Materials at High Temperature Vol 18, Issue 2, 2001
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The oxidation of thin foils of FeCrAl(RE) alloys in moist air
H. Al-Badairy and G.J.Tatlock
Materials Science and Engineering, Department of Engineering, University of Liverpool, Liverpool L69 3GH, UK
The effect of varying levels of water vapour in air on the oxidation of FeCrAl plus Reactive Element (RE) alloys has been investigated at temperatures of 1200°C and 1300°C. Thin (<125 _m) foils of three different commercial alloys (Kanthal AF, PM2000 and Aluchrom YHf) have been tested to failure in laboratory air and air containing 3.2 vol.% water vapour. At this point the protective aluminium oxide scale fails and the alloys go into breakaway oxidation with the formation of chromium- and then ironrich oxide. Our results would suggest that high levels of water vapour in the atmosphere speed up the initial oxidation and reduce the overall lifetime of the thin components. The important factor for the total scale failure, the formation of non-protective iron oxide, is the depletion of Al levels to a critical value, below which no protective alumina scale can form; and this occurred slightly faster in an air + 3.2 vol.% water vapour environment.
Keywords: oxidation, thin foils, FeCrAl(RE), moist air
Effect of an applied stress on the growth kinetics of oxide scales formed on Ni-20Cr alloys
G. Calvarin-Amiri1, A.M. Huntz1, and R. Molins2
1LEMHE, CNRS, UMR 86-47, Université Paris XI, 91405 Orsay, France
2ENSMP, UMR CNRS 76-33, Centre des Matériaux, 91003 Evry, France
The effect of a tensile load on the oxidation rate and mechanism of Ni–20Cr was studied at 600 and 900°C by comparing samples oxidised in air either classically or under tensile creep. The objective was to understand the simultaneous effect of the environment and mechanical loading. Applying a tensile load does not modify the oxide layer nature but the oxidation rate is enlarged due to the increase of oxygen diffusion via fast diffusion paths generated when the strain is higher than a critical value. Thus, at 600°C, oxygen in excess promotes internal oxidation and the oxide layers are thicker than when formed without any load. At 900°C, for small oxidation times, the spinel layer is enlarged, while for longer times, the spinel layer progressively disappears and the excess of oxygen diffusing inward reacts with chromium to form a thicker Cr2O3 layer.
Keywords: Ni–20Cr, oxidation rate, creep, strain effect, oxidation mechanism, oxidation–deformation interaction
The relationship of stress and temperature on high-temperature corrosion fracture mechanism of waspaloy in various catalyst environments†
Phillip Dowson and Charles P. Stinner
1Manager, Materials Engineering, Elliott Company, Jeannette, Pennsylvania, USA
High temperature catalyst from the fluid catalytic cracking process can, under certain conditions, lead to oxidation attack of power recovery turbine materials. This oxidation attack can cause blade failures in the location of the blade/disk root attachment where the stress intensity at the oxide location exceeds the critical stress intensity necessary for fracture to occur. The relationship of stress intensity and temperature and how the catalyst environment effects the fracture toughness of the material are addressed. The influence of catalyst on the mechanical properties of waspaloy is illustrated from a series of stress rupture tests that were conducted for several different sets of “environmental conditions”. The results show that, under certain conditions, the presence of catalyst can lead to rapid failure. An attempt was made to ascertain what species may have been responsible for the rapid failures. Preventative measures that have been developed by Elliott company are highlighted, such as a steam barrier system, protective coatings, and a new superalloy, RK1000.
Keywords: stress, temperature, corrosion fracture, waspaloy
Crack growth behavior of ferritic steel for USC boilers in pressurized superheated steam†
Masahiro InuiI1,Yutaka Watanabe2*,Tatsuo Kondo3, Koshi Suzuki4 and Kimio Kano5
1Machine Intelligence and Systems Engineering, Graduate School of Engineering, Tohoku University, 01 Aoba, Aramaki, Aoba-Ku, Sendai 980-8579, Japan.
2Machine Intelligence and Systems Engineering, Graduate School of Engineering, Tohoku University, 01 Aoba, Aramaki, Aoba-Ku, Sendai 980-8579, Japan.
3Machine Intelligence and Systems Engineering, Graduate School of Engineering, Tohoku University, 01 Aoba, Aramaki, Aoba-Ku, Sendai 980-8579, Japan.
4Thermal Power Engineering Department, Tohoku Electric Power Company, 3-7-1 Ichibancho, Aoba-Ku, Sendai 980-8550, Japan
5Manager, Thermal Power Engineering Department, Tohoku Electric Power Company, 3-7-1 Ichibancho, Aoba-Ku, Sendai 980-8550, Japan
In this study, effects of superheated steam on cyclic crack propagation behavior of a heat resistant steel were investigated. Crack propagation experiments were carried out on NF616 (9Cr-0.5Mo-2WVNb) in pressurized superheated steam (600°C/10MPa) under cyclic loading either with or without holding time at constant load. Superheated steam environment has two opposing effects on cyclic crack growth, acceleration and retardation. A modified tarnish rupture (TR) model has been proposed to explain the crack propagation behavior. The crack propagation rate estimated based on the TR-type model well agreed with the experimental data.
