Abstract
The mechanical properties of sintered steel are improved by the combination of surface densification which is brought about by surface-working (extruding or rolling), and full annealing. The summarized results are as follows.
(1) In the case of extruding, the most effective reduction in area to improve the mechanical properties of sintered steel is about 11%.
(2) In the case of rolling, the mechanical properties of sintered steel are improved in the condition of smaller one pass reduction and larger amount of total reduction using the rolls of smaller diameter. The flexure strength increases from 75 to 125 kg/mm2, the limit of flexure increases from 0.7 to 1.5 mm and the fatigue strength increases from 12 to 17 kg/mm2, in the condition of 7.5 mm roll diameter, 0.02 mm one pass reduction and 0.2 mm total reduction, which was the best condition in the experiment.
(3) The improvement of mechanical properties of sintered steel in bending stress field may be due to the reasons that a proper density distribution is given by the cold surface-working (the density is almost theoretical at the surface layer and smaller inside) and that the flattened holes are spheroidized by the full annealing.
