Verhalten von Gasturbinenschaufelwerkstoffen unter langzeitiger Kriechermüdungsbeanspruchung

Behaviour of gas turbine blade materials under long-term creep fatigue loading The long-term creep fatigue behaviour of the gas turbine blade material IN-738 LC HIP is investigated by a service-type strain cycle up to a test duration of about 12000 h. Further, it is reported about shorter tests of the same type on alloy IN-100 HIP. The cyclic deformation of alloy IN-738 LC HIP in the long-term region is analysed and the creep fatigue crack initiation is described by the generalized damage accumulation rule using two different methods.     

Field Experience of Platinum Aluminide Coated Turbine Blades

This paper describes the results obtained from an evaluation of several platinum aluminide coated first stage turbine blades returned from the field. The IN-738LC blades had accumulated from 3,900 to 27,500 service hours in Centaur (1) 50 industrial gas turbine engines, operating in a high temperature oxidizing environment. The coating performance and condition were assessed using optical and electron microscopy. The condition of the coating was correlated to blade operating temperatures, which were estimated using the gamma prime coarsening technique. The degradation mechanism of the coating, remaining coating life, and blade repairability were also addressed.     

Improvement of grain structure and mechanical properties of a land based gas turbine blade directionally solidified with liquid metal cooling process

Abstract

The manufacturing process of a directionally solidified (DS) IN738LC turbine blade, produced with the liquid metal cooling (LMC) process was improved based on process modelling. The improvement involved varying the system dimensions in the baffle area and optimising the mould thickness, design and the withdrawal parameters. The grain structure of the DS blades produced exhibits a well defined <001>. texture with a few stray grains near the blade top compared to the previous design. Some blades were given to a two stage heat treatment followed by tension tests at 25 and 650°C as well as creep tests at 152 MPa/982°C and 340 MPa/850°C. The yield and tensile strength of improved DS blades were comparable to conventionally cast (CC) IN738LC blades, while the creep properties and the tensile elongation of the DS blades were significantly improved using the optimised LMC process. The LMC system is under more modifications to produce defect free single crystal turbine blades.     

Hochtemperaturverhalten von Werkstoffen aus neuartig gefertigten Gasturbinenschaufeln

High temperature behaviour of materials for gas turbine blades manufactured in new technology On gas turbine blades for the middle and last stages of stationary gas turbines, the service-relevant material properties were examined. The specimens were taken from forged blades of alloys Nimonic 101 and Udimet 720 and from two fine grain versions of cast blades of alloy IN-792. In the range of service relevant temperatures long-term creep rupture tests, annealing tests, relaxations tests as well as low and high cycle fatigue tests were carried out.     

Experimental studies on process-induced morphological characteristics of macro- and microstructures in laser consolidated alloys

Laser consolidation (LC) developed by National Research Council’s Industrial Materials Institute (NRC-IMI-London) since mid-1990s, is a laser cladding based rapid manufacturing and material additive process that could fabricate a “net-shape” functional metallic shape through a “layer-upon-layer” deposition directly from a computer aided design model without using molds or dies. In order to evaluate the LC processability of different materials, some representative nickel-based superalloys (IN-625, IN-718, IN-738, and Waspaloy), stainless steels (austenitic SS316L and martensitic SS420), and lightweight alloys (Ti–6Al–4V titanium alloy and Al-4047 aluminum alloy) have been investigated. Like other laser cladding based processes, due to process-induced rapid directional solidification, the LC alloys have demonstrated certain unique morphological characteristics. Moreover, the “as-consolidated” LC alloys, in nature, are in the “as-quenched” state, and some precipitation processes from their matrices, which are sometimes critical to the development of mechanical performance of the materials, could be effectively suppressed or retarded. Post-heat treatments, therefore, could necessarily facilitate the process of achieving their required operational microstructures. In this article, a comprehensive investigation was performed including metallurgical soundness and process-induced morphological characteristics of the LC materials, and microstructure development brought by post-LC heat treatments using optical microscope, scanning electron microscope, and X-ray diffraction. The implications on the mechanical performance of the LC materials were discussed as well in order to provide essential information for potential industrial applications of the LC materials.     

Beschreibung des Kriechverhaltens von Gasturbinenwerkstoffen. Teil II: Kriechgleichung für die Werkstoffsorte IN-738 LC

Creep Behaviour of Gas Turbine Materials part II. Creep Equation for Alloy IN-738 LC On alloy IN-738 LC, an extensive creep data collection is evaluated to obtain mean creep curves. On these curves a creep equation optimized for an individual test material is fitted to express the mean creep behaviour of alloy IN-738 LC.     

