A study on tensile and fatigue properties of aged NAS 254N stainless steel at elevated temperatures

The effects of aging on tensile properties and fatigue crack growth behaviors of NAS 254N stainless steel was studied. Yield strength and ultimate tensile strength of the aged specimens were almost the same as the as-received (as-rec.). The fracture strain, however, was decreased significantly by the aging, and the fracture surface of the aged at room temperature (RT) test was intergranular. As test temperature increased, yield strength, ultimate tensile strength and elongation decreased. And a type of serration was observed at 550-650°C As strain rate decreased, yield strength and ultimate tensile strength decreased, but elongation increased. It was observed that tensile strength and strain had a sudden change at one point. And this critical temperatureT _cr was 550°C. The effect of aging time on the tensile strength and strain was also investigated. Tensile strength and strain decreased significantly beyond 100hrs. Fatigue crack growth rate at RT was enhanced by the aging at high stress intensity factor range. This is due to the occurrence of the intergranular fracture in the aged specimen. At 650°C, the fatigue crack growth behavior was almost the same without intergranular fracture.     

Thermography in incremental forming processes at elevated temperatures

Temperature measurement is essential for several forming processes at elevated temperatures. It serves to determine and control the workpiece temperature. Thermography as a non-contact-based technology offers the possibility to capture thermograms of complete workpieces without any time-offset. However, the application of thermography requires the knowledge of the fundamentals of radiation thermometry, in particular the emissivity. This paper presents the results of the application of thermography in incremental sheet forming (ISF) with Joule heating and radial–axial ring rolling as a bulk forming process. Using thermography for the determination of the temperature of the forming zone allows for a real-time closed loop control in ISF with Joule heating. Additionally, the results of the temperature measurement of the surface temperature of radial–axial rolled rings are presented, which can be used as a starting point to make a forecast of the rings’ dimensions in cold state.     

Impact adhesion of iron at elevated temperatures

Small iron spheres were made to impinge normally on iron plates at several different temperatures in gases with various oxygen activities. The spheres welded permanently to the plate when the impact occurred under purified hydrogen and at low impact velocities but rebounded at impact velocities larger than a characteristic value which increased with an increase in either the temperature or the surface cleanliness of the sphere and plate. Permanent adhesion can be prevented, even at low impact velocities, by coating the iron with very thin films of wüstite or with thinner films of Al₂O₃. The linear increase in impact area with impact velocity can be understood from the theory of the hardness indenter, using the appropriate high strain rate flow stress. It is inferred that a bond always forms between colliding bodies but that permanent adhesion occurs only when the elastic energy stored during impact is less than the energy required to break the bond by crack propagation. The stored elastic energy increases with impingement velocity by more than does the energy required for the propagation of a crack to separate the bodies completely. Consequently, there exists a critical velocity beyond which permanent adhesion does not occur.     

An evaluation of antioxidants for vegetable oils at elevated temperatures

The antioxidant efficacy of various oxidation inhibitors in low erucic acid rapeseed oil has been studied in a screening test at 130°C by an induction period method. Hindered monophenols, sulphides, phosphites, aromatic amines and zinc dithiophosphates yielded no or only marginal stabilising effects. Remarkably increased oxidation stabilities were observed with certain hindered bisphenols, polyhydroxybenzenes, zinc and bismuth dithiocarbamates. In high oleic sunflower oil with a lesser portion of multiple unsaturation, these additives induced relatively longer induction periods. Mixtures of zinc dithiocarbamates and 4,4′-methylenebis-(2, 6-di-tert-butylphenol) or octylated di-phenylamine, as well as compositions of all three compounds, exhibited synergistic effects. Under the conditions employed these additives were distinctly superior to commercially recommended formulations. A mechanistic concept of the antioxidant action of zinc dithiocarbamate is briefly discussed.     

Tribological characteristics of silicon nitride at elevated temperatures

A sliding friction-and-wear test for silicon nitride (Si₃N₄) was conducted using a ball-on-disk specimen configuration. The material used in this study was HIPed silicon nitride. The tests were carried out from room temperature to 1000°C using self-mated silicon nitride couples in laboratory air. The worn surfaces were observed by SEM and the debris particles from the worn surfaces were analyzed for oxidation by XPS. The normal load was found to have a more significant influence on the friction coefficient of the silicon nitride than an elevated temperature. The specific wear rate was found to decrease along with the sliding distance. The specific wear rate at 29.4 N and 1000°C was 292 times larger than that at room temperature. The main wear mechanism from room temperature to 750°C was caused by brittle fracture, whereas from 750 to 1000°C the wear mechanism was mainly influenced by the oxidation of silicon nitride due to the increased temperature. The oxidation of silicon nitride at a high temperature was a significant factor in the wear increase.     

