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| 2. |
- Hatami, Sepehr, 1981
(författare)
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Critical Aspects of High Performance Manufacturing of Structural Sintered Steel
- 2008
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Steel is the dominant material for the production of structural parts. By means of powder metallurgy (PM) processing, manufacturers have been able to manufacture steel parts with appreciable dimensional accuracy and proper mechanical properties. Nevertheless, end user demands on part manufacturers are not only suggesting the need for components with better mechanical properties, but also a lower final cost. Such requirements can question the competitiveness of the powder metallurgy process compared with the other well established manufacturing routes. Therefore, to fulfil the end user demands, in this thesis, some critical aspects of the powder metallurgy process for manufacturing structural steel components have been studied. The focus of the study is on compaction and sintering. Two powder metallurgical tool steels (Vancron 40 and Vanadis 10) commonly used in compaction dies were investigated. By means of optical, stereo and scanning electron microscopy in addition to X-ray diffraction analysis, the die materials were characterised and the wear mechanisms identified. Interference microscopy and X-ray photoelectron spectroscopy were used to examine the surface of the dies. The die made of Vancron 40 exhibited a much longer lifetime than the Vanadis 10 die. This discrepancy was mainly due to higher tendencies of oxide formation in the former material as compared to that of the latter. Vancron 40 suffered from mild abrasive, while Vanadis 10 exhibited strong adhesive wear leading to galling.In this thesis, some sintering aspects of water atomised chromium alloyed steel powders have also been dealt with. To simulate microstructures and mechanical properties acquired after sintering, a modern thermodynamic and kinetics software (JMatPro) was used. A good correlation between the simulation results and the available experimental data was found. Having developed a methodology for simulating microstructure development and mechanical properties; carbon content and cooling rate sensitivity analyses were performed. Moreover, to have better understanding of the reactions occurring between the powder compact and the sintering gas atmosphere, with the aid of another thermodynamic computer program (HSC Chemistry) the equilibrium partial pressures of the atmosphere constituents were modelled.
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| 3. |
- Hatami, Sepehr, 1981, et al.
(författare)
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Critical aspects of sinter-hardening of prealloyed Cr-Mo steel
- 2010
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Ingår i: Journal of Materials Processing Technology. - : Elsevier BV. - 0924-0136. ; 210:9, s. 1180-1189
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Tidskriftsartikel (refereegranskat)abstract
- In recent years, growing demand for greater mechanical properties of PM steel components with competitive fabrication cost has led to significant innovations in different fields of powder metallurgy. Recent research has been focused on reaching higher performance with lower cost. To this end, the possibility of combining the conventional sintering and post-sintering processes for a particular powder composition has been introduced. Sinter-hardening is a result of the research conducted along this line. Elimination of any secondary operation such as quench-hardening by incorporating it in the sintering process (i.e. sinter-hardening) is of great interest, as it will lead to lower processing costs and equal, if not higher mechanical performance. However, to ensure the desired mechanical properties of the final component and robustness of the performance, critical aspects of the sinter-hardening process should be rigorously studied. Hence with specific attention to a Cr-Mo steel powder (FL-5305), this study deals with the influence of density on cooling rate, the effect of different sintering temperatures (e.g. 1120 degrees C and 1250 degrees C) on austenite grain size and consequently, hardenability. The microstructure development in sinter-hardened FL-5305 material has been analyzed and predicted by means of the available literature for solid steel and also using the commercial software (JMatPro 5.0) for materials assessment based on thermodynamic and kinetics modeling. Finally, inaccurate carbon control and its adverse impact on excessive formation of cementite have been addressed. (C) 2010 Elsevier B.V. All rights reserved.
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| 4. |
- Hatami, Sepehr, 1981, et al.
