| 1. |
- Vattur Sundaram, Maheswaran, 1987, et al.
(författare)
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Vacuum sintering of chromium alloyed powder metallurgy steels
- 2019
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Ingår i: Metal Powder Report. - : Mark Allen Group. - 0026-0657 .- 1873-4065. ; 74:5, s. 244-250
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Tidskriftsartikel (refereegranskat)abstract
- Modern developments in vacuum furnace technologies have led to an increased interest in vacuum sintering of powder metallurgy (PM) steel components owing to the robustness, productivity and the possibility to integrate heat-treatment into the process at an attractive cost. In the present study, the potential for vacuum sintering of chromium-alloyed PM steels and the effect of the vacuum level on the final properties of the components are evaluated. The studies were performed on Fe-1.8 wt.% Cr powder and the sintering experiments were performed in a dilatometer. Four different vacuum levels: 10, 1, 10−2 and 10−4 mbar were studied and the effect of density and carbon content were also examined. The results indicate an efficient oxide reduction and process robustness at intermediate vacuum levels whereas high-temperature sintering in high vacuum brings the risk of Cr-sublimation. Furthermore, industrial trails were performed at 10 and 1 mbar vacuum levels and their mechanical properties were evaluated. Vacuum sintering proved to be an attractive alternative to process Cr-alloyed steel powders.
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| 2. |
- Vattur Sundaram, Maheswaran, 1987, et al.
(författare)
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Vacuum Sintering Studies On Chromium Alloyed PM Steels
- 2016
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Ingår i: World Powder Metallurgy 2016 Congress and Exhibition, World PM 2016; Hamburg; Germany; 9 October 2016 through 13 October 2016.
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Konferensbidrag (refereegranskat)abstract
- Recent advancements in vacuum furnace technologies have led to an increased interest in vacuum sintering of Powder Metallurgy (PM) steel components owing to the robustness, productivity and the possibility to integrate heat-treatment into the process at an attractive cost. In the present study the potential for vacuum sintering of chromium-alloyed PM steels and the effect of the vacuum level on the final properties of the components is evaluated. The studies were performed on Fe-1.8 wt.% Cr powder and the sintering experiments were performed in a dilatometer. Four different vacuum levels: 1, 10, 10-2 and 10-4 mbar were studied and the effect of density and carbon content were also examined. The results indicate an efficient oxide reduction and process robustness at intermediate vacuum levels whereas high-temperature sintering in high vacuum brings risk of Cr-sublimation. Still, applying proper conditions, vacuum sintering proved to be an attractive alternative to process Cr-alloyed steel powders. The influence of vacuum level on the sintering efficiency of chromium-alloyed PM steels is studied.
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| 3. |
- Hatami, Sepehr, 1981, et al.
(författare)
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The Effect of Alloying on Sinter-Hardening Response of Astaloy Mo and Its Diffusion Bonded Derivatives
- 2010
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Ingår i: World Powder Metallurgy Congress and Exhibition, World PM 2010; Florence; Italy; 10 October 2010 through 14 October 2010. - 9781899072194 ; 3
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Konferensbidrag (refereegranskat)abstract
- In this study, sinter-hardening response of different steel powders based on the Astaloy Mo grade (prealloyed with 1.5% Mo) is investigated. In order to understand the effect of alloying, Astaloy Mo was compared to its diffusion bonded derivatives; which are benefiting from an additional 2 wt.% copper (i.e. Distaloy DH) and 2 wt.% Ni (i.e. Distaloy DC). Tensile specimens of each alloy containing 0.55-0.7 wt.% admixed graphite were prepared. All specimens were sintered at 1120°C for 30 minutes and cooled at a rate of 2.5°C/s within the range of 800 to 300°C. Subsequently, the sinter-hardened specimens were stress relieved at 200°C for 60 minutes. Using X-Ray diffraction technique the residual stresses of all specimens were analyzed. In order to understand the relation between microstructure and the obtained mechanical properties, tensile testing and metallographic investigations were performed.
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| 4. |
- Hryha, Eduard, 1980, et al.
(författare)
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Control of delubrication process for PM components based on prealloyed steel powders
- 2012
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Ingår i: Proceedings of the 2012 Powder Metallurgy World Congress & Exhibition, Powder Metallurgy Association and Japan Society of Powder and Powder Metallurgy,2013.
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Konferensbidrag (refereegranskat)abstract
- Reliable in-situ monitoring system for the on-line control of the processing parameters during delubrication of PM steel compacts is presented. The monitoring system is build utilizing sensors (dew-point, CO2 and O2) commonly used in industry. Delubrication of PM steel compacts, based on Cr-alloyed steel powder, was monitored in a number of process gases (inert, reducing and oxygen containing atmospheres) of the different purity (wet and dry gases). Effects of temperature, holding time, heating rate, flow rate and component density were studied. Optimum conditions for efficient delubrication with minimum effect on PM compact properties at lowest processing cost were established. Based on the specimens analysis and gas monitoring profiles in the case of ethylenbisstearamide-based lubricants it is recommended to perform delubrication with low heating rate at temperature around 450°C in nitrogen atmosphere in order to facilitate efficient delubrication and avoid surface oxide during sintering. Rather low flow rate is needed – enough to provide even weak dynamic gas conditions around every component surface.
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| 5. |
- Hryha, Eduard, 1980, et al.
