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 (-Fe single-phase) exhibited a yield strength (y) of 210 MPa and a maximum tensile strength (UTS) of 850 MPa. Its rupture strain (r) was more than 40%. The specimen with high area fraction of the Fe 2 Nb Laves phase on the grain boundaries () exhibited a higher y of 276 MPa and adequate tensile ductility (r = 29%). The specimen with high-density precipitates of the Ni 3 Nb phase in the grain interior exhibited a much higher y. The value of y 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/cm 2 as was increased from 0 to 89%. The fracture mode appeared to vary from ductile transgranular fracture to cleavage transgranular fracture with increasing. There was no indication of crack propagation within the precipitates of the Laves phase covering the grain boundaries or along the interface between-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.

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The effect of the initial microstructure on the hot workability of a powder metallurgy Ni-based superalloy was investigated in the high-temperature range of 950 °C to 1180 °C and strain rate range of 0.001 to 1.0 s−1. Six samples with different initial microstructures were fabricated by various hot isostatic pressing (HIP) conditions and subsequent treatments such as hot extrusion. The coarse-grained samples exhibited low hot workability regardless of the deformation conditions. In contrast, the hot workability of the fine-grained samples significantly varied depending on the deformation conditions. The hot workability exhibited a peak at the sub-solvus temperature of ~ 1100 °C and decreased at temperatures higher and lower than this temperature. In addition, the hot workability decreased monotonically with increasing the strain rate. The prior particle boundaries (PPBs) acted as cavity nucleation sites and crack paths, especially at lower temperatures and higher strain rates, resulting in early fracture and low hot workability. With decreasing the grain size, the hot workability at the peak temperature improved. The extruded sample with the smallest grain size exhibited the best hot workability, owing to the avoidance of PPB fracture and the acceleration of dynamic recrystallization.

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