Sintering of water-atomized iron and low-alloyed steel powder

Doctoral thesis, 2020

Iron and steel powder grades are extensively used in the powder metallurgy (PM) industry for manufacturing of ferrous structural components, with a major customer in the automotive industry. The powder grades are predominantly manufactured by water-atomization, yielding powder particles with highly irregular morphology and good compressibility, making the powder ideal for the conventional press-and-sinter manufacturing route.

The particles of the as-received powder are covered by a surface oxide layer, about 5-7 nm thick, as a result of the exposure to air or oxygen-containing atmospheres. The oxide layer is readily removed at low temperatures in the range 250-500 °C using hydrogen as a reducing agent, but the reduction process is strongly affected by the initial state of the powder and the alloy composition, as well as several processing parameters such as prior compaction of the powder and the composition of the processing gas. Since it contains a large fraction of the total oxygen content in the powder, removal of the oxide layer is a crucial step in the processing of these powder grades. Further raising the temperature enables the progressive reduction of various oxides as reflected by their thermodynamic stability, with oxides containing Cr, Mn and Si requiring significantly higher reduction temperatures than iron-based oxides. The progress of oxide reduction was studied using thermogravimetric and kinetic analysis and complemented with surface analysis methods.

Parallel to the chemical processes, shrinkage occurs as the powder particles are sintered. This increases the density of compacted powder as particles are bonded to each other and porosity decreases. The dimensional changes during sintering, as observed by dilatometry, are shown to be a function of compaction pressure, the direction relative to the compaction direction as well as multiple processing parameters such as the processing atmosphere, heating rate and sintering temperature where a significant phase-dependence exists. A large fraction of the total sintering shrinkage is attained already at low temperatures in the ferrite phase of the iron-based matrix.

The interaction between the chemical and physical processes are shown to be important for sintering as oxide reduction and oxygen removal are strongly dependent on the initial density of the compacted powder. Overall, oxide reduction is seen as a major goal in modern PM steelmaking in order to facilitate the development of strong, defect-free sinter necks. At the same time, sintering shrinkage and anisotropy must be considered as well to ensure dimensional tolerances. These two phenomena occur simultaneously, with large magnitudes already at temperatures significantly lower than standard sintering temperatures, indicating that the heating stage of the sintering process requires careful control in order to successfully sinter water-atomized iron and steel powder grades.