1997 TMS Annual Meeting: Tuesday Abstracts

MATERIALS, COATINGS AND PROCESSES FOR IMPROVED RELIABILITY OF HIGH TEMPERATURE COMPONENTS: Session IV: Life Prediction, NDE, and Repairs

Session Chairperson: W.S. Walston, GE Aircraft Engines, 1 Neumann Way, M85, Cincinnati, OH 45215

2:00 pm INVITED

STATUS, ISSUES AND CHALLENGES IN LIFE ASSESSMENT METHODOLOGY OF HOT SECTION COMPONENTS OF INDUSTRIAL TURBINES: K.G. Kubarych, Solar Turbines, 2200 Pacific Highway, P.O. Box 85376, San Diego, CA 92186

Abstract not available.

2:50 pm

LIFE PREDICTION BASED ON CRACK GROWTH BEHAVIOR FOR GAS TURBINE NOZZLES: N. Isobe, S. Sakurai Mechanical Engineering Research Laboratory, Hitachi Ltd., Japan; K. Kumada, Hitachi Works, Hitachi Ltd., Japan

Life prediction method for gas turbine nozzles was discussed. In gas turbine nozzles, crackings due to cyclic thermal strain generating with start-up and shut-down of turbines limit the life of components. This thermal strain is usually compression and holded during steady operation time. Therefore, it will be necessary to consider the effect of compressive creep or relaxation to the evaluation of crack growth behavior in nozzles. We conducted crack growth tests at 900°C using a strain wave form including compressive strain hold. Test results showed that crack growth rate in compressive strain hold tests were faster than that in no strain hold tests. A fracture mechanics approach was carried out to evaluate this compression hold effects. By using creep J-integral, a good correlation for crack growth data was obtained. A crack growth analysis considering stress gradient in the component was also carried out and we discussed about its accuracy using inspection data for 25MW class gas turbine nozzles.

3:10 pm

RELIABLE TBC'S FOR THE INDUSTRIAL GAS TURBINE DESIGN: W. Beele, W. Stamm, SIEMENS/KWU, Wiesenstr. 35, D-45473 Mulheim/Ruhr, Germany

This paper reviews the TBC-design as meanwhile established in aircraft technology and highlights the reasons why a reliable industrial gas turbine TBC-design has to differ in various material system properties. The development steps for Industrial Gas Turbine-(IGT-)TBC-systems will be presented as well as fundamental research activities like cyclic oxidation tests with enlarged high temperature aging periods, and first design results will be given in some real components examples. The actual use of TBC's in existing gas turbine designs is characterized by: TBC's were developed and approved by SIEMENS in a large test program including several types of cyclic and static high temperature oxidation and corrosion testing, the investigation of the mechanical properties for modeling issues and the review of the P&W-development steps and results for the aircraft application.

3:30 pm BREAK

3:50 pm

NON-DESTRUCTIVE EVALUATION OF HIGH TEMPERATURE OXIDATION DAMAGE USING ELECTROCHEMICAL TECHNIQUES: D.C. Tamboli, A.K. Rawat, V. Desai, Mechanical Materials and Aerospace Engineering Department, University of Central Florida, Orlando, FL 32816

Electrochemical techniques have been widely used as life prediction tools in corroding structures and coatings for aqueous corrosion. However, there has not been much attention given to the applicability of these techniques in assessing high temperature oxidation damage. In the high temperature oxidation, the metallic substrate is covered with an oxide film which has semiconducting properties. Electrochemical techniques such as electrochemical polarization and electrochemical impedance spectroscopy reveal vital information about the electronic transport properties of this oxide film by monitoring the response of the system to applied D. C. and A. C. potentials respectively. The protective properties of an oxide are largely dependent on the electronic and ionic transport through the oxide layer. In the ex-situ studies, the oxidized specimens are immersed in a highly reversible redox electrolyte. The parameters such as polarization resistance, open circuit potential changes and the oxide band gap potential obtained using these techniques showed good qualitative corroboration with the observed oxidation damage in the alloys studied.

4:10 pm

PREDICTIONS OF MICROSTRUCTURE CHANGES IN COATED TURBINE BLADES DURING SERVICE: X. Qiao, J.E. Morral, Department of Metallurgy and Materials Engineering, University of Connecticut, Storrs, CT 06269-3136

With DICTRA software, it is possible to predict microstructural changes in coated turbine blades during service. In the present work, this technique is illustrated by calculating the microstructures that will form between a nickel base +1 alloy and MCrALY type + alloys. It is shown that a number of different microstructures can form initially, depending on the coating composition. As the average composition of the coating varies during service, the microstructure may change several times. These variations can be illustrated on an "Interdiffusion Microstructure Map."

4:30 pm

EFFECTS OF SUBSTRATE CURVATURE AND ROUGHNESS ON RESIDUAL STRESSES IN OXIDE FILMS: J.K. Wright, R.L. Williamson, Idaho National Engineering Laboratory, P.O. Box 1625, Idaho Falls, ID 83415-2218

Finite element simulations are used to examine residual thermal stresses and strains in protective Al2O3 scales on Fe3Al specimens, both during cooling from oxide formation temperatures and during subsequent thermal cycling. Geometrically, the simulations focus on regions of local curvature, either due to corners or substrate surface roughness. A variety of substrate corner radii and film thicknesses are considered. The effects of substrate material behavior are investigated by not only considering the actual elastic-plastic response of the aluminide substrate, but also the limiting cases of purely elastic and perfectly plastic material behavior. When plasticity is permitted, the substrate is able to deform to accommodate stresses at the corner, and the film is in tension along the outside surface and compression near the interface. These tensile stresses are of concern for coating integrity, since corners are observed experimentally to be sites of oxide cracks and spallation.

4:50 pm

REPAIR WELDING OF 1.0CR-1MO-0.25V BAINITIC TURBINE ROTOR: YoungKun Oh, Kia Motors Corporation, Production Engineering R/D Department, J.E. Indacochea, GwangSoo Kim

Weld repair of ASTM A-470 class 8 high pressure steam turbine rotor steel has been performed to extend the service life of older fossil units. Multipass SAW, MIG and TIG welding have been employed. Microhardness of the base metal was VHN 253, however it dropped to VHN 227 at the heat affected zone close to unaffected base metal for SAW. This area of hardness drop is called "softening zone" and has a width of 0.5~0.6mm. During creep rupture test, failure occurred around the softening zone and rupture time was 772.4hr at 19ksi and 593°C. At ruptured area, spherical types of coarsened carbides, which were revealed molybdenum rich M6C were observed. Based on creep rupture life, SAW and/or high heat input TIG process provide the best creep rupture life and it could operate about 8-10 years.

5:10 pm

MATHEMATICAL MODELLING OF ELECTRON BEAM EVAPORATION: Adam Powell, Gerardo Trapaga, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139; Paul Minson, Intel Corporation, Rio Rancho, NM 87114

Mathematical models of vapor and melt pool transport phenomena in electron beam evaporation are used to design an optimal beam scanning pattern to achieve a desired evaporant flux distribution at the substrate. The vapor transport model uses the Direct Simulation Monte Carlo method to calculate evaporant flux distribution at the substrate from the temperature distribution at the source, the activities of source species, background gases and system geometry. The source temperature distribution is adjusted manually in order to produce the desired flux distribution at the substrate. The melt pool model then calculates the heat flux distribution which the scanning electron beam must impart to the molten source surface in order to produce the desired temperature distribution, accounting for thermal losses to radiation and evaporation. Finally, a surface heat transfer model is used to calculate minimum beam local heating by the scanning beam.