Evaluation of vibration-induced stresses in teeth of rotor disks of compressors of gas-turbine engines

We show that loads induced in turbine blades by vibration make a significant contribution to the dynamic stressed state of the teeth of compressor disks. The dynamic stresses formed in the teeth of the investigated blade wheels can be as high as 42% of the stresses induced in the blades. Together with fretting corrosion, static stresses, and other factors, this may lead to initiation and propagation of cracks in teeth and, as a final result, their fracture. Translated from Problemy Prochnosti, No. 6, pp. 95–101, November–December, 1997.     

Hot-corrosion-resistant duplex coatings for a superalloy

The goal of this program was to evaluate the hot corrosion resistance of a variety of cobalt-, nickel- and iron-based coating alloys containing chromium and sometimes aluminum and/or yttrium. The coatings were deposited onto two nickel-based superalloys by physical vapor deposition from an electron-beam-heated source. As a group, the cobalt-based compositions were found to be much more hot corrosion resistant than the nickel-based alloys. The iron-based coatings had relatively poor hot corrosion resistance.The coefficients of thermal expansion for a number of Co-Cr-Al-Y alloys were measured. Compositions with low (0–6%) aluminum and high (26–35%) chromium contents were found to have a reasonably close thermal expansion match with IN-738. If a duplex overlayer of aluminum was diffused into the outer regions of these coatings, it greatly enhanced their hot corrosion resistance.Ductile-to-brittle transition temperatures in the range 760–871 °C were measured for several Co-Cr-based coatings on IN-738. Measurements made on similarly coated IN-738 test bars showed that stress-rupture properties were generally lower than the baseline data for IN-738 but were above the standard deviation (3σ) curve.     

Verhalten von Gasturbinenschaufelwerkstoffen bei mechanischer Langzeitbeanspruchung unter Heißgaskorrosion. Teil 2: Metallkundliche Nachuntersuchungen

Behaviour of gas turbine blade materials under mechanical long term loading and hot gas corrosion. Part 2: Metallographical and microanalytical investigations Results of metallographical and microanalytical investigations on selected specimens after mechanical long term creep and LCF-loading under hot gas corrosion conditions will be presented. Uncoated and MCrAlY or Al-diffusion coated specimens of the polycristalline materials IN-738 LC and IN-939 and the single crystal alloys M 002 mod., CM-SX-6 and B 1914 were investigated. The corrosion attack increases with increasing impurities of the hot gas. Internal nitridation and sulphidation of base material or coating is observed after deterioration of the protective scales rich in Cr₂O₃ and/or Al₂O₃.     

Effects of aging treatments on the Microstructure of the Ni-base superalloy IN-738

The effects of aging treatments on the microstructure of the Ni-base superalloys IN-738 HC and IN-738 LC were investigated. Changes in (i) size, morphology, and distribution of the γ′-phase, (ii) the amount of M₂₃C₆-carbide, and (iii) the occurrence of additional phases and phase instabilities were quantitatively determined. The results indicate the possibility to deduce an unknown thermal history from the observed microstructure.     

Creep and long-term strength of herringbone lock joints of gas-turbine engines under the combined action of static and cyclic loads

We propose a method for the investigation of creep and long-term strength of statically and cyclically loaded elements of power-generating units. We present the results of numerical analysis of creep and fracture of herringbone lock joints of the blades of the second stage of gas-turbine engines as well as of their stress-strain state and accumulation of damage caused by creep. We determined the periods of time to the termination of the processes of latent and complete fracture of the investigated lock joints. It is shown that the effects of high-frequency cyclic loading on the relaxation of stresses, deformation, and fracture of the lock joint are significant. Kharkov State Polytechnical University, Kharkov, Ukraine. Translated from Problemy Prochnosti, No. 1, pp. 34 – 41, January – February, 1998.     

Mechanical properties of a low-pressure-plasma- applied Co-Cr-Al-Y coating

The low pressure plasma spray process was used to deposit coatings of a Co- 29Cr-6A1-1Y alloy onto IN-738 test specimens and to make free-standing deposits from which specimens were machined. Tensile tests, stress rupture tests and high cycle fatigue test were run on the coated specimens from room temperature to 870°C and results showed no significant effect of the coating process. A strain-to-coating-cracking test was also run on coated specimens and a coating ductile-to-brittle transition was found between 760 and 870°C. Free-standing Co- 29Cr-6Al-1Y tensile specimens were machined from the thick low-pressure- plasma-sprayed deposits. The tensile properties of these deposits were measured from room temperature to 980°C and results showed that the tensile strength of the deposits ranged from 50% above to 100% below the strength of cast IN-738, while elongation went from 0% to 130%. It is shown that the low pressure plasma process produced clean low defect coatings which perform well in mechanical cycling. However, it is also shown that the Co-29Cr-6Al-1Y coating, at low temperatures (less than 540°C), has relatively low ductilities which could cause early cracking in high strain level testing or service conditions. Some scanning electron microscopy replica and metallographic evaluations are also presented to illustrate the tensile cracking behavior of the coating.     

