Heavy-duty Gas Turbine Engine Research Articles

Metallographic characterization of metallic heat shields, accidentally precipitation-hardened with tripled soaking time

Abstract Heat treatment (HT) problems, or the lack of necessary HT, are a regular topic of discussion in the papers published in this section Failure Analysis of Practical Metallography. In this present contribution, a case study is presented that shows how an erroneous furnace run almost caused the scrapping of an entire engine set of Metallic Heat Shields (MHS) of a heavy-duty gas turbine engine. The authors readily admit that the subject components did not fail. They were not even put in engine service yet. That means that this case study does not describe a failure in the original sense of the word. However, from a metallurgical point of view, it is still an interesting case that shows that without proper scrutiny during metallurgical investigation in the laboratory, considerable economic loss could have resulted from a simple fabrication problem, that involved a malfunctioning control system of a heat treatment furnace. It was concluded from the metallurgical investigation that is the subject of this contribution that an almost tripled soaking time during aging, i. e., precipitation hardening, of an entire engine set of MHS did not cause any microstructural alterations that could have detrimentally affected mechanical properties of the subject components.

Liquation Cracking in Row 3 Twin Turbine Vane Segment Caused by Malfunctioning Plasma Coating Gun

Abstract Transverse through-wall airfoil cracking was found in a row 3 twin turbine vane segment of a type used in heavy-duty gas turbine engines for power generation. A laboratory investigation determined liquation cracking (LC), a hot cracking mechanism, as the metallurgical cause of failure. As became apparent only after the investigation, the root cause of failure was a malfunctioning plasma gun used for thermal spraying the thermal barrier ceramic top coat onto the subject part.

Non-destructive Metallurgical Failure Investigation of Erroneously Heat Treated Hot Gas Path Component Using Replica Technique

Abstract Metallic heat shields, used in combustion chambers of heavy-duty gas turbine engines and made of nickel-based superalloys, were accidentally heat treated during fabrication, using wrong parameters. There were concerns about embrittlement. Nondestructive metallurgical material characterization using the replica technique verified embrittlement by secondary phases. In spite of this, it was recommended to the client to use the affected parts as is, this on the grounds that no grain growth was observed and hardness was not elevated. While the case study presented in this contribution may appear trivial at first sight, it interestingly shows how metallographic examinations may sometimes be performed non-destructively, without sacrificing any engine components. The inclined reader should also note the superb quality of the photomicrographs presented in this contribution. They were taken from replica foils, not actual metallographic sections.

Metallurgical Failure Investigation of a Deformed and Cracked Lashing-Load Ring of a Heavy-Duty Gas Turbine Engine

Abstract A severely deformed and cracked lashing-load ring of a heavy-duty gas turbine engine was received from the client as-delivered from the site. The damage was found upon a regular non-destructive inspections (NDT). A metallurgical failure investigation was ordered to determine whether there was microstructural damage due to excessive thermal loads, whether there were any other material defects that could have contributed to the failure, or whether the lifting lug was abused or overloaded in any way, and whether this was the only cause of failure. It was determined that the subject lifting lug failed by tensile/bending overload.

Torsional Overload Fracture of Twist-off Bolts During Assembly

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.

Metallurgical Failure Investigation of Combustion Chamber Leakage in a Heavy-duty Gas Turbine Engine

Abstract 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.

Metallurgical Failure Investigation of Fractured Dog Bone Seal Retainer Ring Fillet Welds in the Turbine Exhaust Casing of a Heavy-duty Gas Turbine Engine

Abstract Short fillet welds used to fasten a large retainer ring to so-called dog bone seals failed in the turbine exhaust casing of a non-OEM heavy-duty gas turbine engine used for power generation. The subject fillet welds fractured due to high cycle fatigue loading. Neither weld imperfections nor any other material defects were found that could have contributed to the failure. It was concluded that an unfavorable design, specifying very short fillet welds for fastening the dog bone seal segments to the retainer ring, was the root cause of failure. In a purely static loading situation, this design would probably not have failed. However, in a dynamic loading scenario as is the case in any gas turbine engine exhaust, such a design is simply not sturdy enough.

