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

Abstract

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.

Iron Aluminides

“…Moreover, an Fe-Al-Nb alloy doped with boron to precipitate a continuous film of Laves phase along the grain boundaries was ductile at comparably low temperatures [Fe-Al-Nb-(B) in Figure 7] (104). The latter finding matches recent reports on steels strengthened by close-packed phases precipitated along grain boundaries (122). Apparently, although these phases are inherently brittle, no cracking occurs along or through these brittle phases as long as they form a more or less continuous grain boundary film on the order of 1-μm thickness.…”

Laves phases: a review of their functional and structural applications and an improved fundamental understanding of stability and properties

“…From room temperature tensile and Charpy impact tests, it was concluded that the Laves phase on the grain boundaries has no negative effect on the room temperature ductility. There was no indication of crack propagation within the Laves phase precipitates or along the interface between c-Fe matrix and Laves phase, and fracture was found to be always transgranular [836].…”

“…While at 1200°C the alloy is single-phase fcc-Fe, it is two-phase with Nb(Fe,Cr,Ni) 2 Laves phase precipitates at 800°C, and becomes three-phase by additional precipitation of c'' (Ni 3 Nb) in the fcc-matrix during cooling to 700°C [829,830]. In a series of investigations, the properties of this alloy were later on further improved by modifications of composition and heat treatments [829][830][831][832][833][834][835][836][837][838]. These studies showed that strengthening in this alloy is especially determined by the grain boundary precipitation of Laves phase and that the coverage of the grain boundaries by precipitates and their morphology can be controlled by small additions of B.…”

Effect of Homogenization Treatment on Microstructure and Eutectic Precipitates in Ce‐Containing 15Cr–22Ni–1Nb Austenitic Heat‐Resistant Stainless Steel

“…[5] These large eutectic precipitates are detrimental to the mechanical properties of austenitic heat-resistant steels, such as thermal fatigue properties, [6,7] creep strength, [8,9] and tensile properties. [10,11] In order to eliminate these large eutectic precipitates and reduce the microsegregation of alloying elements, high-temperature homogenization treatment was conducted on the Ce-containing 15Cr-22Ni-1Nb steel.…”