Susanne Riesenbeck

Hot cracking in the HAZ of laser-drilled turbine blades made from René 80

Abstract Advanced film cooling technologies for hot gas path components in gas turbine engines with still constantly increasing inlet temperatures require special manufacturing processes. Today, laser radiation is the preferred means of drilling a large number of small-diameter cooling holes in turbine parts made of nickel-base superalloys. Most of these materials, however, exhibit a relatively high susceptibility to hot cracking in the heat affected zone (HAZ) adjacent to the recast layer. Comprehensive metallographical examinations have been performed to optimise laser drilling parameter settings to minimize hot cracks. In the case of René 80, there seems to be a pronounced microstructural influence on hot cracking sensitivity, i.e. identical laser parameters may produce quite different crack lengths, dependent upon local microstructure in the immediate vicinity of the drilled hole. Grain size, grain boundary morphology and orientation, and primary carbide distribution apparently have a significant influence. These interdependencies should be taken into account when acceptable crack lengths are specified. As far as conventionally investment cast components are concerned, influencing local microstructural features with economically viable effort may be unrealistic.

Schäden an Brennerkomponenten von Heavy Duty-Gasturbinen

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.

Heißrissbildung in der Wärmeeinflusszone lasergebohrter Turbinenschaufeln aus der Nickelbasis-Superlegierung René 80

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.

Failure of a dampening pin: Combination of dynamic service load and increased notch effect because of surface roughness

Abstract As revealed by an inspection of hot gas path components, a dampening pin for the stage 4 moving blades of an industrial gas turbine for power plant applications brake in the summer of 2003. The failure mode of the fractured pin was high cycle fatigue. The root cause of the failure was probably a combination of the dynamic service load and an increased notch effect caused by an increased surface roughness of the pin. In the case of the notch-sensitive material Udimet 720, this was detrimental to the fatigue limit. The measured surface roughness of the failed part was out of specification and traceable to the use of non-OEM (Original Equipment Manufacturer) components. High temperature corrosion did not seem to have played a significant role in the failure mechanism.

Transformation of Delta Ferrite Into Sigma Phase in Metastable Austenitic Stainless Steels After Long-Term High-Temperature Service Exposure

Abstract A service temperature increase of turbine exhaust casing liners of heavy-duty industrial gas turbines, driven by the need to raise thermal efficiencies, motivated a number of ageing trials. Casing liners are often made of metastable austenitic stainless steels, suitable for high temperature applications. Alloys such as 321SS and 347SS might contain, in the as-cast condition, rather large amounts of delta ferrite, if not further processed by rolling or forging, easily in excess of 15 %. Even rolled sheet and bar might contain significant volume fractions of that phase if welded, up to 15 % or so in the heat affected zone (HAZ) immediately adjacent to the fusion line, and in the weld metal. It is known from the literature and from field experience that delta ferrite might decompose into sigma phase after long-term service exposure at elevated temperatures. This may embrittle the material and can be detrimental to mechanical properties. It could also deleteriously affect creep strength. The main aim of ageing trials described in this paper was to study the phase stability of delta ferrite under simulated service conditions. The results are correlated to metallographic testing results, obtained from examining actual service components of heavy-duty gas turbine engines in the field.

Pitting Corrosion Induced Fatigue Fracture on a Gas Turbine Compressor Blade

Abstract According to VDI Guideline 3822-3 “this type of corrosion is … caused by the formation of anodic regions of slight local expansion on the electrolyte-wetted surface. The presence of protective layers (passive layers) is a prerequisite for the occurrence of pitting corrosion.” Both requirements are met on gas turbine compressor blades if water condensates on the turbine blades when the machine is at standstill and/or if the compressor temperature remains low enough in a running machine to allow aqueous media to persist, i.e. for the condensation not yet to have evaporated. This is the case with the front row of blades, generally up to stage 6. Passive layers are present on the gas turbine compressor blades, since these parts are generally manufactured from martensitic 12–16% chrome steel, as also the case for steam turbine blades and discussed in detail in other case studies in the book. The failed compressor stator blade in this case study is made of X15Cr13, material no. 1.4024. Pitting corrosion on the front row compressor blades was not uncommon before the introduction of high temperature corrosion protection coatings containing aluminium pigments on these blades. After the introduction of protective coatings, this corrosion mechanism was absent on the coated blades. The coating systems contain aluminium spherules which, given the correct coating method, are in contact with each other to form a continuous electrically conducting layer between the surface of the blade and the protective coating, creating a sacrificial anode resulting in the less noble aluminium – as compared to steel – preferentially corroding to protect the blade material. The damaged part discussed in this case study originates from an older model gas turbine commissioned in 1955 already, having accumulated almost 163000 hours of operation when the damage was diagnosed. Depending on the type of operation, this represents a period of operation of 15 to 25 years. Such compressor blades were not coated back then.

Metallurgical Failure Investigation of Overheated and Fractured Investment Cast Nickel-base Bolts Made of Inconel 939

Abstract A number of small investment cast nickel-base bolts used for fastening metallic heat shields in combustion chambers of large industrial gas turbine engines were found fractured upon a regular hot gas path inspection in one particular engine. No similar findings were reported from any other engine in the field. The fracture of a single-digit number of these fasteners in an individual engine did not cause the liberation of the respective heat shields, hence no secondary damage occurred. The subject bolts were fractured in the head region that is also used to apply the fastening torque upon assembly of the heat shields. No evidence of fatigue, creep or hot corrosion damage was found. From the results of the metallurgical failure investigation described in this paper it was concluded that the subject bolts failed by torsional and/or tensional overload. Severe embrittlement by precipitation of secondary phases due to long-term high temperature service exposure contributed to the failure. The bolts ran hotter than intended by design. It is likely that cracks were induced in the bolt heads by over-torquing them upon assembly. Those cracks later grew in service.

TMF Cracking in Metallic Heat Shields of Gas Turbine Combustion Chambers

Abstract 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.

Applikationsbeispiele für ambulante Metallographie mittels Replikatechnik bei der zerstörungsfreien Gefügekontrolle großer Gasturbinenbauteile

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.

Fretting Fatigue Cracking of an Arresting Feature in a Turbine Disk of a Heavy-Duty Gas Turbine Engine

Abstract A check for surface cracks under application of the magnetic flux leakage method (MT testing) was performed on the turbine wheel disk of a heavy-duty turbine gas turbine engine for energy generation. It revealed unacceptable findings on the bearing surface of the sealing plate groove. In order to identify the damage-causing mechanism of the material, a metallographic examination was performed. The findings during non-destructive material testing were caused by a gaping material separation which was also easily detectable to the naked eye. The material separation is a high-cycle fatigue crack induced by fretting. Hence, the damage causing mechanism was fretting fatigue cracking.