Metallurgical Failure Investigation of Fractured Spring Seals

Abstract Elastic sealing elements used in combustion chamber assemblies of heavy-duty gas turbine engines used for power generation fractured from high cycle fatigue failure. The subject assembly was burner rig tested in a special testing facility. It was speculated that the affected spring seals likely failed due to forced excitation during burner rig testing, since no material imperfections or fabrication flaws, such as spot weld imperfections, were detected that could have contributed to the failure.

Experimental investigation of combustion dynamics and NOx/CO emissions from densely distributed lean-premixed multinozzle CH4/C3H8/H2/air flames

Lean-premixed fuel-flexible combustion technology capable of operating on a wide range of fuels, including high or pure hydrogen, is expected to play a crucial role in reducing nitrogen oxides and carbon dioxide emissions from heavy-duty gas turbine engines, ultimately in achieving energy system decarbonization. However, fuel-flexible operations, particularly with high hydrogen content fuels, require a fundamental, drastic change from conventional nozzle architecture. The use of a number of small-scale multitube injectors – also referred to as micromix nozzles – is indispensable to reduce the likelihood of flashback events in ultra-fast premixed hydrogen flames. At present, the vast majority of what is known about the fundamental mechanisms of combustion dynamics and emissions stem from intensive research in large-scale swirl-stabilized flames. The behavior of small-scale multinozzle flames governed by isolated or collective movements of constituent flames remains, however, largely unexplored. To address these questions, we use densely distributed sixty-injector mesoscale flames as a platform to explore the influence of a full range of CH4/C3H8/H2 fuel blends – including all possible combinations of a blend of two different fuels (CH4 + C3H8, CH4 + H2, C3H8 + H2) and three different fuels (CH4 + C3H8 + H2), as well as single-component fuels (CH4, C3H8, H2) – on combustion dynamics and exhaust emissions under a constant adiabatic flame temperature condition. Here, we show that while the nitrogen oxide emissions are almost unaffected by the fuel compositions considered here, the variation of fuel composition has a significant impact on self-excited instabilities, which tend toward higher frequency oscillations with increasing hydrogen concentration. From phase-resolved OH PLIF measurements for a 50/50 mixture of CH4 and H2, we identify the creation, evolution, and annihilation of an array of coherent vortical structures as the mechanism responsible for sound generation and flame surface modulations without strong interactions between adjacent flames. In contrast, the behavior of the 50% C3H8 + 50% H2 case is dictated primarily by periodic merging and separation of neighboring reactant jets, leading to large-scale asymmetric oscillations in the transverse direction. Given the substantial change in transport properties, effective Lewis number-related interpretation provides a reasonable explanation for the different behaviors of a cluster of small-scale flames.

Gamma Prime Solutioning Tests of Alloy 247 Specimens

Abstract Strictly speaking, this contribution is not about a case study in failure analysis. However, the investigation described herein was inspired by a failure case. A heavyduty gas turbine engine used in a power and desalination plant in the Middle East experienced creep damage and cracking in some of its row 4 turbine blades after only some 15,000 operating hours. Microstructural alterations detrimental to the creep strength of the alloy were determined to be the metallurgical root cause of the failure. It was believed that said microstructural alterations were brought about by unusual transient operating conditions in engine service. Heat treatment tests were ordered to verify or disprove this hypothesis. It was established that the peculiar gamma prime morphology found in the failed blades can be produced by very high solutioning temperatures and subsequent rapid cooling. Such conditions in engine service are conceivable if sufficient unburnt fuel enters the turbine and ignites downstream of the combustion chamber (high temperature), and if water used in fuel line purging is injected into the turbine section immediately thereafter (rapid cooling). Solutioning above 1240 °C, followed by a water quench, produced said mono-modal fine γ' morphology.

Failure of Impingement Cooling Plates in Gas Turbine Vanes

Abstract 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.

Metallurgical Investigation of Cold-formed Fillet Pieces Made of Metastable Austenitic Stainless Steel, for the Turbine Exhaust Casing of a Heavy-Duty Gas Turbine Engine

Abstract With a novel class of heavy-duty gas turbine engines, turbine exhaust temperatures are expected to further increase. There are concerns that the propensity to cracking in service of the welded turbine exhaust casing (TEC) liners might also increase. Cold-formed “collars”, featuring large stress-reducing fillet radii, were designed to decrease thermal stresses in the welds, where the struts of the turbine bearing casing are aerodynamically clad by liner plates. Such collars were received from a supplier in the course of a product and process qualification (PPQ). The results of the metallurgical investigation ordered by an internal client are the subject of this contribution. It was found that deformation martensite was present in the subject cold-formed fillet pieces, despite a pre-heat of 150 °C and a post-fabrication 920 °C annealing process. However, the amount of deformation martensite found, at below 5 % volume fraction, was considered not to be detrimental for engine operation.

