The Experts below are selected from a list of 84 Experts worldwide ranked by ideXlab platform
Gerhard Poll - One of the best experts on this subject based on the ideXlab platform.
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starvation and Relubrication mechanisms in grease lubricated oscillating bearings
Tribology International, 2022Co-Authors: Sebastian Wandel, Norbert Bader, Fabian Schwack, Jakob Glodowski, Bela Lehnhardt, Gerhard PollAbstract:Abstract Oscillating rolling element bearings can be found in various industrial applications. A popular example are the rotor blade bearings of wind turbines, which allow the rotor blades to turn around their longitudinal axis. These bearings are predominantly grease lubricated and expected to be running in a state of low velocity starvation. Elastohydrodynamic Lubrication (EHL) films in starved contacts are susceptible to the conditions of inlet lubricant supply. If the bearings are running under moderate starvation no early failure due to wear is expected. If the inlet conditions increasingly lead to a drying out of the contact, resulting in a heavily starved contact, metal-to-metal contact can lead to false brinelling and subsequent bearing failure. Using two different greases bearing experiments are used to gain an understanding of the mechanism of wear initiation. It is found that starvation seems to be a major contribution to wear appearing in the investigated operating conditions (2°-45° osc. angle, 0,2–5 Hz osc. frequency). The degree of starvation in the contact depends on the balance between lubricant displacement by the rolling element and the replenishment of lubricant during operation. The following conclusions could be drawn: Replenishment is highly dependent on the operating conditions such as the oscillation frequency, the oscillation amplitude, and lubricant rheology. For small oscillation angles the ability of the grease to release base oil with high mobility into the contact seams to be essential. For greater oscillation angles, after crossing a limiting angle, a secondary replenishment mechanism seems to become active. This prevents early wear initiation. At critical operating parameters, which mainly include high oscillation frequencies and medium oscillation angles, severe wear is initiated after just a few hundred oscillation cycles. By modifying an existing starvation factor the influences could be visualised and compared to the experimental results.
José M. Torralba - One of the best experts on this subject based on the ideXlab platform.
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microstructural and mechanical characterisation of 7075 aluminium alloy consolidated from a premixed powder by cold compaction and hot extrusion
Materials & Design, 2014Co-Authors: M Jabbari A Taleghani, E Ruiz M Navas, José M. TorralbaAbstract:Abstract The present work concerns the processing of 7075 Al alloy by cold compaction and hot extrusion of a premixed powder. To this end, a premixed Al–Zn–Mg–Cu powder, Alumix 431D, was uniaxially cold pressed at 600 MPa into cylindrical compacts 25 mm in diameter and 15 mm thick. Subsequently, selected green compacts were subjected to either a delubrication or presintering heat treatment. Extrusion of the powder compacts was performed at 425 °C using an extrusion ratio of 25:1. No porosity was present in the microstructures of the extruded alloys. Heat treatment prior to extrusion had a great effect on the degree of alloy development in powder compacts and, as a direct consequence, remarkably affected the extrusion process and the as-extruded microstructures and mechanical properties of the processed materials. Hot extrusion caused banded structures for the alloys consolidated from the green and delubricated powder compacts. The alloy extruded from the presintered powder compact showed a fine, recrystallized microstructure which resulted in a superior combination of mechanical properties for the consolidated material.
Sebastian Wandel - One of the best experts on this subject based on the ideXlab platform.