(författare)
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Delubrication of PM Components Based on Cr-Prealloyed Steel Powder – Chances and Risks
- 2011
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Ingår i: Proc. of EURO PM2011 Congress & Exhibition, 9-12 October 2011, Barcelona, Spain. - 9781899072231 ; 3, s. 105-110
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Konferensbidrag (refereegranskat)abstract
- 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.
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| 6. |
- Hryha, Eduard, 1980, et al.
(författare)
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Effect of active components of sintering atmosphere on reduction/oxidation processes during sintering of Cr-alloyed PM steels
- 2015
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Ingår i: Journal of the American Ceramic Society. - : Wiley. - 0002-7820 .- 1551-2916. ; 98:11, s. 3561-3568
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Tidskriftsartikel (refereegranskat)abstract
- Water atomised steel particles are covered by heterogeneous surface oxide, formed by thin (~6 to 8 nm) iron oxide layer covering most of the powder surface, and particulate features formed by thermodynamically stable oxides containing e.g. Cr and Mn with surface coverage about 5%. Development of sufficiently strong inter-particle necks requires as minimum full removal of the iron surface oxide layer that can be achieved by gaseous reducing agents as CO and H2 as well as by carbon typically admixed in the form of graphite. The study evaluates the effect of concentration of reactive components of the sintering atmosphere, with special focus on carbon monoxide, on the reduction/oxidation and carburization/decarburization processes taking place during the whole sintering process. Results of the thermal analysis, SEM analysis of oxide characteristics, metallographic and chemical analysis of the sintered compacts were correlated with thermodynamic simulation of the oxide stability in applied sintering atmospheres. High oxidation potential of the CO-containing atmospheres in case of Cr-alloyed PM steels was detected during heating stage until ~1000 °C. Oxidation potential is linearly increasing with the increasing content of the carbon monoxide in the processing atmospheres and rather severe oxidation is observed if CO-content exceeds 1 vol.%.
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| 7. |
- Karamchedu, Seshendra, 1987, et al.
(författare)
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Changes in the surface chemistry of chromium-alloyed powder metallurgical steel during delubrication and their impact on sintering
- 2015
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Ingår i: Journal of Materials Processing Technology. - : Elsevier BV. - 0924-0136. ; 223, s. 171-185
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Tidskriftsartikel (refereegranskat)abstract
- Changes induced in the surface chemistry of metal powder during the delubrication stage affect the subsequent sintering process and hence the final properties of the sintered compacts. The different stages during delubrication were characterized by means of in situ visual observation of components made by compacting chromium-alloyed water-atomized steel powder with lubricant additions. A strong correlation is found between this visual observation of changes of the compact appearance during the delubrication and the component chemistry depicted via the monitoring of the exhaust gases composition (CO2, O-2 and H2O), thermogravimetric analysis, and changes in powder surface composition (X-ray photoelectron spectroscopy combined with scanning electron microscopy and energy dispersive X-ray analysis). The compacts were delubricated at different conditions and the effect of different parameters was studied, including temperature (300, 450 and 900 degrees C) and processing gas (N-2 or air). Delubricated compacts were further sintered at 1120 degrees C in N-2/3%H-2 mix to evaluate the effect of delubrication on the properties of the sintered components. The mechanical properties, chemistry and microstructure obtained after sintering show a strong relation to the surface changes induced during delubrication. The results indicate that delubrication at 450 degrees C in dry N-2 preserves the original surface condition of the powder and upon sintering the compacts exhibit optimum properties. Sintered compacts delubricated at 450 degrees C in air and at 900 degrees C in N-2 exhibited inferior properties. In the former case, this was related to the growth of Fe-oxide on the powder surfaces. In the latter case, the initial thin Fe-oxide layer, covering most of the powder surfaces, was reduced and oxygen made available was transferred into stable Cr-Mn-Si-rich oxide particles. Delubrication at 300 degrees C resulted in insufficient lubricant removal along with soot formation contributing to inferior mechanical properties after sintering as well. (C) 2015 Elsevier B.V. All rights reserved.
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| 8. |
- Karamchedu, Seshendra, 1987
(författare)
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Critical Aspects of Delubrication and Sintering of Chromium-alloyed Powder Metallurgy Steels
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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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| 9. |
- Karamchedu, Seshendra, 1987
(författare)
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Delubrication of Chromium Prealloyed Powder Metallurgy Steels
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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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| 10. |
- Karamchedu, Seshendra, 1987, et al.
(författare)
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Effect of active constituents in the atmosphere on the sintering of chromium-alloyed PM steels
- 2016
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Ingår i: World Powder Metallurgy 2016 Congress and Exhibition, World PM 2016; Hamburg; Germany; 9 October 2016 through 13 October 2016. - 9781899072484
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Konferensbidrag (refereegranskat)abstract
- Sintering of chromium-alloyed PM steels in atmospheres containing hydrogen and carbon monoxide has been studied. Thermogravimetric analysis was performed on PM steel compacts in atmospheres containing the gaseous constituents individually and when present together. Oxidation/reduction and carburization/decarburization of the compacts was evaluated based on thermogravimetric analysis complemented with bulk carbon and oxygen analysis of the samples. The results suggest that hydrogen promotes the reduction of the iron oxide layer covering the powder surface below 650 °C, the effectiveness of which improves with an increase in the concentration of hydrogen in the atmosphere. When hydrogen was additionally present, an enhancement in the reduction and carburization behavior was observed in the case of carbon monoxide containing atmospheres.
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