Consolidation of eutectoid steel powder with mechanical milling

The mechanically milled Fe-0.8mass%C alloy powder (MM powder) was packed in stainless steel tubes and hot-rolled to consolidate powder to the bulk material with full density (MM bulk material), and the mechanism during consolidation of the MM powder was investigated by means of tensile testing at the consolidation temperature (973K) using the MM bulk material. The MM bulk material has micro-duplex structure which consists of ferrite and cementite grains of the size; 0.3 μm. The deformation stress of the MM bulk material is small as 80 MPa at 973 K, while high as 1700 MPa at room temperature. Although the total elongation was not so large (about 100%) under the condition of strain rate; 10^-2 to approximately 10^-1/s^-1 at 973 K, the m value was estimated at 0.3. Besides, it was confirmed that there were few dislocations within ferrite grains after tensile testing. These results demonstrate the fact that the MM bulk material deforms through the grain boundary sliding. Since the grain size in MM powder before consolidation (about 0.1 μm) is smaller than that in the MM bulk material (about 0.3 μm), MM powder is through to undergo superplastic deformation based on grain boundary sliding during the consolidation by hot-rolling.