Metallurgical Failure Analysis of Cracked First Stage Vane of Heavy-Duty Industrial Gas Turbine Engine for Power Generation

Abstract 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.

Metallurgical Failure Investigation of SCC in Leaking Piping System in a Combined Cycle Gas Turbine Power Plant

Abstract 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.

High Cycle Fatigue Failure of Burner Feeder Line in a Heavy-duty Gas Turbine Engine

Abstract A number of fuel gas supply pipes or feeder lines of heavy-duty gas turbine engines for power generation were found cracked in two sister engines of a power plant, this after approximately 100 000 operating hours. Cracks originated from the outer diameter (OD) of the feeder pipes, at the fusion line of the fillet weld that joined the subject pipes to the main burner bodies. The incident components failed by high cycle fatigue (HCF) fracture. The failure mechanism is purely mechanic. No evidence of corrosion or any other contributing mechanism was found. Design, manufacture, such as welding and coating, burner assembly, and burner installation are all unrelated to the failure. The root cause of the failure is excessive dynamic service loading.

Hydrogen Induced Stress Corrosion Cracking of Fuel Oil Premix Burner Nozzles in a Heavy-duty Gas Turbine Engine

Abstract 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.

Prediction of CO and NOx Pollutants in a Stratified Bluff Body Burner

The major exhaust gas pollutants from heavy duty gas turbine engines are CO and NOx. The difficulty of predicting the concentration of these combustion products originates from their wide range of chemical time scales. In this paper, a combustion model that includes the prediction of the carbon monoxide and nitric oxide emissions is tested. Large eddy simulations (LES) are performed using a compressible code (OpenFOAM). A modified flamelet generated manifolds (FGM) approach is applied with an artificially thickened flame approach (ATF) to resolve the flame on the numerical grid, with a flame sensor to ensure that the flame is only thickened in the flame region. For the prediction of the CO and NOx emissions, pollutant species transport equations and a second, CO based, progress variable are introduced for the flame burnout zone to account for slow chemistry effects. For the validation of the models, the Cambridge burner of Sweeney et al. (2012, “The Structure of Turbulent Stratified and Premixed Methane/Air Flames—I: Non-Swirling Flows,” Combust. Flame, 159, pp. 2896–2911; 2012, “The Structure of Turbulent Stratified and Premixed Methane/Air Flames—II: Swirling Flows,” Combust. Flame, 159, pp. 2912–2929.) is employed, as both carbon monoxide and nitric oxide [Apeloig et al. (2016, “PLIF Measurements of Nitric Oxide and Hydroxyl Radicals Distributions in Swirl Stratified Premixed Flames,” 18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics, Lisbon, Portugal, July 4–7.)] data are available.

Fretting Fatigue Cracking of a Center Guide Bolt Supporting the Combustion Chamber in a Heavy-duty Gas Turbine Engine

Abstract The slotted center guide bolt of the center guide feature of the lower part of the outer shell of an annular combustion chamber was found fractured in a heavy-duty gas turbine engine used for power generation, after approximately 5.500 operating hours. The incident was a one-off event and not a recurring incident. No similar events were reported from the fleet; hence the failure was not considered a field issue. The metallurgical root cause investigation that was ordered to determine the failure mechanism revealed that the incident center guide bolt failed by fretting fatigue cracking, a high cycle fatigue (HCF) phenomenon.

Austenitic Stainless Steel Bolt Failure by Stress Corrosion Cracking

AbstractDuring a minor inspection of a heavy-duty gas turbine engine, the hexagonal head of a bolt used for fixing a metal plate in the air intake was found on the floor of the air intake casing [1, 2]. The respective washer was not found. It is believed to have been ingested into the engine, having caused moderate compressor damage. The inspection was started after 83.500 operating hours. Only one fractured bolt was found. The subject bolt fractured due to stress corrosion cracking (SCC), induced by pitting corrosion and possibly assisted by crevice corrosion.

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.