The mechanical properties of drawn dual phase steel wire

Abstract

Dual-phase steels are a class of composite high strength, low alloy steels. The outstanding properties of ferrite/martensite dual-phase steels include its very high workhardening rates and resistance to fatigue failure. The high work-hardening rates make dualphase steels ideal for large strain cold forming applications, since high strengths may be achieved with less deformation. Dual-phase steels have been shown to be especially suited ~~ wire drawing. In this study, Fe/xC/2Si alloys were heat treated to obtain different ferrite/martensite morphologies and relative volume fractions, then the work-hardening rates at true plastic strain £=0.006 were determined. Wires were then drawn from 4.2 to 1.4 mm in diameter, which corresponded to a strain of £=2.2, and tested in tension and fatigue. The early work-hardening rate increased with increased volume fraction of martensite, increased strength of martensite, and decreased ferrite/martensite interface coherency. The wire drawing limit was raised by discouraging void formation at ferrite/martensite interfaces and shear cracking through martensite particles. The lowest void density was obtained in a structure of fibrous martensite with coherent ferrite/martensite interfaces (i.e. produced by intermediate quenching). The fatigue limit of drawn dual-phase wires was -600 MPa, or 25-30% of the ultimate tensile strength, which appears to be higher than that of similar pearlitic steel wire. There was no superior microstructure for fatigue resistance in wires, as any morphology differences which exist in the initial structure are indistinguishable after heavy deformation. Fatigue cracks initiated on the wire surface at inclusions, sites of interface decohesion and martensite particle cracking, and in the interior of the wire at large cracks which formed during wire drawing. Attempts at monitoring small crack growth by replication were unsuccessful.