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starvation and Relubrication mechanisms in grease lubricated oscillating bearings
Tribology International, 2022Co-Authors: Sebastian Wandel, Norbert Bader, Fabian Schwack, Jakob Glodowski, Bela Lehnhardt, Gerhard PollAbstract:Abstract Oscillating rolling element bearings can be found in various industrial applications. A popular example are the rotor blade bearings of wind turbines, which allow the rotor blades to turn around their longitudinal axis. These bearings are predominantly grease lubricated and expected to be running in a state of low velocity starvation. Elastohydrodynamic Lubrication (EHL) films in starved contacts are susceptible to the conditions of inlet lubricant supply. If the bearings are running under moderate starvation no early failure due to wear is expected. If the inlet conditions increasingly lead to a drying out of the contact, resulting in a heavily starved contact, metal-to-metal contact can lead to false brinelling and subsequent bearing failure. Using two different greases bearing experiments are used to gain an understanding of the mechanism of wear initiation. It is found that starvation seems to be a major contribution to wear appearing in the investigated operating conditions (2°-45° osc. angle, 0,2–5 Hz osc. frequency). The degree of starvation in the contact depends on the balance between lubricant displacement by the rolling element and the replenishment of lubricant during operation. The following conclusions could be drawn: Replenishment is highly dependent on the operating conditions such as the oscillation frequency, the oscillation amplitude, and lubricant rheology. For small oscillation angles the ability of the grease to release base oil with high mobility into the contact seams to be essential. For greater oscillation angles, after crossing a limiting angle, a secondary replenishment mechanism seems to become active. This prevents early wear initiation. At critical operating parameters, which mainly include high oscillation frequencies and medium oscillation angles, severe wear is initiated after just a few hundred oscillation cycles. By modifying an existing starvation factor the influences could be visualised and compared to the experimental results.
Seshendra Karamchedu - One of the best experts on this subject based on the ideXlab platform.
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Critical Aspects of Delubrication and Sintering of Chromium-alloyed Powder Metallurgy Steels
2017Co-Authors: Seshendra KaramcheduAbstract:The cost efficiency and performance of Powder Metallurgy (PM) steels can be improved by replacing conventionally used alloying elements such as copper and nickel with chromium. Utilizing chromium imposes a challenge in terms of processing due to its effect on powder compressibility and high oxygen affinity, the latter of which, to a certain extent, is dealt with by introducing chromium in prealloyed form that decreases its activity. Therefore, each stage in the PM processing route has to be reconsidered to satisfy the thermodynamic and kinetic requirements to appropriately sinter components based on chromium alloyed steel powder for manufacturing high-performance PM parts. For conventional PM steels, consolidation is typically achieved by compaction, where the final dimensions of the part are achieved, followed by sintering, where metallurgical bonding between the powder particles takes place. To facilitate compaction, powder is mixed with lubricant, which also improves the tool life; but the lubricant has to be removed prior to sintering. Among the problems encountered during sintering, those concerning delubrication are frequent but are difficult to detect. In the present study, a reliable approach for in-situ monitoring of the delubrication of PM steel compacts is presented. The method is based on continuous monitoring of the process atmosphere using sensors commonly used in the industry (CO2 and O2). Using this method, the effect of various process parameters on lubricant removal has been investigated and the changes occurring in the surface chemistry of the compacts during delubrication and their impact on sintering have been assessed. Based on these studies, delubrication at 450 °C in dry N2 with dynamic gas flow conditions around the sample and a low heating rate were proposed to be optimum for the delubrication of chromium-alloyed PM steels. Sintering of water-atomized chromium-alloyed powder compacts is typically performed in hydrogen-containing atmospheres with nitrogen as the carrier gas. However, attaining good carbon control in these atmospheres is challenging. Hence, the effect of different active constituents in the atmosphere, such as hydrogen, carbon monoxide and propane, on the reduction-oxidation and carburization-decarburization processes during the sintering of chromium-alloyed PM steels was investigated. It was shown that concentration of carbon monoxide above 1 vol.% in the sintering atmosphere results in significant oxidation of the compacts, whereas lower concentrations contribute to counteract the carbon loss and provide for the possibility of carburization during a continuous sintering process. Furthermore, lean atmospheres containing carbon monoxide, hydrogen and hydrocarbons as active constituents, with their total concentration not exceeding 5 vol. %, were shown to be potential candidates for sintering of chromium-alloyed PM steels, since they provide carburization while confining oxidation to acceptable levels.
