft upon disassembly. It was determined on site that they were fractured at the fillet radius below the head, at the thread's end. A metallurgical root cause investigation was ordered to determine the cause of the cracking. Selected dimensional checks, chemical analyses, microstructural examinations, and evaluations of fuel oil analyses conducted by external sources were performed to achieve the required results. Fractographic studies were not possible in this case due to the corrosion products covering the fracture surfaces. The FOPM burner nozzles fractured due to hydrogen-induced stress corrosion cracking (HISCC). No evidence of pre-existing cracks was found. There are no signs of over-torqueing nor of any other damage caused by manufacturing or assembly; hence this failure was caused by engine service and is not fabrication related.

In this contribution, the most interesting and educational failure cases are presented that the author came across during his over twenty years of laboratory practice as manager of the Materials Testing Laboratory of the Berlin Gas Turbine Plant of Siemens’ Power and Gas Division. The case studies are presented and categorised in accordance with VDI Guideline 3822, the German failure analyst’s guide to the subject of how to organise and run a root cause failure analysis. An effort was made to have each of the main four categories of failure causes represented, namely failures due to mechanical loading, corrosive failures, failures due to thermal loading, and tribological failures. Case studies include turbomachinery components that failed due to tensile overload, stress corrosion cracking, intergranular corrosion, hydrogen embrittlement, hot cracking, fretting, erosion, and galling. Affected components include valves, retaining rings, tubing and piping, burners, rotor disks, lifting lugs, and casings. Some of the presented cases were published in the new section “Failure Analysis” of Practical Metallography between October 2011 and the present time. Others were oral presentations at the Metallography conferences and at the annual failure analysis conferences “VDI Jahrestagung Schadensanalyse”, held during that time. The focus of discussion of the failure cases in this paper is the metallurgical evaluation of failure causes. This is the approach taken in many small and industrial laboratories. A holistic approach of a failure case, which includes calculation and simulation methods such as finite element analysis, and which also implies a knowledge of the service stresses intended by design as well as the actual loading situation of the failed part, is not the aim of this contribution.

In the course of a service life extension after 100 000 operating hours, the rotor of a heavy-duty gas turbine was inspected. During the visual inspection, some compressor wheel disks, particularly in the last stage, showed conspicuous features and flawed material. The only causal damage mechanism worth considering is erosion: only very little deformation was found in the metallographic sections' microstructure immediately underneath the flawed material areas (wear or deformation induced damages would have lead to more deformation) and, what is more, the microstructure showed no evidence of corrosion. In accordance with VDI guideline 3822, the exact designation of the damage mechanism is “Flow wear in the form of sliding jet wear in the two-phase mixture gaseous/solid”.

One out of eight lifting lugs of a large gas turbine engine exhibited indications upon magnetic particle surface crack inspection (MT), that were later verified by ultrasonic testing (UT). The subject part was made of a low-alloy steel casting. It had already been used in service and was supposed to be re-used, together with the other lifting lugs of the set. In order to determine the cause of the indications, non-destructive metallographic examinations by means of the replica technique were performed. In that investigation, the indications were determined to have been caused by crack-like decohesions of material. In one of the examined areas, these imperfections were associated with small casting defects, namely micro shrinkage. This might have contributed to crack initiation. The described small surface cracks at the cylindrical part of the lifting lug itself were probably caused by bending stresses in service. Indications were found only on one out of the entire set of eight parts. There, only the upper side of the short spigot, were axial tension on the surface is to be expected in service when the bollards are in lifting action, was affected. In a second investigation, that same part was sectioned in the area with the largest UT indications. The microstructure of this steel casting is unremarkable. However, several rather large macrocracks were found in the sectioning plane. Scanning electron micrographs confirmed this. There is a moderate carbon segregation in the subject area that is unremarkable. The longest crack in the metallographic section was forced open for microfractography in the SEM. A small pre-existing crack that could not have been caused by service loads was found. Within that area, microfractographic features of transgranular hot tearing were detected, as evidenced by a dendritic appearance of the fracture surface. This is a hot cracking mechanism.

Burners are key components of turbomachinery with internal combustion. They allow for a particularly lean combustion in heavy-duty stationary gas turbine engines for power stations and thus contribute significantly to economical operation. Depending on their design, burners are often subject to thermomechanical stress. This failure analysis is concerned with thermal cycling cracks at the burner outlet. Circumferential cracking, parallel to the component surface, is untypical for this damage mechanism. Thermal cycling and thermal-mechanical fatigue cracks usually spread in a radial direction. Possible causes could be the special kind of centrifugal casting structure or the high thermal tension, which might have built up during burner operation.

