Secondary Austenite Formation During Aging of Hot-Rolled Plate of a TRIP-Aided Mo-Free Lean Duplex Stainless Steel

Abstract

Precipitation and related phase transformation during aging of a hot-rolled Mo-free lean duplex stainless steel (DSS) were investigated. This steel belongs to a class of newly developed TRIP-aided lean DSS. The hot-rolled plate of the present DSS was solutionized at 1100 °C for 5 min, aged in the temperature range of 600–1000 °C for up to 2 h and then water-quenched. The microstructure of the aged DSS was analyzed by electron backscattered diffraction, (scanning-)transmission electron microscopy, and electron probe micro analysis. The Cr2N rapidly precipitated at the δ-ferrite/γ-austenite phase boundary and δ grain interior and boundary, but precipitation of the σ phase and carbides was suppressed due to the absence of Mo. A zone of Cr and N depletion related to Cr2N precipitation formed on the primary austenite side rather than on the δ-ferrite side, resulting in formation of secondary austenite by consuming the primary austenite as aging proceeded. The secondary austenite formation of the present DSS is in contrast to that of conventional Mo-bearing DSS in which the σ phase and secondary austenite precipitate on the δ ferrite side. The fractions of Cr2N and secondary austenite continuously increased during up to 2 h aging without σ phase precipitation. The similarities and differences of microstructural evolution of the present TRIP-aided lean DSS and conventional DSS during aging were addressed. The effects of such microstructural evolution on the room temperature tensile properties of the present DSS were also discussed.