Mechanical properties of austenitic heat-resistant Fe-20Cr-30Ni-2Nb steel at ambient temperature

The mechanical properties of Fe-20Cr-30Ni-2Nb (at%) steels with controlled microstructures were examined by conducting tensile tests and Charpy impact tests at room temperature. The solution-treated specimen (γ-Fe single-phase) exhibited a yield strength (σy) of 210 MPa and a maximum tensile strength (σUTS) of 850 MPa. Its rupture strain (εr) was more than 40%. The specimen with high area fraction of the Fe2Nb Laves phase on the grain boundaries (ρ) exhibited a higher σy of 276 MPa and adequate tensile ductility (εr=29%). The specimen with high-density precipitates of the Ni3Nb phase in the grain interior exhibited a much higher σy. The value of σy of Fe-20Cr-30Ni-2Nb steel can be controlled by changing the precipitation morphology in the grain interior. Fractographic observations demonstrate that ductile intragranular fracture occurred in all of the tested specimens. The absorption energy measured by the Charpy impact test decreased from 211 to 31 J/cm2 as ρ was increased from 0% to 89%. The fracture mode appeared to vary from ductile transgranular fracture to cleavage transgranular fracture with increasing ρ. There was no indication of crack propagation within the precipitates of the Laves phase covering the grain boundaries or along the interface between γ-Fe and the Laves phase. Therefore, the Laves phase that precipitated on the grain boundaries would not have a negative effect on the room-temperature ductility.