Low-pressure Turbine Discs Research Articles

Surrogate-modeling-assisted creep-fatigue reliability assessment in a low-pressure turbine disc considering multi-source uncertainty

Surrogate-modeling-assisted creep-fatigue reliability assessment in a low-pressure turbine disc considering multi-source uncertainty

System-level creep-fatigue reliability evaluation by engineering damage mechanics incorporating cumulative damage-damage threshold interference

Reliability assessment for structural system is of great significance in ensuring system safety, providing a scientific guidance for life management. In this work, a creep-fatigue reliability modeling method is developed for system-level perspective on the basis of cumulative damage-damage threshold interference. Probabilistic damage accumulations to given loading cycles are associated with the creep-fatigue models considering multi-source uncertainties and the damage threshold is regarded as a random variable. Particularly, a numerical procedure is proposed for practical application in engineering by combining the Monte Carlo simulation with Gaussian process regression-based surrogate model. Furthermore, a low-pressure turbine disk under creep-fatigue loading condition is taken as an instance to evaluate its reliability considering multiple weakness sites. Results show that the proposed method is better than the independent treatment because it is capable of avoiding the overly-conservative reliability estimation. In addition, the influence of damage summation rules and damage threshold on reliability is quantitatively analyzed in terms of the developed method. The effect of random parameters in structural system on computational efficiency of the proposed numerical procedure is investigated.

A data-driven roadmap for creep-fatigue reliability assessment and its implementation in low-pressure turbine disk at elevated temperatures

High-reliability life design process not only can ensure system safety in service, but also can provide scientific life management during the maintenance period. The objective of the present work is to develop a roadmap for creep-fatigue reliability assessment. Material-level data accumulations and theoretical foundations of creep-fatigue including creep-fatigue constitutive and multi-axial damage models are introduced. Afterwards, a low-pressure turbine disk under a typical creep-fatigue load waveform is applied as a case study to demonstrate how to perform creep-fatigue reliability assessment by using this roadmap in practice. Precise weakness hotspots are identified at the mortise joint of turbine disk. Based on hotspot-based strategy, it is found that the surrogate model assisted by an optimized machine learning method enhances significantly the computational efficiency. Accordingly, the probabilistic creep-fatigue life with considering multi-sources uncertainty obeys lognormal distributions. In the aspect of failure probability analysis, the current probabilistic damage interaction diagram method with creep-fatigue interaction gives conservative reliability assessments and excellent universality as compared to traditional ones mainly used in the low cycle fatigue field. Last but not least, joint failure evaluation of the turbine disk is discussed to comprehensively consider potential failure occurrence in an averaged hot region instead of a single hotspot.

Low-pressure turbine disc fi rtree slots’ life and reliability enhancement by ultrasonic surface hardening

The possibility of life and reliability enhancing of AL-31F low pressure turbine disc’s fir-tree slots by ultrasonic hardening is considered. Having disc’s material properties studied, working stress derivation is executed which was further used for following comparative fatigue tests. Also, Davidenkov method residual stress analysis is carried out which showed 95.3 % change to compression stress for circumferential residual stress and 80.9 % change to compression stress for axial residual stress which proves possibility of fir-tree slots’ life and reliability enhancement by ultrasonic hardening. Comparative fatigue tests with N = 4•10 5 cycles basis showed that the hardened samples standing out the cycle basis during higher oscillatory amplitudes (and, thus, affecting loads) than the non-hardened basic ones.

