High Performance Manufacture of Sintered Steel – Critical Aspects of Tooling Performance and Sintered Microstructure

Doctoral thesis, 2010

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.