Keywords: ferritic
Modelling creep damage in heat affected zone in 321 stainless steel. Part I: Quantitative study of intergranular damage
M. Chabaud-Reytier1, L. Allais1, D. Poquillon2, C. Cäes-Hogrel1, M. Mottot1 and A. Pineau3
1CEA Saclay, DECM/SRMA, 91191 Gif-sur-Yvette, France
2CEA Cadarache, DER/SERSI, Bat. 212, 13108 St-Paul-lez-Durance Cedex, France
3UMR CNRS 7633, Centre des Matériaux, Ecole des Mines de Paris ,BP87, 91003 Evry Cedex, France
During service at high temperature, stabilized stainless steels are well known to present a serious form of intergranular cracking in the Heat Affected Zones (HAZ) referred to as the reheat cracking phenomenon which is very deleterious for the structural integrity of welds made in this steel. The purpose of the present study is to present quantitative results on mechanical behaviour and the damage at high temperature of simulated HAZ which show different sensitivities to intergranular cracking. Creep laws are determined and an intergranular damage model is identified using the local approach to fracture based on mechanical tests performed at 600 °C and metallurgical observations on smooth specimens and two types of notched specimens.
Keywords: austenitic stainless steel, AISI 321, creep, local approach, intergranular damage, modelling, finite element
method.
Modelling creep damage in heat affected zone in 321 stainless steel. Part II: Application to creep crack initiation simulations
D. Poquillon1, M. Chabaud-Reytier2, L. Allais2 and A. Pineau3
1CEA Cadarache, DER/SERSI, Bat. 212, 13108 St-Paul-lez-Durance Cedex, France
2CEA Saclay, DECM/SRMA, 91191 Gif-sur-Yvette, France
3UMR CNRS 7633, Centre des Matériaux, Ecole des Mines de Paris, BP87, 91003 Evry Cedex, France
Stabilized stainless steels like AISI 321 are well known to present a serious form of intergranular cracking in the HAZ referred to as reheat cracking. The purpose of the present part is to validate the creep damage model identified using the local approach methodology on notched bar creep tests (Part I) by checking if these damage laws are able to give accurate fracture predictions. The comparison between experimental data obtained at 600°C and numerical simulations is made for five different conditions with three different simulated HAZ and on two specimen geometries including CT specimen tested under stress relaxation conditions and sharply notched thin wall tubular specimens tested in torsion (Mode II loading). Both of these specimens have not been used for the identification of the damage model coefficients.
Keywords: austenitic stainless steel, AISI 321, creep, stress-relaxation, intergranular damage, Mode II loading, modelling, finite element method.
Microstructural features of mechanical failure in thermal barrier coating systems under static loadings†
S.Takahashi1, M.Yoshiba1 and Y. Harada2
1Department of Mechanical Engineering, Graduate School of Engineering, Tokyo Metropolitan University, Tokyo 192-0397, Japan
2Thermal Spraying Technology R & D Laboratory, Tocalo Co. Ltd, Kobe 658-0013, Japan
In order to clarify qualitatively and quantitatively the failure mechanism of plasma-sprayed thermal barrier coating (TBC) systems from the microstructural viewpoint, in situ observation of the mechanical failure behavior was conducted for TBC systems under the static loadings at ambient temperature; as the most fundamental aspect, by means of an optical microscopy. Several kinds of TBC systems were prepared by using different sorts of ceramic coating materials. Mechanical tensile loading or compressive loading was gradually applied to the plate shape of TBC specimen using a four-point bending test methodology. It was found that the tensile failure behavior of TBC systems depends strongly on the top-coat microstructures as well as heat treatment after the plasma spraying. The compressive failures were also found rather incidental and depended on the strength of top-coat at the interfacial region. Among different TBC systems, those with the finely segmented top-coat exhibited a good spalling resistance.
Keywords: thermal barrier coating system, mechanical failure, failure analysis, ceramic coating
Degradation of gas turbine blade materials in integrated coal-gasification combined cycle plant†
K.Wada1, L.Yan1, M.Takahashi1, K.Takaishi1 and T. Furukawa2
1Toshiba Corporation, Power & Industrial Systems Research and Development Center, 2-4 Suehiro-cho, Tsurumi-ku, Yokohama 230-0045, Japan
2Toshiba Corporation, Thermal Power Plant Division, 36-5, Tsurumichuo 4-chome, Tsurumi-ku, Yokohama 230-0051, Japan
Interest in the integrated coal-gasification combined cycle plant (IGCC) has increased because of its high efficiency. However, degradation of gas turbine component materials has not been studied so far. Therefore, the purpose of this study is to investigate the corrosion behaviors of these materials in an actual coal gas combustion environment. Our test facility was constructed near the coal-gasification test facility (CGT) at the Aioi works of Ishikawajima-Harima Heavy Industries Co., Ltd. Several superalloy (René80H and FSX414) and coating systems (CoCrAlY, CoNiCrAlY and TBC) were exposed in the coal gas combustion environment at 1123, 1223 and 1323 K up to 350 h. After these tests, the weight change of each specimen was measured. Morphologies of products and element distributions at their surfaces were observed with SEM and EPMA. It is concluded that not only high- temperature oxidation and sulfidation but also ash deposition has a remarkable influence on the degradation of gas turbine blade materials.
Keywords: integrated gas turbine blade materials