摘要:

Two commercial oxide dispersion strengthened alumina-forming FeCrAl alloys, Inco alloy MA956 and Kanthal alloy APM, were studied in order to look at the effect of reactive elements on their oxidation behaviour. MA956 has a distribution of Y2O3−Al2O3particles, while APM has a ZrO2—AI2O3distribution. Isothermal oxidation at 1000°C and 1200°C showed a reduced oxidation rate for both alloys compared to that of an undoped FeCrAl alloy. In short-term cyclic tests at 1200°C, both alloys exhibited excellent scale adhesion. Using scanning transmission electron microscopy with X-ray energy dispersive spectroscopy, both Y and Zr, respectively, were found to segregate to the oxide grain boundaries and the metal-scale interface after oxidation at 1000°C and 1200°C. These experimental observations are discussed with regard to a new theory to explain the reactive element effect.

ISSN:0960-3409    

DOI:10.1080/09603409.1995.11689496

出版商:Taylor&Francis    

年代:1995

数据来源: Taylor

摘要:

This paper focuses on the reactive chemical vapour deposition of composite B4C-SiC coatings onto porous graphite substrates from a BCl3−SiCl4−H2vapour phase. The mixed layer seems to exhibit promising properties such as oxidation resistance. However, its preparation as a thin film from the vapour is very difficult owing to the complexity of the system. The experimental parameters such as temperature, number of moles H2, and the amounts of BCl3and SiCl4in the reactive vapour phase are systematically discussed. The results are compared with a thermodynamic approach. Also, this paper reports investigations on boron and boron silicide coatings. Scanning electron microscopy, X-ray photoelectron spectroscopy and electron microprobe analysis were used to characterize the various coatings. The compositions, microstructures and properties such as oxidation behaviour of the coatings were also studied. Optimal experimental parameters of the process were reached in order to achieve a dense, adherent and continuous mixed B4C-SiC layer on bulk graphite substrates with a view to coating further carbon fibres and protecting them from oxidation in air at temperatures less than 1000°C.

ISSN:0960-3409    

DOI:10.1080/09603409.1995.11689497

出版商:Taylor&Francis    

年代:1995

数据来源: Taylor

摘要:

The present paper describes part of a larger study which investigated the effects of accumulated service strain (ε) on the reverse temper embrittlement (RTE) response of a series of large HP and IP turbine CrMoV steel bolts which had been in service for 122 000 h at elevated temperatures or around 490°C. It is shown that accumulated service strains exert significant effects on toughness losses in the CrMoV bolting steel, especially at low levels of strain, and that such effects can be adequately described by the expressionsCn = 7.6×10−2ε−0.8andFATT(K) = 418–13.28ε−0.8whereCn is the normalized Charpy energy andFATTis the Charpy fracture appearance transition temperature. It is suggested that the extent of the RTE response in low alloy steels can be separated into a pure RTE component and a strain-induced or synergistic RTE component. Also, from limited Auger electron spectroscopy data it is suggested that the latter may not be the result of strain-induced enhanced grain boundary phosphorus segregation. Finally, when the data are considered in terms of localized accumulated strain at the bolt thread root, it is shown that brittle failure of the bolts is unlikely, even at strain levels of 1.5%, inasmuch as although toughness levels are low, no macrodefects exist since the creep damage is in the form of isolated cavities of around 1μm in size.

ISSN:0960-3409    

DOI:10.1080/09603409.1995.11689498

出版商:Taylor&Francis    

年代:1995

数据来源: Taylor

摘要:

The complex thermal cycles associated with fabrication of weldments in 1.25Cr0.5Mo steel have been documented. For the weaved welding allowed by the specified procedure, base material locations adjacent to the weld profile have been subjected to up to seven excursions above 800°C during fabrication. The multiplicity of these thermal cycles resulted in a large degree of refinement within the weld metal and heat-affected zone. Creep testing has shown that the weldments produced are highly susceptible to type IV cracking, with damage in the form of cavities and microcracks developing below the specimen surface. This observation, combined with the fact that physical evidence of distress occurs relatively late in life, indicates that traditional methods of structural integrity assessment may be inadequate.

ISSN:0960-3409    

DOI:10.1080/09603409.1995.11689499

出版商:Taylor&Francis    

年代:1995

数据来源: Taylor

摘要:

Isothermal uniaxial cyclic loading experiments have been conducted on specimens of the nickel-based superalloy IN100 at selected temperatures in the range 600–1000°C. Power-law stress—strain relations based on plastic strain (Ramberg-Osgood), total strain and their bilinear equivalents have been used to describe the behaviour. In addition, uniaxial thermomechanical fatigue (TMF) tests have been performed on tubular specimens of the material over the temperature range 650–1050°C. The aim of these tests was to model the response during thermal shock experiments involving rapid heating and cooling cycles between 200°C and 1100°C. Some transient tests have also been carried out on blade-shaped single-wedge specimens of IN 100 to simulate the type of thermal fatigue (TF) cycle experienced by turbine blades during service. A finite element sensitivity analysis has been undertaken to predict the results of both the TMF and TF tests using the two different bilinear descriptions of the isothermal cyclic stress-strain response of IN100. Good agreement between the calculations and the TMF experimental results is obtained whichever bilinear relation is chosen.

ISSN:0960-3409    

DOI:10.1080/09603409.1995.11689500

出版商:Taylor&Francis    

年代:1995

数据来源: Taylor

摘要:

The influence of small concentrations of SO2, HCl or Na2SO4vapour in air on the corrosion of the uncoated nickel-based superalloys IN 792 DS + Hf, CMSX-6 and MA 760 ODS and the coatings RT 22 and LCO 22 has been investigated at 1000 and 1100°C. A level of 1% SO2in air somewhat increased the oxidation of IN 792 DS + Hf and favoured the scale spallation of CMSX-6 at 1000°C. Some precipitates of Ti sulphide were found in the subsurface zones of both alloys. No influence was observed at 1100°C. The oxidation of MA 760 ODS and the two coatings was not affected by SO2at 1000 and 1100°C. The addition of 100ppm HCl to air favoured the spallation of the scales of IN 792 DS + Hf and CMSX-6 at 1000°C. Again, no influence could be observed with these alloys at 1100°C nor with MA 760 ODS or the coatings at either of these temperatures. Contrary to expectations, no synergistic effect was found in air with 1% SO2and 100ppm HCl, but SO2reduced the negative effect of HCl. Catastrophic corrosion occurred with IN 792 DS + Hf and CMSX-6 in the presence of Na2SO4vapour in air at 1000 and 1100°C after an incubation period of more than 100 h. The mechanism of this rapid corrosion could not be clarified and it is still an open question whether the corrosion is caused by acidic fluxing of a liquid Na2SO4−MoO3film or by the reaction between Na2SO4vapour and the scale, thus altering the transport properties of the oxides. MA 760 ODS and the coatings were not tested. Constant extension rate tests with IN 792 DS + Hf and CMSX-6 at 1000°C did not reveal any influence of 100 ppm HCl on the mechanical properties at strain rates of 1×10−6and 3×10−8S−1.

ISSN:0960-3409    

DOI:10.1080/09603409.1995.11689501

出版商:Taylor&Francis    

年代:1995

数据来源: Taylor