Radioisotope method for monitoring the level of wear for protective coatings made from chromium and aluminum on gas turbine blades

A radioisotope method is given for estimating the level of wear for protective coatings made from Al and Cr on gas turbine blades based on measuring the thickness of a coating that may change as a result of oxidation. A two-channel measuring system is developed making it possible to perform rapid analysis. __________ Translated from Izmeritel’naya Tekhnika, No. 10, pp. 36–39, October, 2006.     

Fatigue strength and life of compressor blades for marine gas turbine engines

An experimental procedure has been developed for the investigation of fatigue and crack growth resistance of materials and real compressor blades. Methods for the determination of stress intensity factors in specimens and in blades with cracks have been justified. Investigations have been performed on the influence of manufacturing residual stresses and surface defects in the form of simulators of dents, corrosion pits, and nonmetallic inclusions on the fatigue strength of steels and a titanium alloy. The characteristics of the material crack growth resistance have been studied taking into account the effect of the medium (sea water), stress ratio in a cycle, and programmed mode of loading. The authors also consider fatigue strength of newly-manufactured blades and those in operation, as well as specific features of fatigue crack propagation in blades. They have substantiated a method for predicting the life of blades with cracks. Institute for Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 1, pp. 28–40, January–February, 1999.     

Effect of the types of surface treatment and multilayer coating of turbine blade bodies on their fatigue strength

The technological characteristics of magnetoabrasive treatment of blade bodies used as a finishing operation in blade reconditioning and as a preliminary one in applying thin-layer coatings are studied. Fatigue testing of compressor blades with various types of surface treatment of the blade body and with a multilayer coating has been performed. A multilayer TiN coating is shown to exhibit good adhesion to the base metal, and blades with this coating have a higher fatigue strength and a lower scatter of durability values than after finishing polishing. Translated from Problemy Prochnosti, No. 5, pp. 109–116, September–October, 1999.     

Strain–life curves of steels and methods for determining the curve parameters. Part 1. Conventional methods

The available publications give much consideration to strain–life curves which are usually described by the Basquin–Manson–Coffin equation. The parameters of this equation are related to those of the Ramberg–Osgood equation that represents the cyclic stress–strain diagram. Many different methods have been put forward for the assessment of parameters of these equations on the basis of static strength and plasticity characteristics. Most of the methods rely on a fairly small body of experimental evidence. Using the experimental data on static and cyclic strength and plasticity characteristics of about 200 various steels from the well-known publications, a statistical analysis of parameters of Basquin–Manson–Coffin and Ramberg–Osgood equations has been performed by each of the assessment methods, revealing their advantages and disadvantages.     

Investigation of Fatigue Failures of Titanium Alloy Blades Used in Compressor Modules of Aeroderivative Industrial Gas Turbines

Gas turbine axial compressor blades are designed to withstand cyclic stress loading resulting of modal vibrations. However, when surface suffers from foreign object damage, a fatigue crack is likely to initiate. In this paper two failure case-studies of titanium alloy used for aeroderivative gas turbine compressor blades are presented. Both are related to a failure of first stage compressor blades coming from different turbine power ranges. Observed cracks were found at blade spans corresponding to modal vibrations.     

Gas turbine blade service life assessment with account of fracture stage

The numerical simulation of creep, continuum fracture zone evolution and crack propagation in a gas turbine blade uner cyclic loading conditions have been performed using a semianalytic finite-element method. The blade basic life (before fracture zone appearance) and additional resource (concerned with fracture process) of a blade have been determined. __________ Translated from Problemy Prochnosti, No. 5, pp. 28–36, September–October, 2008.     

To the method of accelerated evaluation of fatigue of metals with hardening coatings

Within the framework of the proposed scheme of an accelerated evaluation of strength of base material–coating compositions under cyclic loading, we show the necessity of its comprehensive realization by experimental and theoretical methods. The use of a high-frequency method for this purpose is substantiated by the example of the proposed experimental procedure. On the basis of the relationships for calculating residual stresses obtained in this work, a calculation model for predicting cyclic strength has been developed. We show its applicability for a preliminary evaluation of the durability of the base material–coating systems and for refinement of the hardening coating technology. Translated from Problemy Prochnosti, No. 2, pp. 79–90, March–April, 2009.     

Second Stage Gas Turbine Blade Premature Replacement Investigation

This article investigates the root causes of the premature failure and replacement of a set of second-stage turbine blades from a heavy industrial gas turbine engine. The investigations included dye-penetrant testing, optical microscopy, X-ray diffractometry (XRD), Environmental Scanning Electron Microscopy (ESEM), and energy dispersive X-ray spectroscopy (EDS) techniques. Moreover, the effect of heat treatment process on restoring the blade microstructure so that the properties were suitable for service was also explored. As a result of the investigation, the second-stage turbine blades premature failure was attributed to the grain boundary secondary phase precipitates. These precipitates were present in the “as-found” condition of the investigated blades.