| 1. |
- 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
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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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| 2. |
- 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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| 3. |
- 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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| 4. |
- 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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| 5. |
- Karamchedu, Seshendra, 1987, et al.
(författare)
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Effect of Delubrication Process Parameters on Surface Characteristics of Cr- alloyed PM steels
- 2012
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Ingår i: Materials for Tomorrow 2012.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Utilizing chromium as an alloying element in structural powder metallurgy (PM) steel parts provides advantages in terms of profitability and recyclability but poses a challenge in terms of processing due to high chromium affinity to oxygen. From the previous investigations it is known that water-atomized chromium prealloyed PM steel powders after reduction annealing are dominantly covered by a thin Fe-oxide layer with fine particulate oxides in between [1-2], meaning good powder sinterability. However, transformation of the surface iron oxide into more thermodynamically stable ones during the consolidation can inhibit the formation of sintering necks which determine the strength of the parts. Hence, the surface chemical changes during each stage of sintering have to be evaluated in order to carefully design the process in a way that proper sintering is ensured and re-oxidation of the powder is avoided. Delubrication in PM processing precedes the high temperature sintering step and aims at efficient removal of the admixed lubricant. At the same time, the process has to be performed in a way to avoid/minimize changes in surface oxide chemistry and amount. In the present work the surface chemical analysis of chromium prealloyed steel Astaloy CrM (Fe-3Cr-0.5Mo) admixed with 0.6 ethylene bis-steramide lubricant is performed after delubrication under different processing conditions. Experimental techniques used were X-ray photoelectron spectroscopy (XPS) in combination with high-resolution scanning electron microscopy (HR SEM) and energy dispersive X-ray spectroscopy (EDX). Investigation of the surface chemical characteristics has been performed on samples delubricated in dry nitrogen atmosphere at 450 and 900°C and in air at 450°C. Delubricated in N2 at 450°C compacts showed the best surface condition similar to that of the base powder. XPS analysis revealed the presence of much higher concentration of Cr and Mn oxides on the surface of the sample delubricated at 900°C. SEM and EDX analysis confirmed the presence of particulate oxides containing the aforementioned elements indicating the growth of undesirable thermodynamically stable oxides at this temperature at the expense of the pre-existing Fe-oxide layer. After delubrication in air at 450°C the thickness of the Fe-oxide layer on the powder particle surfaces increased by a few nanometers in the center and a six fold increase in the oxide layer thickness at the edge of the samples was observed. High magnification SEM imaging on the sample showed presence of regions with the oxide thickness more than 100 nm near the edge of the sample compared to initial 6 nm, characteristic for the base powder. Based on the results from this investigation it is strongly recommended to perform delubrication at~450°C in an inert atmosphere since the as-received surface condition of the powders is retained and no undesirable changes occur.References:1. Chasoglou, Dimitris. PhD Thesis, Surface Chemical Characteristics of Chromium-alloyed Steel Powder and Role of Process Parameters during Sintering. s.l. : Chalmers University of Technology, 2012. ISBN 978-917385-711-6.2. Karlsson, Henrik. PhD Thesis, Role of Surface Oxides in Sintering of Chromium-Alloyed Steel Powder. s.l. : Chalmers University of Technology, 2005. ISBN 91-7291-703-2.
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| 6. |
- Karamchedu, Seshendra, 1987, et al.
(författare)
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Influence of delubrication process on the properties of sintered PM components
- 2013
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Ingår i: Proceedings of Euro PM2013. ; 2, s. 163-168
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Konferensbidrag (refereegranskat)abstract
- Problems associated with improper delubrication are frequently encountered during the manufacturing of sintered components but are very difficult to identify in a continuous sintering process. The effect of delubrication parameters on the final properties of chromium prealloyed PM steels has been studied by varying the delubrication processing conditions (thermo-profile and processing atmosphere) followed by sintering in Ar/10%H2 mixture at 1120 ºC. The best results after sintering were obtained for the compacts delubricated at 450 °C in dry nitrogen. Increasing the delubrication temperature to 900 °C resulted in slightly higher impact energy and as-sintered oxygen content but high carbon loss and lower hardness. Delubrication in air resulted in high carbon losses and the lowest impact energy and hardness values. These effects were related to considerable surface oxide changes during delubrication detected by HR SEM+EDX analyses. Delubrication at 300 °C was insufficient resulting in soot formation and inferior as-sintered mechanical properties.
