Aging Precipitation Behavior of 18Cr-16Mn-2Mo-1. 1N High Nitrogen Austenitic Stainless Steel and Its Influences on Mechanical Properties

Abstract

The solution-treated (ST) condition and aging precipitation behavior of 18Cr-16Mn-2Mo-1. 1N high nitrogen austenitic stainless steel (HNS) were investigated by optical microscope (OM), scanning electron microscope (SEM), and transmission electron microscope (TEM). The results show that the ST condition of 18Cr-16Mn-2Mo-1. IN HNS with wN above 1% is identified as 1 100 °C for 90 min, followed by water quenching to make sure the secondary phases completely dissolve into austenitic matrix and prevent the grains coarsening too much. Initial time-temperature-precipitation (TTP) curve of aged 18Cr-16Mn-2Mo-1. 1N HNS which starts with precipitation of 0.05% in volume fraction is defined and the “nose” temperature of precipitation is found to be 850°C with an incubation period of 1 min. Hexagonal intergranular and cellular Cr2N with >a = 0.478 nm and c = 0.444 nm precipitates gradually increase in the isothermal aging treatment. The matrix nitrogen depletion due to the intergranular and a few cellular Cr2N precipitates induces the decay of Vickers hardness, and the increment of cellular Cr2N causes the increase in the values. Impact toughness presents a monotonic decrease and SEM morphologies show the leading brittle intergranular fracture. The ultimate tensile strength (UTS), yield strength (YS) and elongation (El) deteriorate obviously. Stress concentration occurs when the matrix dislocations pile up at the interfaces of precipitation and matrix, and the interfacial dislocations may become precursors to the misfit dislocations, which can form small cleavage facets and accelerate the formation of cracks.