Effect of Microstructural Characteristics on Mechanical Properties of Austenitic, Ferritic, and γ-α Duplex Stainless Steels

Materials properties of γ (austenite), α (ferrite), and γ-α duplex stainless steels were experimentally examined using samples with different grain sizes (8 to 1000 µm) and different ratios of the γ to α phase (γ proportion: 35 to 78 pct). The mechanical properties (hardness and tensile strength) of the duplex stainless steel were about 1.5 times higher than those of the austenitic and ferritic stainless steels. Two main reasons for the high strength of duplex stainless steel were identified as follows: (i) severe interruption of slip deformation in the γ phase on the α phase; (ii) a high misorientation angle around phase boundaries between the γ and α phases, caused by bonding of the different lattice structures: γ-fcc and α-bcc. The ultimate tensile strength of duplex stainless steel increased with increasing proportion of the γ phase to 50 pct, but decreased with a further increase in the amount of γ phase. The mechanical properties improved with decreasing grain size of the stainless steels, which follows the Hall–Petch relationship; however, the reverse relationship was obtained for ferritic stainless steel, especially with large grain sizes (100 to 1000 µm), in which the size of hard Cr₂₃C₆ precipitates increased with increasing grain size.