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| 7. |
- Karamchedu, Seshendra, 1987, et al.
(författare)
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INFLUENCE OF PROCESS PARAMETERS ON DELUBRICATION OF PM STEELS
- 2011
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Ingår i: Powder Metallurgy Progress. - 1335-8987. ; 11:1-2, s. 90-96
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Tidskriftsartikel (refereegranskat)abstract
- A systematic study of the effect of various process parameters on lubricant removal in the processing of chromium prealloyed PM steels has been conducted. Improper delubrication affects the densification process and can also lead to blistering, sooting and deposition of decomposition products inside the furnace and on the PM compacts. Current work is focused on defining optimum conditions for efficient delubrication with minimum effect on PM compact properties at lowest processing cost. Effect of temperature, holding time, heating rate, process gas composition (inert, reducing and oxygen containing atmospheres), purity (wet and dry gases) and flow rate was evaluated based on in-situ continuous monitoring of the dew-point, carbon dioxide and oxygen content using industrial sensors. For ethylenebissteramide based lubricants, delubrication at temperature around 450°C with a low heating rate in an inert processing atmosphere with rather low flow rates (enough to provide weak dynamic gas conditions) is recommended.
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| 8. |
- Karamchedu, Seshendra, 1987, et al.
(författare)
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Lean atmospheres for sintering chromium alloyed PM steels
- 2013
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Ingår i: Proceedings of Euro PM2013. ; 2, s. 353-358
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Konferensbidrag (refereegranskat)abstract
- Hydrogen and carbon monoxide are effective reducing agents at different temperature ranges and there is only scarce experimental and theoretical data about their interaction and combined reducing effect at dry conditions and low amount. The work presents experimental and theoretical evaluation of the reducing and carburizing activity of lean nitrogen-based atmospheres containing hydrogen, carbon monoxide and small additions of hydrocarbons with their total content in the range of 0.3 to 5 vol%. Effect of carbon content (0 and 0.4 wt%) and temperature profile in the case of sintering of chromium prealloyed PM steels was evaluated. Results indicate that the carbon pickup increases with increasing CO content in the atmosphere, however it brings significant risk of oxidation. Atmospheres with low content of CO and H2 (
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| 9. |
- Karamchedu, Seshendra, 1987, et al.
(författare)
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SINTER-HARDENING RESPONSE OF SINTERED STEEL BASED ON ASTALOY MO AND ITS DIFFUSION BONDED DERIVATIVES
- 2014
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Ingår i: Powder Metallurgy Progress. - 1339-4533. ; 14:3, s. 93-99
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Tidskriftsartikel (refereegranskat)abstract
- This study addresses the sinter-hardening response of sintered steel based on Astaloy Mo grade (prealloyed with 1.5 wt.% Mo) and its derivatives Distaloy DH- diffusion bonded with 2 wt.% copper and Distaloy DC- diffusion bonded with 2 wt.% Ni, respectively. Compacts of these powder grades 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 followed by stress relieving at 200 °C for 60 minutes. In order to understand the relation between microstructure and the obtained mechanical properties, tensile testing and metallographic investigations were performed. It is shown that the distribution of the alloying elements as expected has a crucial impact on the microstructure development where regions lean in Cu or Ni become bainitic while regions with Cu or Ni become martensitic. Also, despite of the fact that Cu is more homogeneously distributed into the DH material, while Ni partly remains in Ni-rich regions in the DC material, both alloys show similar sinter-hardenability. The compensating effects by carbon redistribution upon bainite formation and Ni on C activity are supposed to be key factors in this context.
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