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delubrication of chromium prealloyed powder metallurgy steels
2013Co-Authors: Seshendra KaramcheduAbstract:Consolidation of steel components produced by means of powder metallurgy (PM) is achieved typically through compaction and sintering. During consolidation, final dimensions of the part are already achieved after compaction, while sintering imparts strength to the components. Prior to the compaction stage, lubricants are added to the metal powder to reduce the inter-particle and die-wall friction during pressing and hence improve powder compressibility and ejection of the component from the compaction tool. These lubricants have to be safely removed after compaction since they are source of oxygen and carbon that can negatively affect further sintering process as well as final performance of the sintered components. Lubricants commonly used for PM steels are based on ethylene bis-stearamide (EBS) and their removal is achieved through thermolysis in the initial zone of the sintering furnace. With increasing demands on the achievable densities and performance of PM components, lubricants and possible risks of improper delubrication have received interest. Among the problems encountered during sintering, those concerning delubrication are frequent but difficult to detect. Hence previous studies have aimed at improving the efficiency of lubricant removal and predicting the same. Delubrication being a complex process still poses problems in practice and its proper control requires a system which monitors the delubrication sequence and can be incorporated into a closed loop control. In the present work, a reliable approach for in-situ monitoring of delubrication of PM steel compacts is presented. The method is based on continuous monitoring of the processing atmosphere using sensors commonly used in the industry (CO2 and O2). It was demonstrated that the initial stages of lubricant removal can be monitored using the oxygen sensor based on zirconia ceramics and the later stages can be detected utilizing CO2 sensor based on infrared cell technology. Based on the established methodology, a systematic study on the effect of various process parameters on lubricant removal in the processing of chromium prealloyed PM steels has been conducted. Effect of temperature, holding time, heating rate, process gas composition (inert, reducing and oxygen containing atmospheres) and purity (wet and dry gases), flow rate and graphite addition was evaluated. Additionally, change in surface chemistry of the base powder occurring during delubrication and its effect on sintering have been studied using X-ray photoelectron spectroscopy and scanning electron microscopy combined with the energy dispersive X-ray analysis. Based on the experimental results it is recommended to perform delubrication at ~ 450 °C applying low heating rates in dry-nitrogen atmosphere with flow sufficient enough to provide dynamic gas conditions around the component surface. This is especially important in the case of powder prealloyed with oxygen sensitive elements like chromium.
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delubrication of pm components based on cr prealloyed steel powder chances and risks
Proc. of EURO PM2011 Congress & Exhibition 9-12 October 2011 Barcelona Spain, 2011Co-Authors: Eduard Hryha, Seshendra Karamchedu, Lars NyborgAbstract:Utilization of prealloyed powder grades for production of high-performance PM structural parts require reconsidering of specific stages of sintering process like debinding. Modern lubricants are complex organic compounds, decompositions of which leads to production of a number of species – from complex hydrocarbons to carbon oxides. Composition and amount of decomposition products are strongly determined by the temperature profile and processing atmosphere composition/purity. Uncontrolled delubrication process, especially in the case of the PM components containing elements with high-sensitivity to oxygen, can lead to considerable oxidation. Therefore, in-situ monitoring of the processing parameters during delubrication is presented utilizing commonly used in industry sensors (dew-point, CO2 and O2). Delubrication was monitored in a number of inert and reducing gases of the different purity. Based on the specimens analysis and gas monitoring profiles it is recommended to perform delubrication at the temperatures below 500°C in dry inert atmospheres in the case of ethylenbisstearamide-based lubricants.
M Jabbari A Taleghani - One of the best experts on this subject based on the ideXlab platform.
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microstructural and mechanical characterisation of 7075 aluminium alloy consolidated from a premixed powder by cold compaction and hot extrusion
Materials & Design, 2014Co-Authors: M Jabbari A Taleghani, E Ruiz M Navas, José M. TorralbaAbstract:Abstract The present work concerns the processing of 7075 Al alloy by cold compaction and hot extrusion of a premixed powder. To this end, a premixed Al–Zn–Mg–Cu powder, Alumix 431D, was uniaxially cold pressed at 600 MPa into cylindrical compacts 25 mm in diameter and 15 mm thick. Subsequently, selected green compacts were subjected to either a delubrication or presintering heat treatment. Extrusion of the powder compacts was performed at 425 °C using an extrusion ratio of 25:1. No porosity was present in the microstructures of the extruded alloys. Heat treatment prior to extrusion had a great effect on the degree of alloy development in powder compacts and, as a direct consequence, remarkably affected the extrusion process and the as-extruded microstructures and mechanical properties of the processed materials. Hot extrusion caused banded structures for the alloys consolidated from the green and delubricated powder compacts. The alloy extruded from the presintered powder compact showed a fine, recrystallized microstructure which resulted in a superior combination of mechanical properties for the consolidated material.