Sample cell for EXAFS measurements on molten materials at elevated temperatures

We describe the design, fabrication, and utilization of a simple sample cell for extended x-ray absorption fine structure (EXAFS) measurements at elevated temperature. The cell is rigid, inert, easy to fabricate, cheap and effective in maintaining the critical specimen dimensions of both solid and liquid samples up to 1000 degrees C.     

Incremental forming of Mg alloy sheet at elevated temperatures

In the present study, incremental forming of Mg alloy sheet at elevated temperatures was attempted with local heating apparatus. The device is composed of several halogen lamps and designed to move with forming tool for local heating in deformation zone. In order to investigate the influences of process parameters to incremental formability of AZ31 alloy sheet, a series of incremental forming tests of AZ31 for cone and pyramid type of simple models were carried out under various process conditions. Experiments were performed under various temperatures, feeding depth per cycle and inclination angles and the results were analysed.     

The tribological characteristics of TiCN coating at elevated temperatures

The tribological behaviour of TiCN coating prepared by unbalanced magnetron sputtering is studied in this work. The substrates made from austenitic steel were coated by TiCN coatings during one deposition. The measurements were provided by high temperature tribometer (pin-on-disc, CSM Instruments) allowing measuring the dependency of friction coefficient on cycles (sliding distance) up to 500 °C. The evolution of the friction coefficient with the cycles was measured under different conditions, such as temperature or sliding speed and the wear rate of the ball and coating were evaluated. The 100Cr6 balls and the Si₃N₄ ceramic balls were used as counter-parts. The former were used at temperatures up to 200 °C, the latter up to 500 °C. The wear tracks were examined by optical methods and SEM. The surface oxidation at elevated temperatures and profile elements composition of the wear track were also measured.The experiments have shown considerable dependency of TiCN tribological parameters on temperature. Rise in temperature increased both friction coefficient and the wear rate of the coating in case of 100Cr6 balls. The main wear mechanism was a mild wear at temperatures up to 200 °C; fracture and delamination were dominating wear mechanisms at temperatures from 300 to 500 °C.     

A device for testing a fast nanographite photodetector at high temperatures

The results of tests of a fast photodetector based on a nanographite film grown on a silicon substrate are presented. The temperature dependence of the optoelectric signal from the photodetector in the range 300–800 K has been studied under the vacuum conditions. It has been shown experimentally that, when the temperature increases from 300 to 625 K, the photodetector’s sensitivity decreases by approximately 30%. When the temperature increases to higher values, the optoelectric-signal amplitude drops linearly and is halved at T = 740 K as compared to room temperature. Extrapolation of the experimental data shows that the optoelectric response of the photodetector disappears at T > 1000 K.     

Wear performance of tribologically modified thermoplastic composites at elevated temperatures

The use of injection moulded thermoplastic composites is increasing, but without fillers, reinforcements, or lubricants, they are limited by their thermal behaviour. This study looks at four polymers — polyamide 46, polyphthalamide, polyetherimide, and polyphenylenesulphide — containing first glass fibres as reinforcement, and a second version with added PTFE. Two types of wear test were undertaken, sliding wear (pin-on-disc and block-on-ring), and reciprocally fretting wear. The test materials showed different wear rates both under different tests and at various temperatures.     