Several metallic heat shields of the combustion chamber of a large industrial gas turbine were affected by thermo-mechanical cracking after long service exposure. The base metal Alloy 617 is coated with a thermal barrier system. While microstructural evidence allowed the exclusion of creep damage as the metallurgical cause of failure, other microstructural features led to the conclusion that the affected components were massively overheated and experienced service temperatures of up to 1100 °C. There are several possible reasons for such overheating, ranging from burner failure and lack of cooling to abuse in service. A discussion of these factors is not the subject of this case study.

Kurzfassung Brenner sind Schlüsselkomponenten in Turbomaschinen mit interner Verbrennung. Sie ermöglichen in Großgasturbinen für Kraftwerke eine besonders magere Verbrennung und tragen so wesentlich zum wirtschaftlichen Betrieb bei. Abhängig vom Design sind sie häufig statisch und dynamisch hoch beansprucht. Bei der Fertigung dieser sowohl geometrisch als auch werkstofftechnisch komplexen Bauteile spielen schmelzschweisstechnische Fügeverbindungen eine wichtige Rolle. An drei ausgewählten Schadensbeispielen, die mit gerissenen Schweissnähten in Zusammenhang stehen, wird gezeigt, dass fast immer eine Kombination mehrerer Ursachen schadensauslösend wirkt. Besonderer Wert wird dabei auf die Betrachtung konstruktions-, fertigungs-, montage- und betriebsbedingter Einflüsse und Ursachen gelegt. Die als Datenträger wichtigen Bruchflächen der Schadensteile zeigen in den vorgestellten Fällen Schwingbrüche und Heißrisse als werkstofftechnische Versagensmechanismen.

Kurzfassung In der betrieblichen Qualitätssicherung großer Maschinenbauteile kommt ambulanten metallographischen Prüfverfahren eine wichtige Rolle zu. Allein durch diese Art der Prüfung können zerstörungsfrei, also ohne Probenentnahme, Aussagen zum Gefügezustand bzw. zu Gefügefehlern getroffen werden. Ihrer Größe wegen entziehen sich die meisten Bauteile einer direkten zerstörungsfreien mikroskopischen Untersuchung, sieht man von portablen Mikroskopen mit begrenzter Vergrößerung einmal ab. Im vorliegenden Beitrag werden am Beispiel der Folienabdrucktechnik eine Reihe von Anwendungsmöglichkeiten dieser Untersuchungsmethode aufgezeigt, wobei besonderer Wert auf die Darstellung der Möglichkeiten und Grenzen der lateralen Auflösung von Gefügedetails mittels Replikatechnik gelegt wurde. Die gezeigten Fallbeispiele stammen aus dem Großmaschinenbau, speziell aus der Fertigung großer Industriegasturbinen. Die extremen Temperaturdifferenzen, die in diesen Turbomaschinen herrschen und von ca. −40 °C bis 1500 °C reichen, erfordern vom Konstrukteur eine entsprechende Werkstoffauswahl. Daher reicht die Werkstoffpalette der gezeigten Fallbeispiele vom Grauguss mit Kugelgraphit bis zur Nickelbasis-Superlegierung.

Kurzfassung Moderne Filmkühltechnologien für Heißgasteile von Gasturbinen mit nach wie vor steigenden Eintrittstemperaturen erfordern spezialisierte Fertigungsverfahren. Inzwischen ist der Laser das bevorzugte Werkzeug zum Herstellen einer großen Anzahl kleiner Filmkühlbohrungen in Turbinenteilen aus Nickelbasis-Superlegierungen. Die meisten dieser Werkstoffe sind allerdings in der an die umgeschmolzene Schicht grenzenden Wärmeeinflusszone heißrissempfindlich. Umfangreiche metallographische Untersuchungen, die zur Parameteroptimierung durchgeführt wurden, zeigten für den Werkstoff René 80 einen ausgeprägten Gefügeeinfluss auf die Heißrissempfindlichkeit, d.h. gleiche Laserparameter können je nach lokaler Gefügeausbildung in unmittelbarer Bohrungsumgebung zu unterschiedlichen Heißrisslängen führen. Korngröße, Korngrenzenform und -lage sowie die Ausbildung der Primärcarbide scheinen eine wesentliche Rolle zu spielen. Diese Zusammenhänge sollten bei der Festlegung zulässiger Heißrisslängen berücksichtigt werden, da eine Einflussnahme auf die lokalen Gefügeparameter bei konventionellen Feingussprozessen mit wirtschaftlich vertretbarem Aufwand nicht realistisch erscheint.