Effect of compaction pressure and powder grade on microstructure and hardness of steam oxidised sintered iron

Steam oxidation has proven to be an effective process to improve the proper ties of sintered iron components. The oxide Formed on the surface and in th e interconnected porosity strongly influences both the tribological and mec hanical properties of these materials, For example through the extent of po re closure and the nature and morphology of the oxide produced. In this pap er, the influences of compaction pressure acid powder size on the microstru cture. oxide content, hardness. and surface topography of steam treated sin tered iron are analysed. Specimens prepared from atomised iron powders of d ifferent sizes (< 65, 65-90, 90-125, and > 125 mum) were compacted at four different pressures (300, 300. 500. and 600 MPa), sintered Fur 30 min at 11 20 degreesC and then subjected to a continuous steam treatment at 540 degre esC for 2 h. A clear influence of the processing parameters on porosity was highlighted. tow porosity was always associated with high compaction press ure and greater powder size. Pore size was affected in the same way by comp action pressure, even though the effect of powder size acted in the opposit e sense. Changes in compaction pressure and powder size had no significant effect on pore shape. Decreasing powder size always led to high hardness. T he effect of compaction pressure on hardness is clear evidence of a comprom ise between porosity and blockage of the pore network by oxide. Samples pro duced with smaller powder sizes showed a continuous decrease in hardness as the compaction pressure increased, although for the large powder size ther e was a slight increase to a constant value of ultimate hardness. For the i ntermediate powder size a maximum hardness was obtained as the compaction p ressure increased. X-rai diffraction showed that the oxide layer is compose d of magnetite and haematite.