The effect of thermal stressing on perfluoropolyalkylethers at elevated temperatures

Fultz et al. have reported that the thermo‐oxidative properties of linear PFPAEs can be improved by stressing the fluid at elevated temperature (371°C) in the presence of air. A study of M‐50 steel coupons exposed to unstressed and stressed linear PFPAE fluids at 260 °C and 330 °C each reveal complex surface layers. For the coupon exposed to the unstressed fluid at 260 °C, a subsurface layer is observed between the iron oxide and iron substrate that has been characterized as being composed of FeF₂. In contrast, the coupon exposed to the stressed fluid has a marked increase in the iron oxide thickness ∼2–3 times) when compared to the unstressed sample and shows no evidence of a buried fluorine‐containing layer. An increase in temperature (330 °C) in the stressed fluid O–C test was required to form a subsurface FeF₂ layer. It is proposed that the elimination of the fluorine layer found on the M‐50 substrate increases the upper temperature limit found from the oxidation–corrosion studies. The increase in the oxide layer thickness implies that the FeF₂ layer found in the unstressed sample acts like a diffusion barrier which inhibits the outward movement of Fe⁰ and the decreased rate of iron oxide growth. This revised version was published online in June 2006 with corrections to the Cover Date.     

Microscopic evaluation of vesicles shed by erythrocytes at elevated temperatures

The images of human erythrocytes and vesicles were analyzed by a light microscopy system with spatial resolution of better than 90 nm. The samples were observed in an aqueous environment and required no freezing, dehydration, staining, shadowing, marking, or any other manipulation. Temperature elevation resulted in significant concentration increase of structurally transformed erythrocytes (echinocytes) and vesicles in the blood. The process of vesicle separation from spiculated erythrocytes was video recorded in real time. At a temperature of 37°C, mean vesicle concentrations and diameters were found to be 1.50 ± 0.35 × 10⁶ vesicles per microliter and 0.365 ± 0.065 μm, respectively. The vesicle concentration increased approximately threefold as the temperature increased from 37 to 40°C. It was estimated that 80% of all vesicles found in the blood are smaller than 0.4 μm. Accurate account of vesicle numbers and dimensions suggest that 86% of the lost erythrocyte material is lost not by vesiculation but by another, as yet, unknown mechanism. Microsc. Res. Tech. 76:1163–1170, 2013. © 2013 Wiley Periodicals, Inc.     

The wear performance of yttrium-modified Stellite 712 at elevated temperatures

Cobalt-based alloys are often used for bearing applications, especially at elevated temperatures. One of the newly developed 700 series cobalt-based alloys, Stellite 712, has been demonstrated to possess high resistance to wear and corrosion in aggressive environments. Continuous efforts have been made to further improve this alloy for enhanced resistance to high-temperature wear involving oxidation. Recent studies showed that the improvement of the oxide scale on Co-base alloys by alloying with yttrium was an effective way to diminish wear of the alloys at elevated temperatures.In this work, sliding wear performances of yttrium-free and yttrium-containing Stellite 712 samples at elevated temperatures were evaluated. The mechanism responsible for changes in its wear performance was investigated by studying the effects of alloying yttrium on microstructure and mechanical properties of the bulk alloy and its oxide scale, employing various experimental methods including micro- and nano-mechanical probing, XRD, SEM-EDS, AFM and high-temperature pin-on-disc wear testing. The research demonstrated that alloying a small amount of yttrium (e.g. less than 1%Y) rendered the oxide scale on Stellite 712 stronger with higher adherence to the substrate, which was largely beneficial to the wear performance of the alloy at elevated temperatures. Mechanisms involved are discussed in this article.     

Laser powered heating stage in a scanning electron microscope for microstructural investigations at elevated temperatures

A laser powered heating stage designed for application in high vacuum environment of a scanning electron microscope (SEM) is presented. It was developed to observe and characterize microstructural changes in crystalline materials at elevated temperatures up to 1000 degrees C. The approach utilizes the power output of a commercial infrared diode laser in order to heat up specimens without interference with the electronic system of the SEM. The heating stage can be used in combination with any standard characterization technique applicable for SEMs--electron backscatter diffraction, orientation contrast imaging, x-ray energy dispersive spectrometry, etc. The results of test measurements are presented.     

Test procedure for rapid assessment of frictional properties of engine oils at elevated temperatures

A fifteen-minute test sequence has been developed for screening and ranking base oils and friction modifiers. The tests are run on a high-frequency friction machine developed at the authors' company and based on the Mills-Cameron design. The procedure can distinguish clearly and repeatably between the behaviour of different friction modifiers and with a degree of discrimination which is better than has been achieved in engine mechanical loss tests     

The frictional behaviour of iron and iron-chromium alloys at elevated temperatures

Friction experiments have been carried out in an ultrahigh vacuum system at 20–850 °C to investigate the tribological properties during like on like sliding of iron and iron-chromium alloys containing up to 40% Cr. Most of the tests were performed in a background pressure of 10^?8 Pa, although others were carried out in 10^?4 and 10^?5 Pa oxygen. High temperature microhardness tests were also made on the materials. The results indicate that hardness and coefficient of friction values are not closely related, but that the coefficient of friction may be correlated with the ductility of the metal. The only alloy not to exhibit seizure during sliding in 10^?8 Pa at high temperature was Fe-40% Cr.     

