Microstructure and mechanical properties of Fe-Ni-Cr-Al steel wires produced by in-rotating-water spinning method

Abstract

Nonequilibrium austenite, γ, or duplex austenite + lath martensite,γ+ α′ _L, phase wires with high strengths and large elongation have been produced in Fe-Ni-Cr-Al-C alloy system by the in-rotating-water spinning method in which a melt stream is ejected into a rotating water layer. These wires have a circular cross section and a white luster, and the wire diameter is in the range of 80 to 180 µm. The γ phase has a grain size as small as about 1 to 4 µm. The yield strength, Σ_y, tensile fracture strength, ay, and elongation, ε_p, are about 340 to 655 MPa, 440 to 975 MPa, and 12 to 22 pct for the γ single phase wires and about 465 to 865 MPa, 640 to 1350 MPa, and 2 to 18 pct for the α′_L+γ duplex phase wires. A cold drawing causes significant increases in Σ_y and Σ_f, and the attained values are about 3200 MPa and 4030 MPa for Fe-8Ni-12.5Cr-2.5Al-3C wire drawn to about 95 pct reduction in area owing to the formation of a strain-induced α′_L phase and a remarkable work-hardening ability of γ and α′_L phases. On the subsequent low-temperature annealing around 673 K, the Σ_y and Σ_f increase further to 4000 MPa and 4240 MPa, respectively, probably because of the enhancement of the interaction between dislocations and interstitial carbon atoms. Around the temperature (≃800 K) where the γ phase decomposes into a stable mixed structure of α + ordered bec compound + M₇C₃ on annealing, the ε_p decreases drastically and the fracture surface morphology changes from a dimple pattern to a cleavage pattern. It has been therefore inferred that the high strengths and good ductility of the melt-quenched y and γ + α′_L wires are due to the suppression of the phase transformation of y to a mixed structure of γ + ordered bec compound + M₇C₃ carbide by the melt-quenching technique.