Surface Composition and Microstructure Control during Pressureless Forming and Sintering of Ferrous Powder Metallurgy Materials

This study addresses key issues related to different stages of ferrous PM materials processing. Several analytical tools are applied in the study of surface composition and microstructure control in the processing of ferrous powder including forming, degassing and sintering stages. The tools include X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), mass spectrometry (MS), thermodynamic modelling (Thermo-Calc®) and optical microscopy. <p />Hot degassing was studied as a means of improving powder surface cleanliness. This technique is of interest when applied to high alloy PM materials that contain reactive elements such as Ti and Al. Vacuum degassing of an iron-base superalloy powder at above 350 °C resulted in gas release associated with hydroxide decomposition. This minimum temperature needed for effective degassing was in agreement with theoretical predictions. <p />Liquid phase sintering was applied as a means to enhance shrinkage during sintering of powder mixtures comprised of iron base powder and liquid forming particle agglomerates. Thermodynamic modelling was used in the design of alloys based on the Fe-P-C or Fe-B-C systems, resulting in relative sintered densities of 90% at: 1120 °C for the Fe-P-C system (25% liquid phase) and 1180 °C (25% liquid phase) Fe-B-C system. By using metal injection moulding as forming method, powder packing was improved and 99% relative final density was achieved for Fe-P-C material at 1120 °C (20% liquid phase). Post-sintering heat treatment (850 °C for 24 h) resulted in spheroidisation of iron phosphide and iron carbide that decorated the grain boundaries, which led to partial improvement of the microstructure. <p />Compared to the powder mixtures investigated, high speed steel powder (HSS) possesses significantly improved sintering behaviour because of its ability to undergo supersolidus liquid phase sintering. Thus, HSS powder was compacted by means of a novel shaping technique, designated as Starch Consolidation (SC). Rigid green bodies were attained by thermal treatment and after starch burn out, the HSS green bodies were sintered to full density at 1245 °C in vacuum. However, there was excessive carbide growth for this sintering temperature and therefore a new sintering cycle was developed with final hold at 1230 °C. It yielded good densification (>99%) and somewhat better microstructure. The final microstructure is far from optimised, due to too coarse MC carbides and the presence of M_{<font size="-1">6</font>}C primary carbides. Concerning the SC process, it is demonstrated how optimisation of powder packing is of crucial importance to be able to reach high relative sintered density. Processing practice including correct design and production of powder suspensions is shown to be essential