(författare)
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Galling related surface properties of powder metallurgical tool steels alloyed with and without nitrogen
- 2010
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Ingår i: Wear. - : Elsevier BV. - 0043-1648. ; 269:2-4, s. 229-240
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Tidskriftsartikel (refereegranskat)abstract
- Two types of powder metallurgical tool steels (i.e. with and without nitrogen) are investigated with respect to their galling related surface properties. Despite similar macrohardness values, Vancron 40 (nitrogen alloyed) exhibits superior wear behaviour as compared to Vanadis 10 (without nitrogen); demonstrating approximately 20 times longer life span when used as die material in powder compaction. The main failure mechanism is mild abrasive wear for Vancron 40 and early severe galling for Vanadis 10. One important difference causing the tribological discrepancy is supposed to be associated with the preferred formation of solid lubricant oxides of the Magnéli type on the Vancron 40 surface as compared to Vanadis 10. The VN precipitates in Vancron 40 have half the thermal conductivity compared to that of the VC precipitates existing in Vanadis 10. Hence, as a result of the local accumulation of frictional heat generated during powder compaction (or any other type of forming process), Magnéli phases of V2O5 and/or VO2 are more easily formed and maintained on the Vancron 40 surface. With regard to surface oxides, the tool steels were studied by means of X-ray photoelectron spectroscopy (XPS). The tool steel surfaces were examined by means in situ SEM/AFM and electron backscatter diffraction (EBSD) for local adhesion force measurements and understanding the orientation of the phases, respectively. In addition, pin-on-disc wear tests were performed on the tool steels and the worn surfaces were analysed using scanning election microscopy (SEM) combined with energy dispersive X-ray spectroscopy (EDS).
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| 5. |
- Hatami, Sepehr, 1981
(författare)
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High Performance Manufacture of Sintered Steel – Critical Aspects of Tooling Performance and Sintered Microstructure
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Steel is the dominant material for the production of structural parts. By means of powder metallurgy (PM) processing, manufacturers have been able to manufacture steel parts with appreciable dimensional accuracy and desirable mechanical properties. Nevertheless, end user demands on component manufacturers are not only suggesting the need for components with better mechanical properties, but also a lower final cost. Such requirements can question the competitiveness of the powder metallurgy process compared with the other well-established manufacturing routes.In this thesis the aim is to obtain a deeper understanding of some critical aspects of the powder metallurgy process which can result in the production of higher quality parts at a lower cost. Hence, the emphasis has been placed on three main areas of the Press & Sinter process: tooling performance, microstructure control and optimised component design and process selection. With regard to tooling performance, two types of powder metallurgical tool steels (i.e. alloyed with and without nitrogen) are investigated. These tool steels are commonly used in the manufacturing of compaction dies. The results show that dies made of the nitrogen alloyed (i.e. Vancron 40) material exhibit a much longer lifespan than the nitrogen free (i.e. Vanadis 10) dies. Vancron 40 typically suffers from mild abrasive, while Vanadis 10 exhibits severe adhesive wear leading to galling. This tribological discrepancy is supposed to be associated with the preferred formation of solid lubricant oxides of the Magnéli type on the Vancron 40 surface as compared to Vanadis 10.Furthermore, critical aspects of the sintering operation with specific attention to the sinter-hardening process have been rigorously studied. To this end, the microstructure, mechanical and physical properties of a water atomised Cr-Mo prealloyed steel powder were simulated by utilising a modern thermodynamic and kinetic software (JMatPro). Based on the available literature for solid steel, JMatPro and finite element method (FEM) simulations, other issues such as the influence of density on cooling rate, the effect of different sintering temperatures (e.g. 1120 °C and 1250 °C) on austenite grain size, and consequently hardenability have been thoroughly analysed and predicted. Finally, a computer software (i.e. PM Manager) has been developed to provide designers with an easy to use tool with which the possibilities and limitations of designing and manufacturing a PM component can be readily evaluated.
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| 6. |
- Hatami, Sepehr, 1981, et al.
(författare)
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Modelling of Sintering Atmosphere and Microstructure Development of Chromium Alloyed Steel
- 2008
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Ingår i: Powder Metallurgy Progress. - 1335-8987. ; 8:2, s. 115-120
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Tidskriftsartikel (refereegranskat)abstract
- In order to take advantage of recent achievements in furnace technology and monitoring equipments, it is necessary to have a rigorous understanding of the chemical reactions that take place inside the sintering furnace. If overlooked, there is a risk for insufficient control of the sintering condition and the outcome will be scatter in PM parts’ final alloying content and mechanical properties. At the same time, end user demands require tight tolerances in product performance for sintered goods. In this study, different mixtures of iron based chromium prealloyed Astaloy CrM powder with the nominal composition of 3 wt.% Cr and 0.5 wt.% Mo were studied. The investigated powder mixtures were the plain powder, admixed with lubricant and admixed with lubricant and graphite (0.45 wt.%). Specimens were sintered in N2 and 10%H2/90%N2 atmospheres at 1120°C. Chemical reactions that are possible to occur in the above mentioned atmospheres and between the aforementioned powders mixtures are thoroughly examined from a thermodynamic point of view. Calculations have been performed to find the equilibrium partial pressures of the atmosphere constituents using a thermodynamic software package, HSC Chemistry 6.0. In addition, by means of modern thermodynamic and kinetic modeling software, JMatPro 4.0, the carbon activity for the alloying systems in question has been calculated. The simulation concept used for microstructural predictions of chromium alloyed sintered steel, developed by Nyborg et al. is applied to the alloying systems studied.
