COMPARISON OF MECHANICAL PROPERTIES OF MANGANESE STEELS BASED ON SPONGE AND ATOMISED IRON POWDERS

Low carbon ferro-manganese and graphite powders were admixed to Hoganas sponge, NC 100.24, and water atomised, ABC 100.30 and ASC 100.29, iron powders - to produce three variants of Fe-3Mn-0.8C. These were pressed into tensile and bend specimens at 660 MPa, sintered in semi-closed containers for 1 hour in dry nitrogen or hydrogen at 1120 or 1250°C and cooled at 64°C/min. Neither detectable densification nor swelling took place, whatever the powder and sintering conditions. Green and sintered densities were expectedly highest for ABC 100.30 base at ~7.1g.cm -3 , >0.2 g.cm -3 larger than for the sponge-based alloy. Similarly, Young's modulus in the former material attained ~130 GPa, being ~117GPa in the latter. Yield strengths were higher for the atomised iron base alloys and for 1250°C and nitrogen sintering, from 370 - 440 MPa (1120°C – NC 100.24) to ~ 520 MPa (1250°C - ABC 100.30). Tensile and bend strengths were somewhat higher for nitrogen sintering and, generally by ~10%, higher for specimens pressed from sponge iron, resulting in ~730 MPa tensile strength for sintering in nitrogen at 1250°C, to be compared to the strongest ABC 100.30 base at ~650 MPa. The higher plasticity of the sponge-based Mn steel, reaching >4%, and hence fracture strength and dimple rupture, are associated with the increased surface area available to the Mn vapour for alloying. This reduces the Mn content at sinter necks, thereby maintaining a much better Mn-C balance for forming pearlite, and ensures cleaner, more cohesive prior particle boundaries, the favoured microcracking paths.