A case of industrial safety appraisal for extension of service life of GTK-10-4 gas turbines used at gas transmission stations

It is shown that the extended life and enhanced operational reliability of parts and subassemblies of the most popular GTK-10-4 gas transmission plants are determined by the enhanced efficiency of the control over technical condition and operational safety of turbine plants in conformity with industrial safety requirements imposed on gas pipeline compressor stations. It has been established that the materials of parts and subassemblies of gas turbine plants with different, especially with maximal operating time, shall be exposed to NDT for the purpose of determining the actual mechanical characteristics of these materials with different operating time and calculating residual life. The analysis of damageability and operating conditions has helped to identify parts and subassemblies for repair or replacement with the highest frequency of unacceptable defects. These parts and subassemblies have been shown to include base members of the axial compressor (AC), a turbine housing, an axial compressor rotor, high- and low-pressure turbine (HPT and LPT) discs, a 12-part holder, the housing of the holder of HPT and LPT guiding blades, a sealed baffler, and working and guiding AC, LPT and HPT blades. The most typical operational defects have been enumerated and analyzed. It has been determined that the primary task of the industrial safety appraisal for extending the life of GTK-10-4 with limit-exceeding operating time is to thoroughly examine HPT and LPT discs with more than 130,000 hours of operating time and establish by DT methods characteristics of materials for evaluation, taking account of their degradation, and residual life of critical turbine elements. In addition, it has been shown that the service life of HP turbine discs can be extended by replacing the disc material (EP-428 12% chromium steel) with a material with a higher linear expansion factor that somewhat exceeds the expansion factor of EI-893 nickel alloy used to melt out working blades.

Phased array ultrasonic signal compressive detection in low-pressure turbine disc

As the core component of nuclear units, the inspection of low-pressure (LP) turbine disc is of particular importance to assure their quality and safety. Nevertheless, the complex structure makes the position of transducer very limited and the huge size brings about a significant amount of data. Recently, it has been suggested that compressive sensing (CS) may be promising to implement the sampling far below the Nyquist rate. In this work, a novel CS-based procedure specifically dedicated to phased array ultrasonic signals in LP turbine disc NDE is proposed, aiming to achieve high compression rate with low signal distortion. Taking the effect of sampling frequency into account, the simulation is first conducted on the signals derived from CIVA platform. An excellent recovery is reached using measurement points less than the Nyquist sampling limitation required, which confirms the miraculous power of the CS. Then experimental verification is presented on a proportional mockup with EDM notches and the reconstruction performance is compared between five greedy algorithms, like used in the simulation. Although not as impressive as the results obtained from simulated data, the Orthogonal Matching Pursuit (OMP) algorithm verifies its feasibility and effectivity in ultrasonic array signal compression with sufficient recovery accuracy. Finally, the performance of OMP-based CS is compared with the gold standard for signal compression, namely, the discrete wavelet transform (DWT).

Fracture mechanics analysis of damaged turbine rotor discs using finite element method

This paper presents evaluation fracture mechanics parameters in low pressure turbine components. Critical locations such as keyway and dovetail area are experiencing stress concentration leading to crack initiation. Stress intensity factors were evaluated using the J-Integral approach available within ANSYS software code. The finite element method allowed the prediction of the point of crack initiation and the crack propagation using the orientations of the maximum principal stresses. Special attention in this investigation is focused to develop analytic expressions for stress intensity factors at critical location of low pres-sure steam turbine disc.

Crack Orientation and Depth Estimation in a Low-Pressure Turbine Disc Using a Phased Array Ultrasonic Transducer and an Artificial Neural Network

Stress corrosion cracks (SCC) in low-pressure steam turbine discs are serious hidden dangers to production safety in the power plants, and knowing the orientation and depth of the initial cracks is essential for the evaluation of the crack growth rate, propagation direction and working life of the turbine disc. In this paper, a method based on phased array ultrasonic transducer and artificial neural network (ANN), is proposed to estimate both the depth and orientation of initial cracks in the turbine discs. Echo signals from cracks with different depths and orientations were collected by a phased array ultrasonic transducer, and the feature vectors were extracted by wavelet packet, fractal technology and peak amplitude methods. The radial basis function (RBF) neural network was investigated and used in this application. The final results demonstrated that the method presented was efficient in crack estimation tasks.