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| 7. |
- Hatami, Sepehr, 1981, et al.
(författare)
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Optimised selection of processing routes for sintered parts manufacture: A component-oriented approach
- 2009
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Ingår i: Advances in Powder Metallurgy and Particulate Materials - 2009, Proceedings of the 2009 International Conference on Powder Metallurgy and Particulate Materials, PowderMet 2009. - Princeton, N.J. : Metal Powder Industries Federation. - 9780981949611 ; , s. 139-1323
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Konferensbidrag (refereegranskat)abstract
- To stay competitive, PM manufacturers have to be able to accept a wide range of components with different features and properties. Thus, it is of utmost importance for manufacturers to adapt their common process routes to new requirements as rapidly and economical as possible. The procedure of identifying the right route among all the alternatives requires abundance of experience and knowledge and is a crucial and time-consuming decision; usually made by a team of engineers. In this study, by considering the mechanical properties and design specifications of the final component, a simple methodology is developed to aid the engineer(s) in selection of the optimum PM process route. In order to sort the possible routes according to cost, a cost estimating module has been devised. The total concept is applied to a case study and its accuracy has been evaluated.
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| 8. |
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| 9. |
- Hatami, Sepehr, 1981, et al.
(författare)
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Surface preparation of powder metallurgical tool steels by means of wire electrical discharge machining
- 2012
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Ingår i: Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science. - : Springer Science and Business Media LLC. - 1073-5623. ; 43:9, s. 3215-3226
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Tidskriftsartikel (refereegranskat)abstract
- The surface of two types of powder metallurgical (PM) tool steels (i.e., with and without nitrogen) was prepared using wire electrical discharge machining (WEDM). From each grade of tool steel, seven surfaces corresponding to one to seven passes of WEDM were prepared. The WEDM process was carried out using a brass wire as electrode and deionized water as dielectric. After eachWEDM pass the surface of the tool steels was thoroughly examined. Surface residual stresses were measured by the X-ray diffraction (XRD) technique. The measured stresses were found to be of tensile nature. The surface roughness of the WEDM specimens was measured using interference microscopy. The surface roughness as well as the residual stress measurements indicated an insignificant improvement of these parameters after four passes of WEDM. In addition, the formed recast layer was characterized by means of scanning electron microscopy (SEM), XRD, and X-ray photoelectron spectroscopy (XPS). The characterization investigation clearly shows diffusion of copper and zinc from the wire electrode into the work material, even after the final WEDM step. Finally, the importance of eliminating excessive WEDM steps is thoroughly discussed.
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| 10. |
- Hatami, Sepehr, 1981, et al.
(författare)
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Tempering of 3Cr–0.5Mo sintered steel: influence on mechanical properties
- 2012
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Ingår i: Powder Metallurgy. - 0032-5899 .- 1743-2901. ; 55:4, s. 302-308
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Tidskriftsartikel (refereegranskat)abstract
- Prealloyed 3Cr–0.5Mo steel powder (FL-5305) is considered as an outstanding sinter hardenable grade. Although the FL-5305 powder, at conventional sintering conditions, renders excellent tensile strength, yield stress and hardness, it suffers from low toughness. In this study, with the aim of increasing toughness without a considerable loss of hardness, different tempering treatments have been evaluated. To this end, tempering treatments have been performed at 200, 400, 550 and 650°C for different times. The influence of tempering on hardness, tensile properties as well as impact energy has been investigated. As compared to the hardness (20 HRC) and impact energy (14 J) of a conventionally sintered material, sinter hardening plus tempering at 650°C for 30 min resulted in a significant increase of impact energy (17 J), while the hardness remained the same. Tempering at 400°C for 2 h led to temper embrittlement and hence the lowest impact properties.
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