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

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 degrees C for 5 min, aged in the temperature range of 600-1000 degrees 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 delta-ferrite/gamma-austenite phase boundary and delta grain interior and boundary, but precipitation of the sigma 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 delta-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 sigma phase and secondary austenite precipitate on the delta ferrite side. The fractions of Cr2N and secondary austenite continuously increased during up to 2 h aging without sigma 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. Graphic

Automated summary

The research investigated the precipitation and phase transformation during the aging of a newly developed Mo-free lean duplex stainless steel. The study found that Cr2N and secondary austenite precipitated during aging, while the formation of sigma phase and carbides was suppressed due to the absence of Mo. Additionally, a zone of Cr and N depletion formed, leading to the consumption of primary austenite and formation of secondary austenite.