Development of localized corrosion damage on low pressure turbine disks and blades: II. Passivity breakdown

Passivity breakdown of Type 403 stainless steel (SS), low pressure steam turbine (LPST) blade alloy, and A470/471 low alloy steel (LPST disk/rotor steel) has been studied in deaerated borate buffer solution. Measurement of the apparent breakdown potential as a function of potential sweep rate has demonstrated that the apparent breakdown potential varies linearly with the square root of potential sweep rate, as predicted by the Point Defect Model (PDM). The calculated areal concentration of condensed cation vacancies at the point of separation of the barrier layer from the metal substrate is in satisfactory agreement with that estimated from the crystallographic structure at the metal/barrier layer interface (CrxFe1−x/Cr2O3 for Type 403 SS and Fe/Fe3O4 for A470/471 steel), thereby demonstrating the validity of the PDM. The calculations of breakdown potential, induction time, and their distributions from the PDM using parameter values obtained by fitting the PDM to experimental data represent the first quantitative characterization of passivity breakdown behavior of Type 403 SS. The experimental distributions in the breakdown potential for Type 403 SS in deaerated borate buffer solutions (pH=8.1±0.1) containing various chloride concentrations at ambient temperature and 90°C are in satisfactory agreement with the relevant calculated distributions.

Development of localized corrosion damage on low pressure turbine disks and blades. III: Application of Damage Function Analysis to the prediction of damage

Integral damage functions for Type 403 stainless steel (SS) in borate buffer solution containing chloride ion have been experimentally determined. The pits are of an open morphology, thereby rendering effective determination of the damage functions using optical microscopy. Extensive pit coalescence caused the measurement of the damage functions to be less accurate than desired and, in some instances, rendered their measurement impossible. Accordingly, the Deterministic Extreme Value Statistics (DEVS) protocol of Damage Function Analysis (DFA) is adopted as a convenient means of presenting pitting damage in terms of the growth of the deepest pit in the population. A similar analysis is reported for A470/471 steel, where an insufficient population of shallow, pan-shaped pits precludes DFA, which requires a large population. In this case, too, DEVS is used to extrapolate maximum pit depth to a longer time (750h) after calibrating the model on shorter term data (24h and 240h). The experimental maximum pit depth distribution for 750h is in good agreement with that predicted from the calibrated DEVS, thereby demonstrating the veracity of the method. The maximum pit depth distributions on Type 403 SS and A470/471 steel surfaces do not display the increasing trend with increasing applied potential and chloride concentration. The large depth distribution and/or a large ohmic potential drop inside a pit under a thick adherent layer of corrosion product, probably have “buried” the effects of applied potential and chloride concentration.

Development of localized corrosion damage on low pressure turbine disks and blades: I. Passivity

Passivity on steel surfaces plays a crucial role in the development of pitting corrosion damage. In this work, the passivity on Type 403 stainless steel (SS), low pressure steam turbine (LPST) blade alloy, and A470/471 low alloy steel (LPST) disk/rotor steel has been studied in borate buffer solution. The highly defective barrier layers on Type 403 SS and A470/471 steel, formed at potentials in the passive region, exhibit n-type semiconductor behavior, due to the predominance of oxygen vacancies and/or cation interstitials as crystallographic point defects. The defective Cr2O3 barrier layer on the Type 403 SS surface has a greater donor concentration and a smaller thickness than does the barrier layer (defective Fe3O4) on the A470/471 steel surface. Increasing oxygen and chloride concentration in the electrolyte increase the donor concentration in the passive film on Type 403 SS. In the passive regions of Type 403 SS and A470/471 steel, the steady-state current density is nearly independent of, and the steady-state barrier layer thickness increases linearly with, increasing film formation potential. These experimental observations are interpreted in terms of the point defect model (PDM) for the passive state. Mechanistic analysis of electrochemical impedance spectroscopy (EIS) data by the impedance model developed from the PDM, is also performed, and the validity of the impedance model is demonstrated.