Mechanisms of sliding wear of metals and alloys at elevated temperatures

Sliding wear at elevated temperature is an important material removal mechanism in large number of engineering applications such as metal forming operation, gas turbine engines, etc. The material loss during sliding at elevated temperature is governed by the antagonistic effect of wear process and oxidation. The objective of the present work is to give an outline of the current status and future trends of wear at elevated temperature of selected metallic materials. Starting with a brief overview of present level of understanding of the elevated temperature wear of various metals and alloys, the initial parts of the paper describes various important development in recent years on elevated temperature sliding wear. The salient features which have helped us to gain in depth scientific knowledge of elevated temperature wear are discussed in the light of recent developments. The overview is further substantiated by detailed study and observation in recent times in particular, the work done at the Vienna University of Technology (Institute of Microtechnique and Precision Engineering) and at the Austrian Center of Competence for Tribology. Specific examples from the recent literatures are described to exemplify the mechanisms of formation of various types of layers during high-temperature wear. Some thoughts on the future directions for research are also outlined.     

Ionic liquids as lubricants for steel–aluminum contacts at low and elevated temperatures

Room temperature ionic liquids (ILs) are high performance fluids with a wide thermal stability range. In this work we present the first study of ILs as lubricants under a wide range of temperature conditions (−30, 100, and 200 °C). The tribological performance of the imidazolium ionic liquids 1-hexyl, 3-methyl (L106) and 1-octyl, 3-methyl (L108) imidazolium tetrafluoroborates have been compared with that of a mineral oil (MO) and the synthetic ester propylene glycol dioleate (PGDO) in pin-on-disk aluminum–steel contacts. ILs show lower friction and wear values than conventional oils at all temperatures. The lubricating performance depends on thermal stability, polarity of the molecules, their ability to form ordered adsorbed layers and the tribocorrosion processes, which take place at the interface. While the conventional oils MO and PGDO fail above 150 °C due to thermal decomposition, the longer alkyl chain L108 provides an effective surface separation at all temperatures. L108 only shows friction and wear increments at −30 °C in the presence of water, due to severe abrasion. While the more polar, shorter alkyl chain L106 shows severe wear at 200 °C due to aluminum fluoride wear debris formation by tribocorrosion reactions. The time for tribocorrosion to take place has been determined from friction increments and wear debris generation. Wear mechanisms are discussed on the basis of SEM, EDS, and XPS results.     

Sliding wear performance of reinforced A413 alloy at elevated temperatures

Dry sliding wear behaviour of Al–Si A413 alloy with and without intermetallics has been studied at ambient and elevated temperatures. It is observed that as the temperature is increased, the wear rate decreased. The reduction in wear rate is mainly attributed to the formation of glazing layers at elevated temperature and is observed in both A413 alloy with and without intermetallics. The wear due to oxidation is predominant during high temperature sliding.     

Wear properties of dry bearing liners at ambient and elevated temperatures

Dry bearings have been used increasingly over the last few years to replace conventional grease-lubricated metallic bearings in aerospace applications, particularly where maintenance is difficult or in hostile environments where fluid lubrication is impossible.The friction and wear properties of a wide range of experimental and commercial dry bearing liners have been examined in conditions of reciprocating line contact on a modified pin-on-ring apparatus at temperatures up to 150 °C. The development of the apparatus is briefly described. Accelerated wear data are provided in a few hours as opposed to the several months required for full-scale bearing test rigs. The specific wear rates differed appreciably under ambient conditions, varying by approximately two orders of magnitude. The application of additional heat generally increased the wear rate although some materials had an optimum performance at temperatures above ambient. In general liners containing synthetic reinforcing fibres, e.g. polyamide or polyester, appear to exhibit superior wear properties to those containing glass fibres. Comparisons are drawn between the data obtained from these accelerated tests and the results of those obtained on journal bearings.