Critical Aspects of High Performance Manufacturing of Structural Sintered Steel

Licentiatavhandling, 2008

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.