Using phased array ultrasonic technique for the inspection of straddle mount-type low-pressure turbine disc

Discovery of stress corrosion cracking (SCC) in the blade attachment region of nuclear power plant low-pressure steam turbine discs leads to industry concern about this problem. Reliable inspection methods for the detection of SCC in steam turbine blade attachment area are essential to the rotor run/repair/retire decision making process. The conventional broad beam ultrasonic inspection technique usually has inadequate resolution for flaw sizing and tendency for false calls. But the phased array technique by improved detection capability and resolution could increase coverage of the examination areas with less scanning required. Using phased array ultrasonic technique to straddle mount-type low-pressure turbine disc in nuclear power plant located in Korea was studied in this paper.

Prediction of residual stress distribution after turning in turbine disks

The state of a surface region after machining is definitely affected by cutting parameters, such as cutting speed, feed rate, tool nose radius, tool rake angle and the presence of a cutting fluid, which plays a major role in determining friction at the tool–chip interface. The aim of the present study is to develop a finite element model based on the general-purpose nonlinear finite element code MSC.Marc by MSC.Software Corporation. This software is capable of simulating the cutting process of low-pressure turbine disks of aircraft jet engines from its very beginning to steady-state conditions. Basically, the present analysis is a coupled thermo-mechanical dynamic-transient problem, based on the update Lagrangian formulation; no pre-defined path is given for the separation of the chip from the workpiece, since material deformation occurs as a continuous indentation performed by the rigid tool. In addition to the cutting parameters, the main inputs in this analysis are material constitutive data, the friction coefficient at the toolchip interface and the cutting tool temperature. All the relevant variables, like stresses, strains, temperatures, chip shape and residual stresses, are predicted in a wide range of cutting conditions. The results from the model are compared to some basic theories of metal cutting and to an experimental study, concerning orthogonal cutting of steel AISI 316L. Concerning the specific case of turning process of nickel alloy Inconel 718 low-pressure turbine disks, the calculated residual stress are compared to experimental measurements from real machined disks.

Determination of the Toughness of In-Service SteamTurbine Disks Using Small Punch Testing

Knowledge of the material toughness is crucial in assessing the integrity of heavy section steel components. Conventional tests to determine the toughness involve extraction of large blocks of materials and therefore are not practical on in-service components. On the other hand, conservative assumptions regarding toughness without regard to actual data can lead to expensive and premature replacement of the components. Previous EPRI studies have demonstrated the use of a relatively nondestructive technique termed the “small punch test” to estimate the fracture appearance transition temperature (FATT) and fracture toughness (KIc) of high-temperature turbine rotor steels and nuclear reactor pressure vessel steels. This paper summarizes the results of research into the feasibility of extending the small punch test to characterize the toughness of the 3 to 3.5% NiCrMoV (3–3.5NiCrMoV) low alloy steel used for fossil and nuclear power plant low-pressure (LP) steam turbine disks. Results of the present study show that the small punch transition temperature, Tsp, is linearly correlated with FATT, so that measurement of Tsp permits estimation of the standard Charpy FATT through empirical use of the correlation. The statistical confidence prediction uncertainty bands for the correlation were found to be narrow enough to make the small punch- based FATT estimation practical for this alloy. Additionally, independent KIc measurements made by PowerGen, UK, on some of the same test materials were in excellent agreement with measurements made here, indicating that the small punch KIc measurement can be reproducible across laboratories. Limited testing for fracture initiation toughness showed, as has been demonstrated for other materials, that the small punch test-based initiation fracture toughness (KIc) determination was within ±25% of the ASTM standard measurement of KIc, suggesting that the test method can be used for direct determination of fracture initiation toughness.

Stress Corrosion Cracking of Steam Turbine Rotors

Abstract In the wake of the catastrophic failure of a low-pressure (LP) turbine disk at the Hinkley Point Nuclear Station in 1969, considerable research and development has been devoted to the prob...

