Abstract The formation of β_(Mn) type structure in several Fe-Mn-Al duplex steels with com-positions varying from 33 to 37％ Mn and 5 to 9％ Al has been systematically stud-ied by microscopic examination, X-ray diffraction analysis, microhardness indentation test,hardness measurement and impact test. The temperature range of transformation for (δ+γ) to β_(Mn) lies between 500°and1000℃. The lower temperature limit is lowered by the addition of Al and the upperlimit raised by Mn and Al. The morphology of β_(Mn) formed depends on the composition and heat treatment ofthe alloys. β_(Mn) needles are preferentially originated from δ/δ and δ/γ grain boundaries,ferrite grains, etc., and then grow into rods. At higher temperatures β_(Mn) forms inblocks along the δ/γ boundaries and later grows into δ-ferrite grains. A relief effect onthe polished surface has been observed after the formation of β_(Mn) needles and it is sug-gested that this β_(Mn) is formed by the bainitic transformation mechanism. It has beenfound that the impact value decreases and the hardness increases with increasing β_(Mn)content.