Prediction of Failures of Low-Pressure Steam Turbine Disks

Localized corrosion phenomena, including pitting corrosion, stress corrosion cracking, and corrosion fatigue, are the principal causes of corrosion-induced damage in electric power-generating facilities and typically result in more than 50 percent of the unscheduled outages. In this paper, we describe a deterministic method for predicting localized corrosion damage in low-pressure steam turbine disks downstream of the Wilson line, where a condensed, thin electrolyte layer exists on the steel disk surfaces. Our calculations show that the initiation and propagation of stress corrosion cracking (SCC) is not very sensitive to the oxygen content of the steam, but is sensitive to the conductivity of the condensed liquid layer and the stresses (residual and operational) that the disk experiences in service.

Low pressure turbine disk assessment a case study : McCann, D.R.; Jirinec, M.J.; Quinn, J.R. Proceedings: EPRI Steam Turbine and Generator NDE, Life Assessment, and Maintanance Workshop, Charlotte, North Carolina (United States), 16–19 Jul. 1991. pp. 34.1–34.12 EPRI-TR-101333 (1992)

Low pressure turbine disk assessment a case study : McCann, D.R.; Jirinec, M.J.; Quinn, J.R. Proceedings: EPRI Steam Turbine and Generator NDE, Life Assessment, and Maintanance Workshop, Charlotte, North Carolina (United States), 16–19 Jul. 1991. pp. 34.1–34.12 EPRI-TR-101333 (1992)

Stress Corrosion Cracking Behavior of Two Ni-Cr-Mo-V Steels in Caustic Solutions and Pure Oxygenated Water

Abstract The purpose of this work is to evaluate the stress corrosion cracking (SCC) of two Ni-Cr-Mo-V Italian-made alloys that are materials used for low-pressure (LP) turbine discs in light water...

Mechanical and metallurgical properties of retired 3.5NiCrMoV low pressure steam turbine discs

A study has been made of chemical composition, metallurgical structure and mechanical properties of many low-pressure turbine discs that had been retired from service, due to in-service cracking. Determination of the chemical composition involved both bulk chemical analyses and trace element analyses. The metallurgical examination involved determination of material microstructure, grain size, and grain boundary composition (Auger analysis). The mechanical properties measured included tensile strength, notch toughness, and fracture toughness. It was found that variations in bulk chemical composition do not affect the susceptibility to stress corrosion cracking and no abnormal composition that had contributed to in-service cracking was identified. The turbine discs that had cracked in service had widely varying tensile strengths, indicating that stress corrosion susceptibility of these particular discs were independent of their yield strength in the range 734 to 1131 MPa (106 to 164 ksi). The Charpy V-notch impact tests and the fracture toughness tests performed showed that the 3.5NiCrMoV steels generally had good toughness. Temper embrittlement of these steels led to reduced toughness and was related to increased phosphorus segregation to the prior austenite grain boundaries. The steels examined had similar tempered bainite microstructures, but varying grain sizes. There was no significant microstructural difference among the materials, and there was no evidence of inherent microstructural defects or abnormalities that may have contributed to the in-service cracking.

The Effect of Temperature, Specimen Size, and Geometry on the Fracture Toughness of a 3 Pct NiCrMoV Low Pressure Turbine Disc Steel

The variation of fracture toughness with temperature and specimen size of a 3 pct NiCrMoV LP disc steel has been investigated over the temperature range -100 to +100 °C using compact tension and single-edge-notched bend geometries. A number of large ‘half-disc’ three point bend specimens were also tested. Toughness increased up to a transition temperature coinciding with the onset of stable ductile tearing prior to instability. Below this temperature fracture could be described by established linear elastic or post yield fracture analyses. Above this temperature failure was by plastic collapse. The transition temperature decreased with decreasing specimen size, and at similar thicknesses was lower for the bend geometry than for the compact tension so that it was not possible to predict the fracture behavior of the full size service component from small scale tests in the transition region. A further complicating feature was the extreme scatter of some duplicate test results below the transition temperature. The implications for toughness testing in the transition region are discussed. The data obtained in this work have been combined with published data for similar steels to derive an equation which describes the variation of fracture toughness with temperature for steels of this type.